您需要将MainWindow_2所在的命名空间的引用添加到MainWindow_1的代码中。如果MainWindow_2在一个单独的程序集中，您可能也需要引用该程序集。



[OP已更新代码问题]

MainWindow_2 win2 = new MainWindow_2.Show（）



应该是



MainWindow_2 win2 = new MainWindow_2（） .Show（）

You need to add a reference to the namespace that MainWindow_2 is in to the code for MainWindow_1. If MainWindow_2 is in a separate assembly, you may need to reference that assembly as well.

[OP has updated question with code]
MainWindow_2 win2 = new MainWindow_2.Show()

Should be

MainWindow_2 win2 = new MainWindow_2().Show()

只需按Ctrl +。在 MainWindow_2 中的行 MainWindow_2 win2 = new MainWindow_2（）;

它将解决你的名字空间（如果dll已经是项目的一部分）。

Just press Ctrl + . on MainWindow_2 in the line MainWindow_2 win2 = new MainWindow_2();
It will resolve your name space (if the dll is already part of the project).