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Mocking A Generic Method And Generic Constraint

14 May 2009 by Stuart Cam

I just stumbled across a problem in my integration testing around a particular scenario.

The problem occurs when attempting to create a mock instance for a class that has a generic method with a generic constraint - either at the class or method level.

Consider the following code:

public interface IFoo<T> where T : IBar
{
  T Get();
}

We have added a constraint to the interface IFoo such that T must implement IBar. So far so good, apart from the 'standard' Rhino Mocks code below throws an exception:


public class MyBar : IBar {
  //...
}

//...

var mockInstance = Mocks.DynamicMock<IFoo<MyBar>>();

This fails with an error:

System.TypeLoadException: Access is denied: 'IFoo`1[MyBar]'.

The problem is that the underlying BCL has a bug in Reflection Emit which causes it to generate invalid code. Oh dear. This has already been raised as an issue with MS - but they are failing to resolve it. Go and make your feelings heard!

Ayende has a workaround in Rhino Mocks 3.4 and above, so now you can write the following:

var mockInstance = Mocks.DynamicMockWithRemoting<Foo<MyBar>>();

This pushes the creation of the mock object through some additional hoops to bypass the BCL bug. Smart!

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Categories: .NET | C Sharp | TDD


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