Using WPF User Control in application

I'm having difficulty using a User Control in a application.

Application:

Xaml:

...


Xaml.cs:
namespace MyApp
{
    public partial class MyAppMain : Window
    ...
}

User Control:

Xaml:

             x:Name="FileLookup"
             ...

             
Xaml.cs:
             
namespace MyApp
{
     public partial class FileLookup : UserControl
}

Integrating User Control into App:

Xaml:

       
        ...       

       
       
   



Regardless of where in the application I insert , I get an indication that the FileLookup can't be found. Where am I going wrong?