WPF MessageBox Layout

You can not change the appearance of the WPF Message Box because it is not a window created by .Net using the WPF components, this window is basically a call to the Windows API, in this case the MessageBox itself of user32.dll

I would say your best alternatives are to even create your own window or use some component ready for it.

If you do not want to use the WPF option you can do the same form and assign the buttons and features and create the DialogResults.

Use WPF toolkit, then just change the layout of it with Style like any other Control:

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