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Uno.Features.AppCloseHandler

Preventing Window Closing

Tip

This article covers Uno Platform–specific behavior. For the full API documentation, see AppWindow.Closing Event.

The AppWindow.Closing API lets you respond to or prevent standard app window closing requests—such as clicking the window’s close button or pressing Alt+F4. Note that this does not block the user from terminating the app by force (e.g., via Task Manager or kill).

Platform Support

Feature Windows App SDK Android iOS Web (WASM) Desktop (Windows) Desktop (macOS) Desktop (Linux)
AppWindow.Closing ✔️ ❌ ❌ ❌ ✔️ ✔️ ✔️

Note

On platforms where this feature is not supported, the Closing event will still be raised, but setting args.Cancel = true has no effect.

Usage Example

MyWindow.AppWindow.Closing += OnAppWindowClosing;

private void OnAppWindowClosing(AppWindow sender, AppWindowClosingEventArgs args)
{
    // Replace with your own logic, such as checking for unsaved changes
    bool cancelClose = ShouldWindowStayOpen();

    // Cancel the close request if needed
    args.Cancel = cancelClose;
}

Important

The AppWindow.Closing event must be handled synchronously. Asynchronous operations (e.g., showing a ContentDialog) are not allowed and will not delay the closing process.