public void AddAfterMapping() { string log = string.Empty; var msg1 = "original mapping should have run"; var msg2 = "and also this one"; var mapper1 = new PropertyMapper <IView, IViewHandler> { [nameof(IView.Background)] = (r, v) => log += msg1 }; mapper1.AppendToMapping(nameof(IView.Background), (h, v) => log += msg2); mapper1.UpdateProperties(null, new Button()); Assert.Contains(msg1, log); Assert.Contains(msg2, log); var originalIndex = log.IndexOf(msg1); var additionalIndex = log.IndexOf(msg2); Assert.True(originalIndex < additionalIndex); }