I tried to set the CornerRadius of the window but it didn't work

[ApaulMyLittleAirport]

Window prop:
HasSystemDecorations="False" CornerRadius="10" Background="yellow" BorderThickness="2" BorderBrush="red"

[maxkatz6]

It's not supported. You would need to use native API (AppKit on mac or WinAPI on win) to change these properties.
Though I am not sure if it's even possible on mac and linux.

Alternatively you can make borderless window (set SystemDecorations to none) and that you can put a Border control with custom corner radius. You will lose some features and native look, but that's it.

[ApaulMyLittleAirport]

它不受支持。您需要使用本机 API（Mac 上的 AppKit 或 win 上的 WinAPI）来更改这些属性。虽然我不确定这在Mac和Linux上是否可行。

或者，您可以创建无边框窗口（将“系统装饰”设置为“无”），并且可以放置具有自定义角半径的边框控件。你会失去一些功能和原生外观，但仅此而已。
Thank you very much for your help, I really benefited a lot, but it seems that after I set it up, there is still no red rounded border.
SystemDecorations="None" CornerRadius="10" Background="yellow" BorderThickness="12" BorderBrush="red"

[DasLixou]

When you are using SystemDecorations="None" then you can just make the window transparent and put a border with roundings in it.

[ApaulMyLittleAirport]

Thank you very much for your advice. I seem to have failed the test. I will try again.