dotnet/wpf · error · InvalidOperationException

SR.NoAddChild

Error message

SR.NoAddChild

What it means

NavigationWindow overrides AddChild to explicitly forbid adding object children through the XAML content-model, since its content must be set via the Content property / navigation. Any attempt to append a child (e.g. XAML '<NavigationWindow><Button/>') throws.

Solutions

1. Set NavigationWindow.Content to a single element instead of adding children.
2. Put your UI in a Page and navigate to it (Source or Navigate()).
3. Wrap multiple elements in a container (Grid/StackPanel) assigned to Content.
4. Remove child elements from NavigationWindow XAML and host them in the navigated page.

Example fix

// before
<NavigationWindow ...>
  <Button Content="OK"/>
</NavigationWindow>
// after
<NavigationWindow x:Name="navWin" Source="MainPage.xaml" ... />
// or navWin.Content = new Button{ Content="OK" };

if (navWindow is NavigationWindow) { navWindow.Content = element; } else { navWindow.AddChild(element); }

void SetContent(Window w, object c) { if (w is NavigationWindow) ((NavigationWindow)w).Content = c; else w.Content = c; }

try { navWindow.Content = element; }
catch (InvalidOperationException) { /* fall back to Page navigation */ }

When it happens

Trigger: Calling navigationWindow.AddChild(obj) directly, or declaring child elements inside a NavigationWindow in XAML, which routes through AddChild during parsing.

Common situations: Migrating from Window to NavigationWindow while keeping child-element XAML; programmatic UI construction code calling AddChild on a NavigationWindow.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Animation_NoTextChildren
• SR.ParserCanOnlyHaveOneChild
• Animation_ChildMustBeKeyFrame
• Animation_ChildMustBeKeyFrame
• Animation_NoTextChildren

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/3a08890a1f38e188. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Navigation/NavigationWindow.cs:838

        //
        //  Protected Methods
        //
        //------------------------------------------------------
        #region Protected Methods
        /// <summary>
        /// Creates AutomationPeer (<see cref="UIElement.OnCreateAutomationPeer"/>)
        /// </summary>
        protected override AutomationPeer OnCreateAutomationPeer()
        {
            return new NavigationWindowAutomationPeer(this);
        }

        /// <summary>
        ///  Add an object child to this control
        /// </summary>
        protected override void AddChild(object value)
        {
            throw new InvalidOperationException(SR.NoAddChild);
        }

        /// <summary>
        ///  Add a text string to this control
        /// </summary>
        protected override void AddText(string text)
        {
            XamlSerializerUtil.ThrowIfNonWhiteSpaceInAddText(text, this);
        }

        /// <summary>
        ///     This even fires when window is closed. This event is non cancelable and is
        ///     for user infromational purposes
        /// </summary>
        /// <remarks>
        ///     This method follows the .Net programming guideline of having a protected virtual
        ///     method that raises an event, to provide a convenience for developers that subclass
        ///     the event. If you override this method - you need to call Base.OnClosed(...) for

View on GitHub (pinned to 81131a70a4)