Using the C#/Win32 code generator to enhance your WinUI 3 app

With the new C#/WIn32 code generator it has become super easy to generate interop code for Windows APIs. Previously we had to go read the doc and figure out the proper C# signature, or browse and hope it is covered there. We could also reference an external package like PInvoke.Kernel32, PInvoke.User32 etc. etc. but it would also pull in a lot more dependencies and APIs than you probably need.

Now with the code generator, it's as simple as adding the nuget package, and then add the method name you want to a "NativeMethods.txt" file, and you instantly have access to the method and any structs and enums you need. It also uses all the latest C#9 features for improved interop code.

You can read the full blogpost on this here:


Using the C# Win32 Interop Generator with WinUI 3

The latest WinUI 3 Preview 4 release does have several improvements around improving your window, but there is also a lot still missing. However most of those features can be added by obtaining the window's HWND handle and calling the native Win32 methods directly.

So let's set out to try and control our window using CsWin32.


First, we'll create a new WinUI Desktop project:



Next we'll add the Microsoft.Windows.CsWin32 NuGet package the project (not the package project!) to get the Win32 code generator. However, note that the current v0.1.319-beta version has some bugs that you'll see when WinUI is referenced. So instead we'll use the daily build which has been fixed (you can skip this if you're reading this and a new version has been published). Go edit the project file, and add the following to get the latest daily from the nuget server that hosts these builds: (also shout-out to Andrew Arnott for quickly fixing bugs as they were discovered)

    <PackageReference Include="Microsoft.Windows.CsWin32" Version="0.1.370-beta">
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>


Next we'll add a new text file to the root of the project named NativeMethods.txt.

Screenshot 2021-02-19 100955

We're now set up to generate the required methods.

The first Win32 method we need is the ShowWindow method in User32.dll, so add the text ShowWindow to its own line in the NativeMethods.txt file.

Note that you should now be able to get auto completion and full intellisense documentation on the method Microsoft.Windows.Sdk.PInvoke.ShowWindow. Pretty cool huh?

Screenshot 2021-02-19 094339

If you use VS16.9+, you can also expand the Project Name -> Dependencies -> Analyzers -> Microsoft.Windows.CsWin32 -> Microsoft.Windows.CsWin32.SourceGenerator and see the code that gets generated.

Screenshot 2021-02-19 094156

None of this is coming from a big library. It's literally just an internal class directly compiled into your application. No external dependencies required, and if you're building a class library, no dependencies to force on your users either.


Controlling the WinUI Window

As you can see in the screenshot above, the first thing we need to do is provide an HWND handle. You can get this from the Window, but it's not obvious, as we need to cast it to an interface we define. Let's create a new static class called WindowExtensions to help us do this:

using System;
using System.Runtime.InteropServices;
using WinRT;

namespace WinUIWinEx
    public static class WindowExtensions
        internal interface IWindowNative
            IntPtr WindowHandle { get; }

        public static IntPtr GetWindowHandle(this Microsoft.UI.Xaml.Window window)
            => window is null ? throw new ArgumentNullException(nameof(window)) : window.As<IWindowNative>().WindowHandle;


Now we want to call the ShowWindow method. If you read the documentation for this method: you'll see takes a number indicating the desired window state, like 0 for hide, 3 for maximize, 5 for show, 6 for minimize and 9 for restore.

        private static bool ShowWindow(Microsoft.UI.Xaml.Window window, int nCmdShow)
            var hWnd = new Microsoft.Windows.Sdk.HWND(window.GetWindowHandle());
            return Microsoft.Windows.Sdk.PInvoke.ShowWindow(hWnd, nCmdShow);

        public static bool MinimizeWindow(this Microsoft.UI.Xaml.Window window) => ShowWindow(window, 6);
        public static bool MaximizeWindow(this Microsoft.UI.Xaml.Window window) => ShowWindow(window, 3);
        public static bool HideWindow(this Microsoft.UI.Xaml.Window window) => ShowWindow(window, 0);
        public static bool ShowWindow(this Microsoft.UI.Xaml.Window window) => ShowWindow(window, 5);
        public static bool RestoreWindow(this Microsoft.UI.Xaml.Window window) => ShowWindow(window, 9);


Because these are all extension methods, this now enables us to make simple calls inside our Window class like this:



The CsWin32 code generator just makes this really easy. I've taken all of this a few levels further and started a WinUIEx project on GitHub, where you can see the above code and much more, like setting your windows to be always-on-top or adding a tray icon to your app, so you can minimize to the tray.

You can find the repo on github here: