I have created a UserControl Popup window and called that xaml file into another file. Imported the namespace.

When I am trying to build this project an error at this below given code:

<Popup x:Name="POP" IsOpen="False" PlacementTarget="{Binding}" Placement="Center" AllowsTransparency="True">
      <a:PopUpWindow x:Name="pp" />

Error at

<a:PopUpWindow x:Name="pp" /> 
Could not create an instance of type 'PopUpWindow'.

Can you provide more of your XAML? It sounds like it doesn't recognize a:PopUpWindow as a valid type, perhaps. Does your PopUpWindow have a default constructor that's different, or anything?