MaterialDesignInXAML/MaterialDesignInXamlToolkit · error · InvalidOperationException

Content cannot be passed to a dialog via the OpenDialog if D

Error message

Content cannot be passed to a dialog via the OpenDialog if DialogContent already has a binding.

What it means

AssertTargetableContent rejects passing content via the OpenDialog/Show APIs when DialogContentProperty already has a WPF Binding expression. The host cannot overwrite a bound DialogContent without breaking the binding, so ShowInternal calls AssertTargetableContent early (DialogHost.cs:265) and the check at :798 throws if a BindingExpression exists.

Source

Thrown at src/MaterialDesignThemes.Wpf/DialogHost.cs:797

    /// <summary>
    /// Callback fired when the <see cref="DialogClosed"/> event is fired, allowing the event to be processed from a binding/view model.
    /// </summary>
    public DialogClosedEventHandler? DialogClosedCallback
    {
        get => (DialogClosedEventHandler?)GetValue(DialogClosedCallbackProperty);
        set => SetValue(DialogClosedCallbackProperty, value);
    }

    protected void OnDialogClosed(DialogClosedEventArgs eventArgs)
        => RaiseEvent(eventArgs);

    #endregion

    internal void AssertTargetableContent()
    {
        var existingBinding = BindingOperations.GetBindingExpression(this, DialogContentProperty);
        if (existingBinding != null)
            throw new InvalidOperationException(
                "Content cannot be passed to a dialog via the OpenDialog if DialogContent already has a binding.");
    }

    internal void InternalClose(object? parameter)
    {
        var currentSession = CurrentSession ?? throw new InvalidOperationException($"{nameof(DialogHost)} does not have a current session");

        currentSession.CloseParameter = parameter;
        currentSession.IsEnded = true;

        //multiple ways of calling back that the dialog is closing:
        // * routed event
        // * the attached property (which should be applied to the button which opened the dialog
        // * straight forward IsOpen dependency property 
        // * handler provided to the async show method
        var dialogClosingEventArgs = new DialogClosingEventArgs(currentSession, DialogClosingEvent);
        OnDialogClosing(dialogClosingEventArgs);
        _attachedDialogClosingEventHandler?.Invoke(this, dialogClosingEventArgs);

View on GitHub (pinned to 98edec3a0b)

Solutions

Remove the Binding on DialogContent in XAML and pass content via Show(content) / Show(content, identifier) instead.
If the binding is required, do not pass content to Show; rely on the bound view model and just toggle IsOpen or call Show(null).
Audit XAML for 'DialogContent=' bindings and reconcile with the chosen open-dialog pattern.

Example fix

<!-- before -->
<materialDesign:DialogHost DialogContent="{Binding MyContent}" />

<!-- after (use imperative Show) -->
<materialDesign:DialogHost Identifier="RootDialog" />
// await DialogHost.Show(new MyView(), "RootDialog");

Defensive patterns

Strategy: validation

Validate before calling

var binding = BindingOperations.GetBindingExpression(dialogHost, DialogHost.DialogContentProperty);
if (binding is not null)
    throw new InvalidOperationException("Remove the DialogContent binding before using Show(content).");

await dialogHost.Show(content);

Prevention

Pick ONE dialog pattern: either bind DialogContent in XAML or pass content via Show—not both.
Audit XAML for 'DialogContent=' bindings before switching to imperative Show.
If a binding is needed, call Show(null) and toggle IsOpen instead of passing content.

When it happens

Trigger: XAML binds DialogContent (e.g. DialogContent="{Binding ...}") and code then calls Show(content), ShowInternal(content, ...) or DialogSession.UpdateContent(content) with a non-null content object.

Common situations: Designer/data-template bound DialogContent while app code tries to inject a runtime control; migrating from IsOpen binding pattern to async Show pattern without removing the XAML binding; partial views where both a binding and imperative Show are wired.

Related errors

AI-assisted analysis of MaterialDesignInXAML/MaterialDesignInXamlToolkit@98edec3a0b (2026-08-13). Data as JSON: /api/errors/e7f289b1011e1bb2. Report an issue: GitHub.