MessageDialog missing in C# WPF

Question

I am new in WPF C#.

I want to use a MessageDialog, but I can't instantiate it because I can't find its namespace reference.

Please note that I can use a MessageBox.

I take a screenshot here:

What should I look to solve the problem?

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Update I tried to use a ModalWindow as suggested by some user who deleted his answer, but I have the same problem.

Answer 1

MessageDialog is a UWP class, so it is only available to Windows 10 Store applications. As far as I know it does not exist in WPF.
The namespace is Windows.UI.Popups.