PrismLibrary/Prism · error · InvalidOperationException

Prism applications only support the use of PrismWindow, but…

Error message

Prism applications only support the use of PrismWindow, but found '{window.GetType().FullName}'.

What it means

Prism.Maui requires the application's active window to be a PrismWindow (its own Window subclass carrying CurrentPage state). GetCurrentPage throws InvalidOperationException with the concrete offending type name when windowManager.Current is any other Window - typically a plain Microsoft.Maui.Controls.Window created by the default MAUI template.

Solutions

1. Change CreateWindow to return a PrismWindow: `protected override Window CreateWindow(IActivationState state) => new PrismWindow(...);`
2. Let Prism create the window automatically (remove custom CreateWindow override) and configure the app via PrismAppBuilder.
3. If Shell is required, don't use Prism's IWindowManager page extensions, which assume PrismWindow.

Example fix

// before
protected override Window CreateWindow(IActivationState s) => new Window(startupPage);
// after
protected override Window CreateWindow(IActivationState s) => new PrismWindow(startupPage);

if (Application.Current?.Windows.Any(w => w is not PrismWindow) ?? false)
    throw new InvalidOperationException("All windows must be PrismWindow for Prism.Maui");

bool IsPrismHosted(IWindowManager wm) => wm.Current is PrismWindow;

try { var page = windowManager.GetCurrentPage(); }
catch (InvalidOperationException ex) when (ex.Message.Contains("PrismWindow"))
{
    // fix CreateWindow to return a PrismWindow
}

When it happens

Trigger: Overriding CreateWindow to `return new Window(page);` instead of PrismWindow; using MAUI Shell (Shell windows) with Prism's WindowManager extensions; another library swapping the application's main Window.

Common situations: New MAUI projects created from the default template then adding Prism, without removing the default `new Window(this)` in MauiProgram/App; mixing Shell and Prism navigation.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• The current window ' ' is not a PrismWindow.
• No Window has been set in the Application
• No current page has been set.
• Expected Navigation Failed. No Root Window has been created.
• The object must be of type

AI-assisted analysis of PrismLibrary/Prism@358118cd64 (2026-09-15). Data as JSON: /api/errors/0c912340ee21d148. Report an issue: GitHub.

Appendix: source

Thrown at src/Maui/Prism.Maui/Navigation/IWindowManagerExtensions.cs:40

    /// <summary>
    /// Gets the <see cref="IDialogService"/> for the currently displayed <see cref="Page"/>.
    /// </summary>
    /// <param name="windowManager">The <see cref="IWindowManager"/>.</param>
    /// <returns>The <see cref="IDialogService"/> for the current <see cref="Page"/>.</returns>
    public static IDialogService GetCurrentDialogService(this IWindowManager windowManager)
    {
        var page = windowManager.GetCurrentPage();
        var container = page.GetContainerProvider();
        return container.Resolve<IDialogService>();
    }

    private static Page GetCurrentPage(this IWindowManager windowManager)
    {
        var window = windowManager.Current;
        if (window is null)
            throw new InvalidOperationException("No Window has been set in the Application");
        else if (window is not PrismWindow prismWindow)
            throw new InvalidOperationException($"Prism applications only support the use of PrismWindow, but found '{window.GetType().FullName}'.");
        else if (prismWindow.CurrentPage is null)
            throw new InvalidOperationException("No current page has been set.");
        else
            return prismWindow.CurrentPage;
    }
}

View on GitHub (pinned to 358118cd64)