MaterialDesignInXAML/MaterialDesignInXamlToolkit · error · ArgumentNullException

window

Error message

window

What it means

ArgumentNullException(nameof(window)) in GetFirstDialogHost(Window) (DialogHostEx.cs:182). Triggered when a ShowDialog extension method on Window is called with a null Window reference, before any visual-tree traversal runs.

Source

Thrown at src/MaterialDesignThemes.Wpf/DialogHostEx.cs:182

    /// <summary>
    /// Shows a dialog using the parent/ancestor <see cref="DialogHost"/> of the a given <see cref="DependencyObject"/>.
    /// </summary>
    /// <param name="childDependencyObject">Dependency object which should be a visual child of a <see cref="DialogHost"/>, where the dialog will be shown.</param>
    /// <param name="content">Content to show (can be a control or view model).</param>
    /// <param name="openedEventHandler">Allows access to opened event which would otherwise have been subscribed to on a instance.</param>
    /// <param name="closingEventHandler">Allows access to closing event which would otherwise have been subscribed to on a instance.</param>
    /// <param name="closedEventHandler">Allows access to closed event which would otherwise have been subscribed to on a instance.</param>
    /// <exception cref="InvalidOperationException">
    /// Thrown is a <see cref="DialogHost"/> is not found when conducting a depth first traversal of visual tree.  
    /// </exception>
    /// <returns></returns>
    public static Task<object?> ShowDialog(this DependencyObject childDependencyObject, object content, DialogOpenedEventHandler openedEventHandler, DialogClosingEventHandler closingEventHandler, DialogClosedEventHandler closedEventHandler)
        => GetOwningDialogHost(childDependencyObject).ShowInternal(content, openedEventHandler, closingEventHandler, closedEventHandler);

    private static DialogHost GetFirstDialogHost(Window window)
    {
        if (window is null) throw new ArgumentNullException(nameof(window));

        DialogHost? dialogHost = window.VisualDepthFirstTraversal().OfType<DialogHost>().FirstOrDefault();

        if (dialogHost is null)
            throw new InvalidOperationException("Unable to find a DialogHost in visual tree");

        return dialogHost;
    }

    private static DialogHost GetFirstDialogHost(Window window, object? dialogIdentifier)
    {
        if (window is null) throw new ArgumentNullException(nameof(window));

        DialogHost? dialogHost = window.VisualDepthFirstTraversal().OfType<DialogHost>().FirstOrDefault(x => x.Identifier is not null && x.Identifier.Equals(dialogIdentifier));

        if (dialogHost is null)
            throw new InvalidOperationException($"Unable to find a DialogHost with identifier '{dialogIdentifier}' in visual tree");

View on GitHub (pinned to 98edec3a0b)

Solutions

1. Null-check the Window before calling ShowDialog and surface a clearer error or skip.
2. Acquire the window via Window.GetWindow(this) only after the control is loaded (subscribe to Loaded).
3. Prefer DialogHost.Show(content, identifier) which routes via Identifier and avoids needing a Window reference.

Example fix

// before
Application.Current.MainWindow.ShowDialog(content); // MainWindow may be null at startup

// after
var window = Window.GetWindow(this);
if (window is not null)
    await window.ShowDialog(content);
// or: await DialogHost.Show(content, "RootDialog");

if (window is null)
    throw new ArgumentNullException(nameof(window));

await window.ShowDialog(content);

static bool CanShow(Window? window) => window is not null && window.VisualDepthFirstTraversal().OfType<DialogHost>().Any();

When it happens

Trigger: Calling window.ShowDialog(content) where 'window' is null (e.g. Window.GetWindow(this) returned null because the control is not yet in a loaded Window, or a field was never assigned).

Common situations: Calling ShowDialog from a control before it is loaded into a Window; using Application.Current.MainWindow before it is set; refactor leaving a Window field null.

Related errors

• session
• session
• childDependencyObject
• session
• owner

AI-assisted analysis of MaterialDesignInXAML/MaterialDesignInXamlToolkit@98edec3a0b (2026-08-13). Data as JSON: /api/errors/d23290ec408b853c. Report an issue: GitHub.