public void GetMessage() { var writerMock = MockRepository.GenerateMock <TextMessageWriter>(); writerMock.Expect(mock => mock.Write("Expected: ExpectedName but was: First, Second")); writerMock.Replay(); var constraint = new EnumValueInfoListContentsConstraint("ExpectedName"); PrivateInvoke.SetNonPublicField(constraint, "actual", _list); constraint.WriteMessageTo(writerMock); writerMock.VerifyAllExpectations(); }