窗体背景随机更换的一个C#实例，每次打开窗口都有不同的背景，怎么样？虽然挺简单的一个示例，不过还算实用吧，对新手尤其有用处。-Form the background of random replacement of a C# example, each time you open the window has a different background, how? Although a very simple example, but it pretty useful, particularl