public void TryValidate_ShouldShowUnassignedMessageBox_WhenValueIsNullOrEmpty_AndReturnFalse(string value) { mSut.Value = value; IMessageBoxService messageBoxService = Substitute.For <IMessageBoxService>(); Assert.IsFalse(mSut.TryValidate(messageBoxService)); messageBoxService.Received(1).ShowError("A valid rendering class must be assigned."); }