将旧的C＃文件导入新的WPF应用程序后，“使用System.Windows;”会显示错误消息

After importing an older C# file into a new WPF appliction, "using System.Windows;" gives an error message. This wasn't just one C# file, it was a number of them and I put them into a new project.After importing an older C# file into a new WPF