TypeLoadException: Internal method from another assembly does not have an implementation

[heldergoncalves92]

Work State

Moq version: 4.10.0.0 (C#)
TargetFramework: net5.0

I have the following interfaces in the assembly ProjectA:

public interface IView {
    void Method_1();
    internal void InternalMethod();    
}

public interface IViewModule {
     IView Host();
}

Then in another project, ProjectB, that consumes ProjectA, I just have visibility to Method_1. That's fine and everything seems to work. Except for tests that are failing.

In ProjectB I have another interface that inherits from IView.

public interface IExtendedView : IView {
    // Other methods   
}

Test that fails

Now I will show you the code that will create the exception:

void MockViewModule() {
    var viewModule = new Mock<IViewModule>();
    var view = new Mock<IExtendedView>();
    
    // It crashes in the next line
    viewModule.SetupGet(x => x.Host).Returns(view.Object);
}

Error Message

Message: 
    System.TypeLoadException : Method 'InternalMethod' in type 'Castle.Proxies.IExtendedViewProxy_5' from assembly 'DynamicProxyGenAssembly2, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' does not have an implementation.

Stack Trace: 
    TypeBuilder.CreateTypeNoLock()
    TypeBuilder.CreateTypeInfo()
    AbstractTypeEmitter.CreateType(TypeBuilder type)
    AbstractTypeEmitter.BuildType()
    InterfaceProxyWithoutTargetGenerator.GenerateType(String typeName, Type proxyTargetType, Type[] interfaces, INamingScope namingScope)
    <>c__DisplayClass6_0.<GenerateCode>b__0(String n, INamingScope s)
    BaseProxyGenerator.ObtainProxyType(CacheKey cacheKey, Func`3 factory)
    InterfaceProxyWithTargetGenerator.GenerateCode(Type proxyTargetType, Type[] interfaces, ProxyGenerationOptions options)
    DefaultProxyBuilder.CreateInterfaceProxyTypeWithoutTarget(Type interfaceToProxy, Type[] additionalInterfacesToProxy, ProxyGenerationOptions options)
    ProxyGenerator.CreateInterfaceProxyTypeWithoutTarget(Type interfaceToProxy, Type[] additionalInterfacesToProxy, ProxyGenerationOptions options)
    ProxyGenerator.CreateInterfaceProxyWithoutTarget(Type interfaceToProxy, Type[] additionalInterfacesToProxy, ProxyGenerationOptions options, IInterceptor[] interceptors)
    CastleProxyFactory.CreateProxy(Type mockType, IInterceptor interceptor, Type[] interfaces, Object[] arguments)
    Mock`1.InitializeInstancePexProtected()
    PexProtector.Invoke(Action action)
    Mock`1.InitializeInstance()
    Mock`1.OnGetObject()
    Mock.get_Object()
    Mock`1.get_Object()

---

I would like to understand if there's a way to avoid this exception.

[stakx]

Initial suspicion: possibly related to / caused by the same underlying problem as #991.

[heldergoncalves92]

Are you sure @stakx? I just added the stack trace, and it seems to be quite different from #991.

[stakx]

No, I'm not yet sure. (That's why I wrote "initial suspicion", it was more of a semi-informed guess aka gut feeling.) You can ignore my above post, it was mostly a reminder for myself for when I get time to look at this in detail. 🙂

[stakx]

@heldergoncalves92 – in case you haven't solved this yet yourself, you'll need to add the following somewhere in your ProjectA (the project that defines IView):

[assembly: InternalsVisibleTo("DynamicProxyGenAssembly2")]

Alternatively, add a default implementation for InternalMethod directly in the interface, instead of only declaring it.

Reason: It isn't possible for DynamicProxy (the proxying library on top of which Moq 4 is built) to create a full implementation class of IView if InternalMethod isn't accessible to it. Once you make IView's internals accessible to DynamicProxy, it can then implement the method in a dynamically generated subtype (proxy class).

The InternalsVisibleTo attribute is explained a bit in the Quickstart.