Override Main in a WinUI Application

I was writing a WinUI 3 application that uses the Entity Framework and the Microsoft Hosting Extensions for IoC.

My migrations were not working. After a lot of troubleshooting I figured out that the EF migrations tool has some specific assumptions on the order of initializing the Host, the Services and the Application itself. Such fine grained control is not possible using the default generated XAML entry point.

➡ How to properly override the default generated XAML entry point?

The default generated Main entry point

If we check the executable for the application we can see that the Main entry point is actually in a class called Program. That class is generated by the XAML compiler along with the App class. If we take a look in the obj folder of the App project, there is a file named App.g.i.cs which contains some partial code generated for the App class but also code that is conditionally included based on the presence of the DISABLE_XAML_GENERATED_MAIN compilation constant, defining a new class called Program with the Main entry point definition.

#if !DISABLE_XAML_GENERATED_MAIN
    /// <summary>
    /// Program class
    /// </summary>
    public static class Program
    {
        [global::System.Runtime.InteropServices.DllImport("Microsoft.ui.xaml.dll")]
        private static extern void XamlCheckProcessRequirements();

        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.UI.Xaml.Markup.Compiler"," 3.0.0.2309")]
        [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
        [global::System.STAThreadAttribute]
        static void Main(string[] args)
        {
            XamlCheckProcessRequirements();
            
            global::WinRT.ComWrappersSupport.InitializeComWrappers();
            global::Microsoft.UI.Xaml.Application.Start((p) => {
                var context = new global::Microsoft.UI.Dispatching.DispatcherQueueSynchronizationContext(global::Microsoft.UI.Dispatching.DispatcherQueue.GetForCurrentThread());
                global::System.Threading.SynchronizationContext.SetSynchronizationContext(context);
                new App();
            });
        }
    }
#endif

Using our own Main entry point

The easiest way to achieve our goal is obviously to define the DISABLE_XAML_GENERATED_MAIN constant, and include your own Program.cs.

The first change to make is to App.csproj project definition file.

+		<!-- Use our own Main entry point so we can control the HostBuilder -->
+		<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+		<DefineConstants>DISABLE_XAML_GENERATED_MAIN</DefineConstants>
+		<StartupObject>App.Program</StartupObject>

We define the DISABLE_XAML_GENERATED_MAIN compile time constant and provide our own Program.cs. The majority of the Program.cs content is copied from the default one, which will ensure that the proper initialization and checks are made as if we were using the default entry point. Any additional pre- or post- initialization work needs to be placed appropriately.

ℹ NOTE 1:

AllowUnsafeBlocks is not really needed for the custom entry point but is required to use LibraryImport which is preferred to DLLImport.

ℹ NOTE 2:

In the recent versions of the WindowsAppSDK, both XamlCheckProcessRequirements() and InitializeComWrappers() don't do anything. It is better however to keep them for maximum compatibility as they are still there in the default generated entry point.