.NET MAUI - Toggle fullscreen for MacCatalyst desktop app

.NET MAUI Mac Catalyst apps run on macOS, but use UIKit (iOS frameworks) rather than AppKit (native macOS frameworks). To toggle full screen, the same behavior as clicking the green ”zoom” button in the window chrome, we need to bridge into AppKit’s NSApplication and NSWindow using the Objective-C runtime.

#if MACCATALYST

    var nsAppClass = ObjCRuntime.Class.GetHandle("NSApplication");
    var nsApp = ObjCRuntime.Runtime.GetNSObject(nsAppClass);
    var sharedApp = nsApp?.PerformSelector(new ObjCRuntime.Selector("sharedApplication"));
    var keyWindow = sharedApp?.PerformSelector(new ObjCRuntime.Selector("keyWindow"));
    keyWindow?.PerformSelector(new ObjCRuntime.Selector("toggleFullScreen:"), keyWindow);

#endif

If you want to see it in action, check out this demo project containing a Mac App.

Here’s how it works.

var nsAppClass = ObjCRuntime.Class.GetHandle("NSApplication");

Gets a handle (pointer) to the Objective-C class NSApplication. This is AppKit’s application singleton class that manages the macOS app lifecycle. Since Mac Catalyst doesn’t directly expose AppKit types to C#, we access it dynamically through the Objective-C runtime.

var nsApp = ObjCRuntime.Runtime.GetNSObject(nsAppClass);

Wraps the raw class handle in an NSObject that we can interact with from C#. This gives us a managed object representing the NSApplication class itself, which we get with the next line.

var sharedApp = nsApp?.PerformSelector(new ObjCRuntime.Selector("sharedApplication"));

This calls the sharedApplication class method on NSApplication, which returns the singleton instance of the running application.

var keyWindow = sharedApp?.PerformSelector(new ObjCRuntime.Selector("keyWindow"));

This retrieves the keyWindow property from the shared application, i.e. the window that will receive any keypresses. This returns an NSWindow instance. Then. . . finally. . .

keyWindow?.PerformSelector(new ObjCRuntime.Selector("toggleFullScreen:"), keyWindow);

Calls toggleFullScreen: on the NSWindow. This is the same method macOS invokes when you click the green full screen button. The method takes a sender parameter (we pass the window itself), and toggles between full screen and windowed mode. This mean you can call it again to revert the full screen.

And with that, you now have a fullscreen Mac app using .NET MAUI.

What about Windows?

If you want to implement the same functionality within MAUI for Windows, I highly recommend this blog post.