Question

I’m using NUnit and Moq to test a class which uses a generic WCF service client wrapper I wrote, and I’ve run into an error I can’t figure out. I have the following interface:

public interface IService
{
    void Call();
}

…implemented by the following service client:

public class ServiceClient : ClientBase<IService>, IService
{
    public void Call()
    {
    }
}

…and wrapped by this class with the following generic constraints:

public class ServiceClientWrapper<TClient, TService>
    where TClient : ClientBase<TService>, TService
    where TService : class
{
    public virtual TService CreateServiceClient()
    {
        return (TService)Activator.CreateInstance<TClient>();
    }
}

To make it testable I have a wrapper factory which I can mock. The wrapper factory interface is this:

public interface IServiceClientWrapperFactory
{
    ServiceClientWrapper<TClient, TService>
        CreateServiceClientWrapper<TClient, TService>()
        where TClient : ClientBase<TService>, TService
        where TService : class;
}

I test this set up using this code:

// A mock IService to return from my mock service wrapper:
var mockService = new Mock<IService>();

// A mock client wrapper to return from my mock wrapper factory:
var mockClientWrapper = 
    new Mock<ServiceClientWrapper<ServiceClient, IService>>();
mockClientWrapper
    .Setup(mcw => mcw.CreateServiceClient())
    .Returns(mockService.Object);

// A mock wrapper factory to inject into a client object:
var mockClientWrapperFactory = new Mock<IServiceClientWrapperFactory>();
mockClientWrapperFactory
    .Setup(mcwf => mcwf.CreateServiceClientWrapper<ServiceClient, IService>())
    .Returns(mockClientWrapper.Object);

// Get the mock client wrapper from the mock wrapper factory - boom!
mockClientWrapperFactory.Object
    .CreateServiceClientWrapper<ServiceClient, IService>();

The error is:

GenericArguments[0], ‘TService’, on
‘ServiceClientWrapper`2[TClient,TService]’
violates the constraint of type
parameter ‘TClient’.

The constraints are the same wherever I’ve stated them, it compiles and runs just fine, the error isn’t thrown if I implement IServiceClientWrapperFactory and run it without Moq… any ideas?

Answer 1

I played with the code a bit, and I get the same result. As far as I can tell, there isn’t anything wrong with your definitions.

It seems that it might be a bug in Moq, when implementing mocks that have various combinations of generic constraints on them.

I tried removing various constraints (and commenting out or modifying code, as required). It seems if I remove TService from where TClient : ... , TService, then I don’t get this error.

This seems further corroborated by the fact that there have been multiple similar bugs in the past:

• http://code.google.com/p/moq/issues/detail?id=127
• http://code.google.com/p/moq/issues/detail?id=177

FYI, the version I used to repro your problem was: Moq.4.0.10827, NET40 (not NET40-RequiresCastle)