您已在XAML中为MouseLeftDown定义了一个处理程序，但您的代码中没有处理程序。

You have defined a handler for the MouseLeftDown in your XAML but you have no handler in your code.

您可以右键单击处理程序名称SwitchImage，VS将创建一个你的空处理程序。

You can right click the handler name SwitchImage and VS will create an empty handler for you.