Some thoughts on Event-Based Components

The German software engineer Ralf Westphal currently spreads some knowledge about an alternative model for programming components and especially communication between them. Due to their nature they are called Event-Based Components. After some discussion with colleagues at SDX I want to share some of my thoughts on that with you (of course for further discussion as well).
The aim of Event-Based Components (EBC) is to create software components that are really composable without specific topological dependencies. You can compare EBCs with elements in electronic circuits. But first things first…

Interface-Based Components style

Normally we’re developing components in .NET as IBCs: Interface-Based Components. That means client classes have topological and functional dependencies to interfaces (or directly to other classes), which provide some sort of functionality. Well developed, such a dependency could be resolved with a Dependency Injection container like StructureMap:

class Program
    static void Main(string[] args)
        // Bind (here: StructureMap)
        ObjectFactory.Initialize(x =>

        // Resolve and Run
        IBusinessClient client = ObjectFactory.GetInstance<IBusinessClient>();

interface IBusinessClient
    void BusinessOperation(int personId);

class BusinessClient : IBusinessClient
    private readonly IDataAccessComponent _dataAccessComponent;

    public BusinessClient(IDataAccessComponent dataAccessComponent)
        _dataAccessComponent = dataAccessComponent;

    public void BusinessOperation(int personId)
        Person p = _dataAccessComponent.GetPerson(personId);
        // do something ...

interface IDataAccessComponent
    Person GetPerson(int id);

class DataAccessComponent : IDataAccessComponent
    public Person GetPerson(int id)
        return  // ...some person...

That’s pretty standard so far, isn’t it? In Ralf’s opinion this programming style lacks real composability of the components. Due to the topological dependency the clients is bound to a specific interface and no arbitrary component can perform the functionality. Instead a component has to implement the specific interface. You’re not able to use components which could provide the functionality, but don’t implement the interface…

Event-Based Components style

Ralf suggests Event-Based Components to the rescue. Components in this programming style can be compared to components in electronic circuits. Methods act as input pins of a component and can be called by other components. Events/delegates act as output pins and establish a connection to other components that should be used by the component or to provide calculation results. The output pins can be bound to any arbitrary method that meet the signature. Thus the dependency is still functional, but not topological any more.
The example above in EBC style could look as follows:

class Program
    static void Main(string[] args)
        // Build
        var client = new BusinessClient();
        var dataAccess = new DataAccessComponent();

        // Bind
        client.GetPerson = dataAccess.GetPerson;

        // Run

class BusinessClient
    public Func<int, Person> GetPerson { get; set; }

    public void BusinessOperation(int personId)
        Person p = GetPerson(personId);
        // do something ...

class DataAccessComponent
    public Person GetPerson(int id)
        return  // ... some person ...

This example shows the components BusinessClient and DataAccessComponent interacting as EBCs in a very simple form by using the Func<T> delegate type and thus enabling symmetric communication. Ralf encourages the use of standard input/output pins as Action<object>, which leads to asymmetric communication, because the DataAccessComponent would need to declare an output pin for providing the Person as result of GetPerson(). For the sake of simplicity I haven’t followed this principle here.

So the example uses a Func<T> delegate and no event. But you can still think of it as Event-Based Component, just because events are nothing more than multicast delegates. I could have used events instead of the simple delegate as well, but I’m quite fine, because I don’t need the functionality of multiple subscribers here.

As you can see from the example, just like IBCs the EBCs have some kind of initial Bootstrapper phase. This is the time when the components are composed. The output pins of a component’s client (BusinessClient in this example) are connected with the input pins of the component itself (here: DataAccessComponent).


When I first saw EBCs I thought: „Dude, this is damn cool, isn’t it?“. Indeed this kind of programming style first feels strange and alternate and thus for me it’s really interesting. But are there some real benefits as well?

I think one big benefit of EBCs is their composability. A client hasn’t to know the interface of a component from which he wants to use some functionality. A component on the other side is not forced to implement an interface to provide some functionality, but it’s still retaining loose coupling. Even without interfaces the components are still independent from each other and have great testability.

Other benefits I see are the exchangeability and the topological independence. Components are not bound to a specific topological context in form of interfaces and thus are independent from topological changes on the interfaces. You can exchange the components easily by replacing the binding section with any other setup phase and can binding other methods to them. Especially your components are not forced to use (or implement) some kind of interface from which they will perhaps use just one single functionality…

Last but not least I see a very easy way to intercept calls and adding functionality without changing the components themselves. If you use events as output pins you can add some more event handlers in the binding phase. Thus you can easily integrate Logging, Tracing etc. into your applications. Of course you can achieve this with IBCs as well, I just say that EBCs are suiting very well for those requirements.


Besides those benefits in my opinion there are some significant drawbacks as well.

First of all is the additional complexity which comes with EBCs. Composing EBCs can become complex, at least in projects of significant size. Due to binding methods and events together on the fine instead of interfaces on the coarse, there have to be much more binding statements. In fact you can think of an event’s signature as a one-method interface that has to be fulfilled from components. Furthermore (again especially in projects of a reasonable size) you will loose intelligibility and  overview over your system and the component interaction. Any arbitrary component can provide a functionality and there is no way to navigate between layers and components as clients and suppliers of functionality. Explicit interfaces are much more comprehensive than such „implicit“ specifications.  Perhaps in the future there will be tools that simplify composition between EBCs and navigation through EBCs, but until there’s such a tool I consider this as serious drawback.

Another drawback of EBCs is the loss of interfaces as formal contract of coherent functionality. Of course you can define interfaces and let your components implement them, but while clients are not compelled to use them they loose much of their value. Interfaces force components to implement a certain set of functionality completely and make this set explicit. Clients have to refer this contract explicitly. Explicit contracts lead to intention revealing and this is a good thing!


So in my opinion EBCs have benefits as well as shortcomings. I think they are worth investigating and knowing them, but at the moment I don’t see that they will establish well and become a replacement for IBCs. First there is the higher complexity, which could perhaps be solved by tools and some sort of „DI container“ for EBCs in the future. But second, being explicit and define formal contracts through explicit interfaces is no bad thing. Of course it’s not cheap as well, but I don’t see that this justifies the application of EBCs on the small scale. On the big scale there are other solutions like BizTalk, NServiceBus etc. to achieve the goal of pluggable components which have features like scalability as well. So perhaps there are delimited scenarios for using EBCs (like component topologies that change often), but I would not suggest to use them in general.

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A look at: Contract Driven Development

Today I want to take a look at this paper (PDF) entitled „Contract Driven Development = Test Driven Development – Writing Test Cases„. Like the paper on SDD I found this essay on my research for a synergetic development approach that takes both DbC and TDD into account.

The paper was written by A. Leitner et al. at the ETH Zurich in 2007, when important steps towards automatic testing had been already made. One of the co-authors of the paper is Bertrand Meyer, the inventor of the Eiffel programming language which can be seen as cradle of DbC. Hence it’s not surprising that the paper and the described tool are based on Eiffel.

Content of the paper

The paper describes Contract Driven Development (CDD) as new approach to lower the effort for writing tests with intensive use of contracts. This approach is based on a mechanism to extract test cases completely automatically from failure-producing runs of a component, where contracts act as test oracle.

In the introduction the paper takes a short look at current developments towards automatic testing and shows the drawbacks that come with many tools. Automatic testing tools don’t know the semantics of a program and the insights a programmer has and thus those tools cannot distinguish between meaningful and meaningless inputs. CDD wants to overcome this drawback. It relies on the observation that developers are often writing implicit test cases when manually running a program to determine the correct behavior of their code. But many developers don’t extract those test cases as automatic unit tests and thus those tests cannot be run in a reproducible way.

The paper presents CDD as method that captures those implicit test cases automatically and extracts them into explicit tests. One important limitation is that it only captures test cases which produce a failure e.g. through a broken contract or other exceptions. With this approach the resulting test suite contains tests for mistakes made by the developers in the past. The developer builds up this test suite by running the application with corresponding input values. The authors state that the test suite would have similar properties to a suite as result from TDD when you write contracts before the implementation of a feature. You can find my personal thoughts on this below.

The paper describes a mechanism by which CDD observes program executions and detects the last uninfected state when a failure occurs. In this case CDD takes a snapshot of this state and a new test case for the failing component is created automatically. The snapshot serves as starting context for the test case by which the failure can be reproduced.

With this process the developer has to provide the inputs triggering a failure only once. Then CDD jumps in, saves the values that lead to the failure and extracts a test case to reproduce the failure. The developer can go forward with his implementation and can choose to fix the bug later on while the failure is saved as reproducible test case. Contracts can help a lot in this process since they can act as oracle for expected behavior and can express assertions that go beyond some technical exceptions. With this the benefit of CDD is mainly driven by a extensive utilization of contracts.

The authors finish with a conclusion of their work:

This article explains the fundamentals of the Contract Driven Development approach. A tool autonomously observes the developer while he is working on a program and extracts test cases from failures either provoked by the developer (in the spirit of test driven development) or by mistake (leading to a regression test). The approach is novel in that complete test cases are extracted not only from the information provided by the system under test, but also from non-permanent clues given by the programmer during development.

My thoughts

Personally I feel a little ambivalent about the CDD approach. First I like the idea of making implicit test cases which are run by the developer explicit by extracting them automatically. This includes the developer’s knowledge about the context of an application into the generated test cases and thus goes beyond the technical aspect of automatic testing. It takes some work on writing test cases from the developer’s shoulder, but it doesn’t have as many advantages as one may think…

Expected functional behavior can be defined by contracts (postconditions, invariants) which act as test oracle for CDD. But it’s a limitation as well. Important characteristics of code are difficult to define with contracts while they can be easily defined with explicit unit tests (aspect of expressiveness). Thus CDD is limited in its expressiveness as well.

Furthermore CDD extracts tests for failing runs only. But what about the other side of the medal? For successful runs no test case will be generated automatically which leads to the problem that if anything in the code changes there is no possibility to check the successful tests without running them manually again. Thus CDD is not suitable for complete regression testing.

Last but not least the authors state that a test suite created with CDD is very similar to TDD (beside other parallels to TDD). Moreover the title of the paper gets it to the simple formula „CDD = TDD – Writing Test Cases“. Here I strongly disagree! CDD and TDD have very different properties. TDD takes things like code design and specification into account, is well-suited for documentation purposes and creates a real regression testing suite. None of those aspects is in scope of CDD, so TDD goes far beyond CDD.

At the end I think CDD is an interesting approach, but the benefits are bought dearly and eaten up by the drawbacks. One of the most serious drawbacks for me (personally) is the design aspect. If you rely on CDD you are not driving your API design to be clear and lucid. There will be no unit tests for which you want to have loosely coupled components and furthermore CDD will everytime run hard against your concrete infrastructure.
To make a final advice: The only scenario where I would use CDD is for automatic test generation when running the software in production. If something fails in production with CDD you as developer could get immediate feedback about what went wrong and you could immediately reproduce the failure by running the extracted test case. This would be a nice support for bugfixing and it would be really valuable. But for testing purposes in the development process I don’t see a chance for CDD.

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A look at: Agile Specification-Driven Development

While investigating the synergy of TDD and DbC I asked myself if there are not already development processes that combine both principles. And on my research I stumbled over this paper (PDF) named Agile Specification-Driven Development on which I want to look in this blog post.

Content of the paper

The paper (published in 2004) from J. Ostroff, D. Makalsky and R. Paige describes the agile approach of Specification-Driven Development which combines features of both TDD and DbC.

The paper starts with an introduction that states the TDD and DbC can enhance each other. Tests and contracts both are some kind of specification with benefits and drawbacks on each of them. This is equivalent to my points on types of specification some blog posts ago.

Further on the authors describe plan-driven development and DbC as plan-driven development approach. Thus they show DbC as process coupled to the design phase when mapping the complete set of requirements to the software. DbC with its contracts as specifications closes the gap between requirements and code. In the following section DbC in general with some of its benefits is described and the „quality-first“ DbC design method from Bertrand Meyer is briefly explained.

Chapter 3 describes TDD as agile development method to replace plan-driven up-front design by „stressing the development of working code over documentation, models and plans“. Striking aspects and benefits of TDD are shown as well as tests as a form of specification. The authors make a good point on what they call collaborative tests which address the interaction/collaboration of code components. Thus collaborative tests are related to UML sequence/collaboration diagrams. Collaborative tests go beyond the single-component-based TDD process which is an important insight. The next section shows the drawbacks of tests as example-driven approaches which I have shown before as well when discussing universality of tests and contracts. And the authors describe the „problem“ of contract checking: program verification as difficult task and runtime checking of the assertions which needs unit tests to stress the contracts. This again mirrors my thoughts on checking correctness.

The SDD process

Chapter 4 contains the description of the Specification-Driven Development (SDD) approach as combination of TDD and DbC. It starts with a motivation for this movement:

„There are surprising commonalities between TDD and DbC, particularly: both contracts and tests are specifications; both TDD and DbC seek to transform requirements to compilable constructs as soon as possible; both TDD and DbC are lightweight verification methods; both methods are incremental; and both emphasise quality first in terms of units of functionality. We claim that it is not necessary to choose between the two approaches a priori, and that there are substantial benefits to using TDD and DbC together in a project.“

Agile Specification-Driven Development

The picture on the left side shows the statechart of the SDD approach. It’s important to see that SDD doesn’t dictate where to start with development. The authors state that „it is the developer’s choice whether to start with TDD or DbC based on project context“ while „the emphasis is always on transforming customer requirements into compilable and executable code“.

Note that SDD takes three sub-processes into account: DbC on the right, TDD on the bottom and collaborative tests/specifications on the left. You can switch between these three processes at any time, it’s up to your interpretation and you are responsible to find a valid workflow…

The authors say that „SDD provides more than TDD or DbC individually, as it eliminates some of the limitations with each approach“. And for them „SDD is more than the sum of TDD and DbC, as there are synergies between the approaches“.

Another point is made on contracts as test amplifiers. Contracts can help to drive the production of tests since they show requirements to call a code component and the conditions that should hold in return. And tests should validate/invalidate pre- and postconditions to show the correctness and the appropriateness of contracts.

The description of SDD ends with some observations and the recommendation to prefer writing unit tests before contracts. However it doesn’t make a statement whether to start with TDD or with collaborative tests and how the process should look in concrete.

The paper closes with a short conclusion and a table as summarization of those conclusions.
Two synergetic advances of SDD are pointed out:

  1. Contracts are test amplifiers,
  2. Contractual and collaborative specifications provide lightweight verification of the design.

Thoughts on SDD

SDD mirrors some of my thoughts on the synergy of DbC and TDD and I think it’s a movement in the right direction.

Otherwise for me the paper makes too vague statements and it’s only scratching on the surface. SDD doesn’t describe a clear process of how to manage DbC, TDD and collaborative tests. The given statechart is nothing more than a collection of those three principles and doesn’t state how they can be used in combination. For me it has a too „academic“ touch and isn’t worth much for a practitioner. Just to say „use these parts in combination and decide for yourself“ doesn’t help, especially beginners need clear rules and a clear process to get started to use DbC and TDD in conjunction. Thus I can’t agree with the authors that SDD is more than the sum of the parts. Perhaps it is, but the paper doesn’t clarify this.

Furthermore the paper doesn’t make a strong statement about the combination of tests and contracts. One sentence says that tests should exercise the contracts by validating and invalidating each pre- and postcondition. With this the appropriateness and correctness of contracts should be ensured. But manually writing tests for the contracts doesn’t make sense in my opinion. This would mean to duplicate the contract-based specification logic with an equivalent test-based specification logic. The contracts already contain the necessary information for the tests and thus they should be used as test oracle by automatic test generators like Pex. Moreover the paper doesn’t give any usage advices for tests and contracts. It doesn’t clarify for which specifications you should write contracts and where you should use tests. But this is a central question which has to be addressed by a synergetic development process.

In summary SDD is a first step in the right direction and fits into my thoughts on the synergy of TDD and DbC. But SDD as described in the original paper is too vague to form a new development process based on TDD and DbC. It lacks clear statements about the combined usage of TDD and DbC, but just those statements are needed for the establishment of a new development process.

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Comparison: DbC and TDD – Part 4

Let’s start the last part of my little blog post series regarding the comparison of DbC and TDD. This post is about code changes and the support of clean code principles by either DbC and TDD.

Code changes

Changing code is a big challenge for ensuring software quality and software design as well. It’s an important but often underestimated topic. If new features should be implemented or existing requirements change you should be able to change your code in a way that ensures correctness for the behavior of the new and the existing code base and you should be able to keep a consistent design.

With TDD you set up a nice test suite and tests can be run in a reproducible way. If requirements change or are extended and you adapt/extend your code, with TDD you first have to rewrite and/or extend your tests to fit the new set of requirements. But then by running your adapted test suite you can catch unexpected side effects from your code changes. This regression testing is a great safety net for those changes. Without such a test suite how could you be sure that the expected behavior still holds after the code has changed? Tests in a TDD way are great for continuous integration. When a developer in your team changes code and performs a check-in, with clear unit tests you are able to validate the changes during a continuous integration build. With a constraint on the minimum value of allowed code coverage this gives you a great confidence that a code change has no side effects which affect the expected behavior of your components. Moreover TDD ensures a more consistent API design over code changes. By writing tests before implementing new requirements you continuously adapt your design from a client view.

A drawback of tests is the effort to manually write and adapt tests. When behavior changes and old tests fail you have to investigate why those tests fail, thus what the old behavior has been and if this behavior has changed or if your implementation is simply wrong. With huge test suites maintainability can become a very time-consumptive task. Another important issue with TDD and automated tests in general is that any developer who joins your project must have a good comprehension for the TDD process and for automated tests. For many developers TDD is not very intuitive at the beginning and it’s overwhelming them by completely exchanging their development styles and habits. Becoming familiar with TDD could be an important gap for developers and giving all developers in a project the same comprehension for the process can be difficult (especially if they don’t see the benefits of TDD). That’s a question of accountability and project lead. There have to be project guidelines which include testing standards and there must be mechanisms to watch the adherence of those standards.

DbC can’t give you huge support for code changes out-of-the-box. If you only rely on runtime checking of the contracts you could not make a statement if an implementation still holds the contractual specification when code changes. Moreover you have to manually adapt your contracts when you change an implementation. There’s no direct mechanism besides the limited static checking that ensures the correctness of your implementations in terms of the defined contracts. The only solution is to set tests in place that validate the contracts for exemplaric input/output pairs. With such a test suite you can check in a reproducible way if your implementation still mirrors your contracts. But note: I don’t say that tests for contracts should be written manually. In my opinion it doesn’t make much sense, because both tests and contracts are a form of specification and with manually testing a contract you would duplicate the specification code! Moreover contracts already contain the relevant information which is needed to set up a test automatically. Solutions like Pex can jump in here to generate tests from scratch or from parameterized unit tests, using contracts as test oracles for accepted input values (preconditions) and expected output (postconditions).

Contracts have a gap for developers as well. Contracting can get messy and confusing if you are not able to give the developers on your project the very same comprehension of DbC: how and where to use contracts, limits for contract definitions and so on. As further drawback there is no possibility to check the percentage of contracting in a fashion of code coverage in TDD. A developer with no commonsense for the process could leave contracts and nobody would recognize and react. Again it’s the responsibility of the project lead to give guidelines on contracting and to observe the adherence of those guidelines.

Clean code principles

TDD and DbC both give great support to some important clean code principles and thus improve the software quality far beyond the bug prevention aspect.

  • YAGNI (You Ain’t Gonna Need It):
    DbC cannot support you here, but the TDD process greatly supports YAGNI. For each new feature you write a test which leads to an implementation that reflects the test suite and which contains only those features that are really necessary.
  • SRP/SoC (Single Responsibility Principle/Separation of Concerns):
    Both DbC and TDD support those principles. TDD has less impact, but in my opinion with TDD you early discover the dependencies of your components and thus you are encouraged to keep components clean. DbC is more offensive in terms of SRP and SoC. It’s difficult to write contracts for components with many responsibilities and thus DbC directly enforces a separation of concerns.
  • KISS (Keep It Simple and Stupid):
    Both TDD and DbC add some value here. With TDD due to the incremental evolution an API is kept simple and client-friendly. For DbC the same rules apply as for SRP/SoC: It’s easy to define simple and stupid methods with clear contracts, but it’s hard to define contracts on complex and messy components.
  • DRY (Don’t Repeat Yourself):
    I don’t see support from TDD, but DbC influences the DRY principle. Because DbC introduces contracts that clarify benefits and obligations in client/supplier communication it prevents clients from checking return values and parameters redundantly. For example a postcondition says „the return value will never be null“. Thus there is no need for a client of the method to check the return value before handing it to another method that expects a non-null value as argument. This would be encouraged when you do defensive programming, but with DbC you can really prevent such redundancies.
  • OCP (Open/Closed Principle):
    I don’t see big influence from both DbC and TDD here. OCP states: „Software entities should be open for extension, but closed for modification“. This means that your components should be easily extensible without the need for code modifications. OCP increases the level of abstraction and the code complexity, but it makes components easily extensible. With TDD you drive your API only for a current feature set and there seems to be no impact on extensibility. DbC with its contracts can’t help you here as well in my opinion.
  • ISP (Interface Segregation Principle):
    Both DbC and TDD support the ISP. DbC influences ISP by the enforcement of SRP/SoC. When you set contracts on an interface with many responsibilities/members it would be painful to write invariants that must be maintained by each interface member. But it’s  pretty easy if you have clear and simple interfaces. For TDD the impact is less, but it’s there since you early discover dependencies and you drive your API in a „segregated“ direction when implementing tests for new features.
  • DIP (Dependency Inversion Principle):
    Support for this principle mainly comes from TDD, but DbC can add little value as well. With TDD to test a component in isolation you have to exchange dependencies of this component with test doubles like mocks or stubs. Thus you need abstractions/interfaces for those dependencies which enforces DIP. Then with Dependency Injection you are able to inject a test double at runtime of a test. The influence of DbC is more subtle. DbC encourages the use of interfaces at whole by enforcing uniform behavior over all implementations of an interface. Thus the use of interfaces instead of concrete implementations in terms of DIP is encouraged as well.
  • LSP (Liskov Substitution Principle):
    TDD can’t help you here, but DbC adds great value. The LSP states that in class hierarchies it must be possible to treat a derived object as if it would be an object of the base class. Thus the specialized object must behave in the same way as the base class object. This principle of clear sub-classing is a basic part of DbC. Inheritors of a base class or implementors of an interface are not allowed to add new preconditions, but they can extend postconditions and invariants with additional contracts. Thus DbC guarantees uniform behavior in terms of the LSP.
  • CQS (Command-Query Separation):
    TDD doesn’t encourage you to do CQS. But with DbC you need to separate commands as pure methods and queries in order to use commands in contracts. Thus DbC leads to command-query separation which is a good thing since it keeps your API clean (SoC) and your components simple (KISS).


This and the last blog posts gave a comparison of DbC and TDD by taking several aspects into account: specification, design, documentation, code coupling, universality, expressiveness, correctness checking, code changes and influence on clean code principles. While writing this little series of blog posts things became much clearer to me. When starting with DbC some months ago I just thought: „Well, forget TDD, DbC with static checking is all what we need and what we should use„. But that was thought too short. TDD goes far beyond testing and writing unit tests. At its heart TDD is more about design, documentation and specification and it’s really valuable in those (and other) terms.

Both DbC and TDD have advantages and drawbacks and both act on their own terrains with interesting overlaps. And it’s absolutely not a mutual exclusive choice between both principles. However TDD and DbC should be used in conjunction to utilize the advantages of both principles. In fact there should be a clear TDD process that makes use of contracts. As the previous blog posts showed DbC can support TDD, but there’s no possibility to replace it in any way.

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Comparison: DbC and TDD – Part 3

This blog post is a continuation of the comparison between DbC and TDD that started with a dedicated look at code specification and covered other aspects in part 1 and part 2. It takes a look at further points and shows the characteristics (commonalities and differences) of TDD and DbC towards these aspects.


One of the most important differences between DbC and TDD is their different universality, because it influences expressiveness as well as verifiability of correctness.

A significant limitation (or better characteristic) of TDD is that tests are example-driven. With a normal manually written unit test you provide exemplaric input data and check for certain expected output values after the test run. Thus you have concrete pairs of input/output values to express a certain feature/behavior that you want to test. Since these values are only examples there has to be a process of finding relevant examples which can be very difficult. One step towards correctness is high code coverage. You should find examples that examine all paths of your code components. That can be a mess since the amount of possible code paths grows exponentially with the count of branches (if, switch/case, throw, …). But even if you find examples that give you 100% code coverage you cannot be sure that the component acts right for all possible input values. On the one side there could be hardware-dependent behavior (e.g. with arithmetic calculations) that leads to differences or exceptions (division-by-zero, underflow/overflow etc.). On the other side perhaps you get 100% code coverage for your components during the TDD process, but this doesn’t help much on integration with other code components or external systems that could act in complex ways. This problem goes beyond TDD. Thus unit tests in a TDD fashion are nice by their flexibility and simplicity, but they cannot give you full confidence. Solutions like Pex can help you here, but this is part of another story (that I hope to come up with in the near future).

DbC on the other side introduces universal contracts that must hold for every value that an object as part of a contract can take. That’s an important aspect to ensure correct behavior in all possible cases. Thus contracts have a higher value in terms of universality than tests (but they fall back in other terms).


The expressiveness of a component’s behavior and qualities as part of its specification is an important aspect since you want to be able to express arbitrary properties in a flexible and easy way. Universality is one part of expressiveness and has just been discussed. Now let’s look at expressiveness on a broader scope.

TDD has a high value on expressiveness besides the exemplaric nature of tests. You are free to define tests which express any desired behavior of a component that can be written in code. With TDD you have full flexibility, but you are also responsible to get this flexibility under your control (clear processes should be what you need). One aspect that goes beyond the scope of TDD is interaction/collaborative testing and integration testing. The TDD process implies the design of code components in isolation, but it doesn’t guide you in testing the interactions between components (and specification of behavior which relies on those interactions). TDD is about unit testing, but there is a universe beyond that.

Their universal nature makes contracts in terms of DbC a valuable tool. And moreover by extending the definition of a code element they improve the expressiveness of these code elements, what’s great e.g. for the role of interfaces and for intention revealing. But they have downsides as well. First they are tightly bound to a certain code element and are not able to express behavior that spans several components (e.g. workflows and classes interacting together). And second they have a lack of what I would call contentual expressiveness. With contracts you are able to define expectations with arbitrary boolean expressions and that’s a great thing. But it’s also a limitation. For example if you have an algorithm or complex business operation then it’s difficult to impossible to define all expected outcomes of this code as universal boolean postcondition. In fact this would lead to full functional specification which implies a duplication of the algorithm logic itself (in imperative programming) and this would make no sense! On the other side if you use example-driven tests you would not have a problem since you should know what values to expect on a certain input. Furthermore no side effects are allowed inside of a contract. Hence if you use a method to define a more complex expectation this method must be pure (= free of visible side effects). The background of this constraint is that contracts mustn’t influence the behavior of the core logic itself. It would be a mess if there would be a different behavior depending on the activation state (enabled/disabled) of contracts (e.g. for different build configurations). This has a limitation if you want to define certain qualitites with contracts like x=pop(push(x)) for a stack implementation, but it has advantages as well, since it leads to the enforcement of command-query separation by contracts.

Checking correctness

Of course you want to be able to express as many behaviors as possible to improve the specification of your code components. But expressiveness is not leading anywhere when it’s impossible to figure out if your components follow the defined specification. You must be able to stress your components against their specification in a reproducible way to ensure correct behavior!

With TDD correctness of the defined behavior (= tests) can be checked by actually running the tests and validating expected values against the actual values as outcome of a test run. The system-under-test is seen as blackbox and behavior correctness is tested by writing values to the input channels of the blackbox and observe the output channels for correct values. This could include techniques like stubbing or mocking for handling an object in isolation and for ensuring reproducible and verifiable state and behavior. This testability and reproducibility in conjunction with a well implemented test harness is important for continuously checking correctness of your code through regression testing. It’s invaluable when performing continuous integration and when code is changed, but this aspect will be covered in the next blog post. However the exemplaric nature of tests is a limitation for checking correctness which has been shown above when discussing universality.

At first DbC as principle doesn’t help you in terms of checking correctness. It introduces a fail-fast strategy (if a contract is not met, it’s a bug – so fail fast and hard, because the developer has to fix the bug), but how can you verify the correctness of your implementation? With DbC bugs should be found in the debugging step when developing code, thus by actually executing the code. And common solutions like Code Contracts for .NET come with a runtime checking component that checks the satisfaction of the defined contracts when running the code. But this solution has a serious shortcoming: It relies on the current execution context of your code and hence it takes the current values for checking the contracts. With this you get the same problems as with tests. Your contracts are stressed by example and even worse you have no possibility to reproducible check your contracts! Thus dynamic checking without the usage of tests makes no sense. However contracts are a great complement of tests. They specify the conditions that must apply in general and thus a test as client of a component can satisfy the preconditions and then validate postconditions or custom test behavior. Another interesting possibility to verify contracts is static checking. Code Contracts come with a static checker as well that verifies the defined contracts at compile-time without executing the code. On the contrary it actually inspects the code (gathers facts about it) and matches the facts as abstraction of the implementation against the defined contracts. This form of whitebox code inspection is done by the abstract interpretation algorithm (there are other solutions like Spec# that do real formal program verification). The advantage of static checking is that it’s able to find all possible contract violations, independent of any current values. But static checking is hard. On the one side it’s hard for the CPU to gather information about the code and to verify contracts. Hence static checking is very time-consumptive which is intolerable especially for bigger code bases. On the other side it’s hard for a developer to satisfy the static checker. To work properly static checking needs the existence of the right (!) contracts on all used components (e.g. external libraries) which is often not the case. And looking at the static checker from Code Contracts it seems to be too limited at the current development state. Many contracts cannot be verified or it’s too impractical to define contracts that satisfy the checker. Thus static checkers are a great idea to universally verify contracts, but especially in the .NET world the limitations of the checkers make them impractical for most projects. Hence a valid strategy for today is to write tests in conjunction with contracts and to validate postconditions and invariants with appropriate tests.

[To be continued…]

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Comparison: DbC and TDD – Part 2

This blog post is a continuation of the comparison and differentiation between DbC and TDD. Please take a look at part 1 which covers the design aspect (and shortly specification again which has been discussed in more detail here). Today’s post takes a look at documentation and code coupling and shows commonalities and differences of TDD and DbC towards these aspects.


One important thing for other developers to understand your code is documentation. Code comments are a first necessity here to basically explain what a code component is intended to do. But comments have the unpleasant habit to run out-of-sync with the real code if you are not carefully and consequently adapting them. Most of all if you write obligations or benefits into comments there will be no check if those requirements hold or if the real implementation matches them.

DbC with its contracts is a much better way to document those specification aspects. Contracts are coupled to the code and e.g. with Code Contracts you get static and runtime checking for them. This checked documentation makes DbC really powerful (if properly used) and avoids the asynchronicity between code and documentation. It shows developers how a component should be used, what requirements the client  has to fulfill and what he can expect in return. The client can rely on these specified qualities which improves the reusability of contracted code components. Furthermore there are possibilities to integrate Code Contracts into the Sandcastle documentation and for Visual Studio 2010 there will be an add-in that immediately shows defined contracts on a component as you develop against it. With that you get great MSDN-like documentation that contains the defined contracts as well as support for your development process when you use contracted code.

Tests in terms of TDD add another aspect of documentation. Due to their exemplary nature and their specification of an element’s behavior those tests are great to show a developer the intent of a code component and give him a guideline to its usage. Since tests can be run and validated in a reproducible way developers are able to rely on defined behaviors as well.

Together with documentation comes the aspect of intention revealing. And both TDD and DbC add some value here. Both express the developer’s intention with a certain component and show far beyond code comments and naming conventions what behavior a client can expect. Developers can use the component in these specified ways and don’t have to manually investigate the component’s implementation.

Code coupling

One drawback of TDD is the locational gap between the code implementation and the tests as specification and documentation source. For sure this has an advantage as well: the code logic isn’t polluted with the specification and thus it’s kept clean. But the disadvantages weigh heavier for me. If developers want to show how a component behaves they have to cross the gap and investigate the tests. This is difficult for developers who are not very familiar with TDD. Furthermore tests don’t give any support for client usage of a component. Of course they give usage examples, but developers can use and especially misuse a component in arbitrary ways. This can lead to serious problems (e.g. inconsistent states) if there are no other mechanisms to prohibit misusage.

DbC on the other side sets contracts directly on the implementation of a code component in place. Thus they have a declarative nature and extend the definition of a code component. Some realizations of DbC like Code Contracts in .NET have drawbacks since they set contracts imperatively into the code, but rewrite the code after compilation to set the contracts in the „right“ places. Thus Code Contracts break the uniformity principle (different semantics should be expressed through different syntax) and pollute the code logic in some way. Other realizations like the Eiffel language have contracts as keywords built into the language which makes a better choice in my opinion. Anyway contracts at the same place as the implementation avoids the drawbacks of a locational gap. And moreover DbC prevents misusage of a component. Contracts are dynamically checked at runtime or statically at compile time and fail early if requirements are not satisfied. That’s a very important concept because it expresses a clear behavior if something goes wrong (existence of a bug) and gives a clear contract for obligations and benefits that hold and are checked in client/supplier communication.

[To be continued…]

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Comparison: DbC and TDD – Part 1

Let’s come to another blog post in preparation of elaborating the synergy of DbC and TDD. My first blog post on this topic covered an initial discussion on specification of code elements. Thereby it has shown different characteristics of DbC and TDD in terms of code specification. Since specification is one really important aspect in comparison of DbC and TDD, it’s of course not the only one. Hence today’s blog post is a starting point for a more general comparison of DbC and TDD with some other important aspects. There are 3 more blog posts that I will come up with during the next weeks, forming a 4-part comparison series.

To say first, Test-Driven Development (TDD) and Design by Contract (DbC) have very similar aims. Both concepts want to improve software quality by providing a set of conceptual building blocks that allow you to prevent bugs before making them. Both have impact on the design or development process of software elements. But both have their own characteristics and are different in achieving the purpose of better software. It’s necessary to understand commonalities and differences in order to take advantage of both principles in conjunction.

Specification again…

The last blog post has already given a dedicated look at the specification aspect and how DbC and TDD can add some value. To summarize both principles extend the code-based specification in their own ways. TDD let’s you specify the expected behavior of a code element in an example-driven and reproducible way. It’s easy to use and allows the expression of any expected behavior. DbC on the other side sets universal contracts in place that extend the definition of a code element and are tightly coupled to it. It’s a great concept for narrowing the definition of a code element by defining additional physical constraints as preconditions, postconditions and invariants. By defining contracts on interfaces DbC strengthens the role of interfaces and enforces identical constraints/behavior over all implementations of an interface. However not every behavior can be expressed by contracts and they’re bound to a single code element. Thus they don’t lessen the position of tests, but can be seen as great complement.

Design aspect

Both DbC and TDD have impact on the design of an API. Well, there is a slight but important difference as the names imply: it’s Design by Contract, but Test-Driven Development. Thus with DbC (contract-first) on the one side you are encouraged to write your contracts as you design your components (at design phase), which perfectly fits with the idea of contracts as extension of a component’s definition. With TDD and the test-first principle on the other side you write a test which maps to a new feature and afterwards you directly implement the code to get the test to green state. Hence TDD is tightly coupled to the development phase in contrast to DbC, which seems to come first. In addition personally I wouldn’t fight a religious war on this naming. If you think of DbC as „Contract-First Development“ or „Development by Contract“ you would have the contract-first principle coupled to the development phase as well. The more important thing is to find a way to effectively use contracts in the development cycle. If you are an advocate of up-front design you would perhaps want to set your contracts at design phase. But if you intensively use TDD it would be difficult to go down this design phase road. However you would set your contracts at development phase in conjunction with the test-first principle. This leads to the question of an effective development model with TDD and DbC and that’s another important story…

But for now let’s come back to the impact of DbC and TDD to the design of an API. With TDD you write a new test for each new feature and then you bring this test to green by implementing some piece of logic. This is some form of Client-Driven Development. Your test is the client of your API and you call your methods from the client’s perspective (as a client would do). If the current API doesn’t fit your needs, you extend or modify it. Thus the resulting API is very focussed on the client’s needs and furthermore doesn’t contain code for unnecessary features, which is a great thing in terms of YAGNI. Another impact of TDD is that it leads to loosely coupled components. Tests in form of unit tests are very distinct and should be coupled to the tested component, but not far beyond that (other components that are called, e.g. data access). Thus there is a certain demand for loose coupling e.g. by DI.

With DbC and contracts on your components you have other impacts. Contracts clarify the definition and intent of your components. When you come up with contracts you strengthen your opinion about a component by setting contractual obligations and benefits. This leads to much cleaner components. Moreover your components will have fewer responsibilities (and thus a better cohesion) since it would be painful to write contracts for components with many different responsibilities. Thus DbC is great in terms of supporting the SRP and SoC. Another impact comes from the „limitation“ of contracts to support only pure methods as part of a contract. If you want to use class methods in contracts (e.g. invariants) of this class you have to keep those methods pure. This leads to the enforcement of command-query separation by contracts, which very often is a good thing in terms of comprehensibility and maintainability.

[To be continued…]

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Specification: By Code, Tests and Contracts

Currently I’m taking further investments in thinking about the synergy of Design by Contract (DbC) and Test-Driven Development (TDD). This process is partially driven by my interests in Code Contracts in .NET 4.0 and other current developments at Microsoft (e.g. Pex). The basic idea I’ve got in mind is to combine DbC and TDD to take best advantage of both principles in order to improve software quality on the one side and to create an efficient development process on the other side. My thoughts on this topic are not too strong at the moment, so feel free to start a discussion and tell me what you think.

Before I come to discuss the synergy of DbC and TDD it’s absolutely necessary in my opinion to understand the characteristics, commonalities and differences of both concepts. This first blog post drives in this direction by looking at a very important aspect of both concepts: the formal and functional specification of code. It’s a starting point for further discussions on this topic (a 4part comparison series follows). So let’s start…

Code-based specification

In essence, DbC and TDD are about specification of code elements, thus expressing the shape and behavior of components. Thereby they are extending the specification possibilities that are given by a programming language like C#. Of course such basic code-based specification is very important, since it allows you to express the overall behavior and constraints of your program. You write interfaces, classes with methods and properties, provide visibility and access constraints. Furthermore the programming language gives you possibilities like automatic boxing/unboxing, datatype safety, co-/contravariance and more, depending on the language that you use. Moreover the language compiler acts as safety net for you and ensures correct syntax.

Since such basic specification is necessary to define the code that should be executed, it has a lack of expressiveness regarding the intent of a developer and there is no way to verify correct semantics of a program with it. As an example interfaces define the basic contract in terms of method operations and data, but looking at C# a client of this interface does not see what the intent of a method is or which obligations he has to fulfill when calling a method or what state he can assume on return of a method. Furthermore interfaces can not guarantee uniform behavior across their implementations. TDD and DbC are there to overcome or at least decrease this lack of expressiveness at some points and to guarantee correct semantics.

Test-based specification

Let’s come to test-based specification using TDD (as well as BDD as „evolutional step“ of TDD). This is inevitably integrated in the TDD cycle. Every test written by a developer maps to a new feature he wants to implement. This kind of specification is functional and example-driven, since a developer defines by exemplary input/output pairs what output he expects as result of the test run under a certain input. With well-known techniques (stubs, mocks) he is able to run his tests in isolation, get reproducible test results and perform state and behavior verification.

Compared to code-based specification, test-based specification in a TDD manner is very valuable when it comes to expression of the behavior of a code element. It gives a set of tests that could at its extreme span the whole behavior of a code element. With their reproducibility the tests are indispensable when it comes to continuous integration to ensure correct behavior of modified code elements. Furthermore tests are a great source for other developers to show the intent of a developer for a method’s behavior and to give demonstration of the usage of a code element. There are other benefits and characteristics of TDD that will be discussed when comparing TDD and DbC altogether in subsequent blog posts.

An important aspect of test-based specification using TDD is that it’s done at a very granular level. For each new feature a test is written and if the present API doesn’t fit the needs, it will be extended or refactored. Thus TDD drives the API design as you go with your tests.

Contract-based specification

Another possibility for specifying code is contract-based specification in terms of DbC, thus defining preconditions and postconditions as method contract and invariants as class contract. With contracts you are able to define the basic obligations that a client must fulfill when calling a method as well as the benefits he gets in return. Furthermore invariants can be used to define basic constraints that ensure the consistency of a class. Thus with DbC a developer is able to define a formal contract for code that makes a clear statement of obligations and benefits in client/supplier communication (aka caller/callee). If a contract fails the behavior is clear as well: by failing fast the developer can be sure that there is a problem (bug) with his code and he has to fix it.

On a technical level there are several possibilities to define contracts. In Eiffel contracts are part of the programming language itself in contrast to Code Contracts that become part of the .NET framework. In any case contracts are directly bound to the code that they are specifying and express additional qualities of the code. Those qualities go beyond the code-based specification. In general contracts allow arbitrary boolean expressions what makes them a very powerful and flexible specification source. Nonetheless contracts only allow partial functional specification of a component. It can be very difficult or even impossible to define the complex behavior of a method (which methods is it calling, what business value does it have, what’s the concrete result for a concrete input, …) or to ensure certain qualities (e.g. infrastructure-related questions like „is an e-mail really sent?“) with contracts. Furthermore it’s impossible to use impure functions in contracts which would be necessary to express certain qualities of code (like expressing inverse functions: x = f-1(f(x)), if f or f-1 are impure). Test-based specification could be used here instead.

But let’s come back to the technical aspect: Contracts are wonderful animals in terms of extending the code-based specification by narrowing the definition of a code element and becoming part of the element’s signature. They can be used to define general conditions, e.g. physical constraints on parameters and return types. Thereby (for example with Code Contracts) the contracts can be inherited to sub-classes while respecting the Liskov substitution principle (LSP). Moreover contracts can be defined on interfaces and thus they are a valuable tool for expressing constraints and qualities of an interface that must be respected by every implementation. With that contracts are wonderfully strengthening the role and expressiveness of interfaces and complementing code-based specification at whole.

With the tight coupling to the code, contracts give immediate support for other developers how an API should be used. They express the developer’s intent (intention revealing) for his code elements, which leads to easier comprehension and usage of those components. They give some kind of checked documentation, which again greatly complements code-based specification and leads to fewer misusage of contracted components.

In contrast to test-based specification, contracts are employed at level of whole components by employing invariants to classes and pre-/postconditions to class methods. Moreover contracts lead to components with very few dependencies (promoting the SRP) since it’s difficult to write contracts for components with many responsibilities.

First conclusion

This blog post is intended to be a first preview of what I want to come up in the next time. It has given an overview of possibilities for specification of code elements with focus on test-based and contract-based specification in terms of TDD and DbC. It has shown some commonalities and qualities of both concepts. In conclusion TDD and DbC are both valuable in terms of the specification of code elements and in revealing the developer’s intent.

In my opinion it’s by no means a „one or the other“ choice. TDD and DbC act on their own terrains with their own benefits and drawbacks. There are overlaps, but in general they can naturally complement each other. It’s time to think about ways to leverage a conjunction of both concepts, isn’t it?

[To be continued…]

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