# Thursday, June 11, 2009

We have just released a new version of Pex, v0.14.040610.2. This version brings a range of Bug Fixes that were reported on the MSDN forums and a couple new features: Fix Suggestions for Invariant Methods and Natural properties  for Stubs.

Fix Suggestions for Invariant Methods

When you use Code Contracts, then Pex can now help you to add missing invariants, just as Pex already inferred missing preconditions in the past. Take for example the ArrayList class that comes with the samples of our recently updated tutorial slide deck. This class has the following fields:


When you add an empty invariant method,


and enable Runtime Checking of contracts, then Pex will create instances of this class using reflection while making sure that the invariants holds (we announced this feature in the last release). However, with an empty invariant method, Pex will be able to create illegal states, e.g. where the _items array is null, which may cause a NullReferenceException, and other issues. To get the following test cases, we let Pex explore the Add method of our ArrayList class:


When you look at the details of the failing test cases, then you will see that Pex now offers the option to add invariants directly into your [ContractInvariantMethod]:


Natural Properties for Stubs

The logic to handle properties with setters and getters generated by Stubs has been slightly improved so that it is possible to force properties to have as if they had a backing field.

When a getter or a setter of a property is invoked, Stubs checks whether the getter delegate or the setter delegates have been set. If not, i.e. no custom user behavior, it queries the fallback behavior for an initial value of the property. If the fallback behavior successfully returns a value, Stubs generates a closure and binds the getter and setter.

For example, here’s a simple IFoo interface with a Bar property:


One could write the following parameterized unit test:


Since we expect the Bar property to behave as it was wrapping a field, we don’t expect the assertion to be raised. When we execute this test with Pex, and the Pex fallback behavior, Pex will choose a value for ‘Bar’, and this value will be ‘stored’ the property. Therefore, we get as expected 2 test cases:


Non default constructor support for Stubs

In previous versions, Stubs could not generate stubs for classes with no default constructor. This is no longer the case, Stubs will generate one public constructor for each public or protected base constructor.

Bug Fixes

  • Various code generation tweaks to make StyleCop happier
  • Visual Studio addin crashes when loading in VS2010
  • Visual Studio addin does not find attributes when they are globally qualified
  • Added missing substitutions for System.Threading.Interlocked members
  • Stubs automatically imports missing references
  • When an object has an invariant method (from Code Contracts), Pex would emit assertions using the private fields
  • Editorial issues to patterns paper
  • Contracts no longer kill the process when the runtime rewriter was not executed (.net 4.0)
  • Fixed bug in generic type guesser
  • Fixed samples for the latest version of Stubs

As always, don’t hesitate to send us your feedback, good or bad, through our MSDN forums.

posted on Thursday, June 11, 2009 7:14:58 PM (Pacific Daylight Time, UTC-07:00)  #    Comments [2]
# Tuesday, June 02, 2009

Inversion of Control (IoC) is a very important practice to make code testable. It usually relies on defining interfaces for each external dependency. Various frameworks and techniques, i.e. Dependency Injection (DI), Service Locator, exist to bind the interface implementations to the components. In this blog post, we will *not* focus on this aspects of IoC but rather on it’s main building block: interfaces. This article describes how Contracts for interfaces improves IoC… regardless of which DI framework/technique you are using. The Contracts for interfaces are a feature of the Code Contracts for .Net tool.

Interfaces are not Contracts

Interfaces are often referred to as Contracts in the context of IoC: they define the methods and properties that a service should implement and are a key factor in achieving the loose coupling: one can build the code against the interface and can plug in various implementation with no risks.

Unfortunately, interfaces are not contracts: while they specify how the methods signatures should be, interfaces do not specify the functional behavior. To illustrate this, let’s take a look at a well-known simple interface of the BCL:


For this interface, I could naively implement as follows:


While my implementation is really really wrong, it fulfills all requirements of the IServiceProvider interface: a method GetService that returns object. Of course, if I would have read the MSDN documentation of GetService, I would have known that object should implement serviceType. Unfortunately, the interface did not tell me anything about that and compilers don’t understand MSDN documentation either.


Contracts for interfaces

Code Contracts provides an API for design-by-contracts (pre-condition, post-condition, etc…). It also supports defining contracts for interface or abstract classes. Contracts for interfaces can be used to specify the functional behavior of interface members. While they also serve as documentation, those contracts can also be turned into runtime checks or leveraged by static checkers.


  • since interface members cannot have method bodies, the contracts are stored in a ‘buddy’ type.
  • The [ContractClass]/[ContractClassFor] attributes bind the interface type and the contract type together.
  • Contract.Result<object>() is a helper method to refer to the result value, since this is not supported in C#.

Let’s take a closer look at the body of IServiceProviderContract.GetService. It contains a pre-condition (Requires) that the serviceType should not be null and a post-condition (Ensures) that the return value should be null or should be assignable to the serviceType:


In this simple case, the contracts captured precisely the specification that we found in MSDN. The critical difference is that they are stored in a format that compilers and runtimes know pretty well: MSIL byte code. The benefits are huge:

  • documentation: the contracts are stored in a programming-language agnostic format that mined and rendered for your favorite programming language,
  • static checking: static analysis tools can (and will) use contract to try to find bugs before you execute the code, or prove that it is correct with respect to the contracts,
  • runtime checking: the runtime checker will instrument all implementations of the interface with the interface contracts automatically. Once you’ve specified how an interface should behave, you do not have to repeat yourself when re-implementing it, the re-writer takes care of that.
  • automated white box testing: tools like Pex can leverage the runtime contract checks to try to find inputs that violate the contracts.
  • IoC/DI framework agnostic: It does not matter which DI framework you use, as soon as you use interface, you could also provide contracts for it.
posted on Tuesday, June 02, 2009 5:14:03 PM (Pacific Daylight Time, UTC-07:00)  #    Comments [1]