dotnet/wpf · error · InvalidOperationException

SR.ContentControlCannotHaveMultipleContent

Error message

SR.ContentControlCannotHaveMultipleContent

What it means

ContentControl.AddChild throws when the control already has non-null Content and AddChild is called with another non-null object. A ContentControl can hold exactly one content element, so adding a second child directly via AddChild is not allowed. Use a panel or an items control if multiple children are needed.

Solutions

1. Set the Content property instead of calling AddChild: control.Content = child (this replaces existing content).
2. If multiple children are required, set Content to a StackPanel/Grid and add children to that panel.
3. Use an ItemsControl-derived type (ListBox, ItemsControl) instead of ContentControl when the content is a collection.
4. Null the Content first if the intent is to replace it, then AddChild.

Example fix

// before
button.Content = "Click";
button.AddChild(new Image()); // throws
// after
var panel = new StackPanel();
panel.Children.Add(new TextBlock { Text = "Click" });
panel.Children.Add(new Image());
button.Content = panel;

bool canAddChild = control.Content == null || child == null;

static bool HasNoContent(ContentControl c) => c.Content == null;

try
{
    control.AddChild(child);
}
catch (InvalidOperationException)
{
    control.Content = child; // replace existing content instead
}

When it happens

Trigger: Calling contentControl.AddChild(child) twice, or calling AddChild on a control whose Content was already set in XAML or via the Content property.

Common situations: Building a Button/Window/Label in code and calling AddChild after setting Content, or migrating XAML where the control already had inline content into code that calls AddChild.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

• Animation_NoTextChildren
• Page can have only one child.
• SR.NoAddChild
• SR.ParserCanOnlyHaveOneChild
• " }} " element found. Expected fixed page element ( }} ).

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/47f97db6d14566fe. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/ContentControl.cs:195

        /// </summary>
        void IAddChild.AddChild(object value)
        {
            AddChild(value);
        }

        /// <summary>
        ///  Add an object child to this control
        /// </summary>
        protected virtual void AddChild(object value)
        {
            // if conent is the first child or being cleared, set directly
            if (Content == null || value == null)
            {
                Content = value;
            }
            else
            {
                throw new InvalidOperationException(SR.ContentControlCannotHaveMultipleContent);
            }
        }

        /// <summary>
        ///  Add a text string to this control
        /// </summary>
        void IAddChild.AddText(string text)
        {
            AddText(text);
        }

        /// <summary>
        ///  Add a text string to this control
        /// </summary>
        protected virtual void AddText(string text)
        {
            AddChild(text);
        }

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