xmlns:sys="clr-namespace:System;assembly=mscorlib" NOTE: System;与assembly中间不能有空格

否则报错，

Error 1 The URI "clr-namespace:System; assembly=mscorlib" is not a valid namespace identifier. D:\Personal\WPF\WpfApplication1\WpfApplication1\MainWindow.xaml 5 9 WpfApplication1