dotnet/maui · error · InvalidOperationException

Both MainPage was set and CreateWindow was overridden to pro

Error message

Both MainPage was set and CreateWindow was overridden to provide a page.

What it means

Thrown during window creation when both Application.MainPage has been set AND the overridden CreateWindow returned a Window that already has a Page (window.Page != _singleWindowMainPage). MAUI cannot decide which page is authoritative, so it refuses rather than silently picking one.

Source

Thrown at src/Controls/src/Core/Application/Application.cs:486

			if (activationState?.State?.TryGetValue(MauiWindowIdKey, out var requestedWindowId) ?? false)
			{
				if (requestedWindowId != null && _requestedWindows.TryGetValue(requestedWindowId, out var r))
				{
					if (r.TryGetTarget(out var w))
					{
						window = w;
					}
					_requestedWindows.Remove(requestedWindowId);
				}
			}

			// create a new one if there is no pending windows
			if (window == null)
			{
				window = CreateWindow(activationState);

				if (_singleWindowMainPage != null && window.Page != null && window.Page != _singleWindowMainPage)
					throw new InvalidOperationException($"Both {nameof(MainPage)} was set and {nameof(Application.CreateWindow)} was overridden to provide a page.");
			}

			// make sure it is added to the windows list
			if (!_windows.Contains(window))
				AddWindow(window);

			return window;
		}

		void IApplication.OpenWindow(IWindow window)
		{
			if (window is Window cwindow)
			{
				OpenWindow(cwindow);
			}
		}

		void IApplication.CloseWindow(IWindow window)

View on GitHub (pinned to f377ff1c5e)

Solutions

Choose one source of truth: either set MainPage (single-window convenience) or override CreateWindow to provide the page, not both.
If you override CreateWindow, stop setting MainPage so _singleWindowMainPage stays null.
If you keep MainPage, remove the Page assignment from your CreateWindow override so window.Page is null.

Example fix

// before
public App() { MainPage = new Page1(); }
protected override Window CreateWindow(IActivationState s) => new Window(new Page2());
// after
public App() { /* do not set MainPage */ }
protected override Window CreateWindow(IActivationState s) => new Window(new Page2());

Defensive patterns

Strategy: validation

Validate before calling

// Pick one source of truth for the first page.
static void ConfigureFirstPage(Application app, Func<Window> createWindow)
{
    if (app.MainPage is not null)
        throw new InvalidOperationException("Do not override CreateWindow to return a Page when MainPage is set.");
}

Prevention

Decide up front: convenience (MainPage) or explicit windows (override CreateWindow).
Remove MainPage assignment the moment you override CreateWindow to return a Window with a Page.
During migration, search the App constructor and CreateWindow together so only one path is active.

When it happens

Trigger: In the window-creation path, _singleWindowMainPage is non-null AND the Window returned by CreateWindow(activationState) has a non-null Page that differs from the single-window main page, triggering the InvalidOperationException.

Common situations: Migrating an app that sets MainPage in the constructor AND overrides CreateWindow to build its own Window/Page; half-finished multi-window migration leaving both code paths active; setting MainPage in OnStart while also overriding CreateWindow.

Related errors

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/1dcd6926b45e1474. Report an issue: GitHub.