Maybe you’ve already seen an exception like this: Castle.DynamicProxy.Generators.GeneratorException: Type is not public, so a proxy cannot be generated. Type: SpielWiese.IReportSender …stack-trace… at SpielWiese.TestSomething.ExpectReportsSend() in Program.cs: line 64 What I’ve tried is to create a mock-object with Rhino-Mock. Since this interface is only intended for usage within the assembly it’s declared as internal interface. So [...]