Application Class and Main method in WPF

I recently ran into an interesting aspect of WPF applications that I had not previously seen.  It started with this error:

Program 'DemoCorp.UI.exe' does not contain a static 'Main' method suitable for an entry point

I had copied an App.xaml file from an existing application into a new application I was creating, because I wanted the resource definitions and thought that would be an easy way to do it.  I then deleted the App.xaml that was automatically generated in the new application, which created the compiler error above.

As it turns out, there's a special output type for the App.xaml file to make it generate the Main method.  Here's an App.xaml in the solution explorer:

If you click on the properties tab to get the file properties for the App.xaml, you'll see something like the following:

To generate a suitable Main method, the Build Action must be set to ApplicationDefinition as shown in this image.  As soon as I figured this out, everything worked great.