What causes "SR.Animation_ChildMustBeKeyFrame" in dotnet/wpf?

Use Int32KeyFrame-derived children (e.g. Int32KeyFrame subclasses like DiscreteInt32KeyFrame, LinearInt32KeyFrame) inside the animation.

dotnet/wpf · error · ArgumentException

SR.Animation_ChildMustBeKeyFrame

Error message

SR.Animation_ChildMustBeKeyFrame

What it means

ArgumentException thrown by Int32AnimationUsingKeyFrames.AddChild (the IAddChild path used for XAML markup children): the child object is not an Int32KeyFrame, and key-frame animations only accept key frames of their specific type as direct children.

Solutions

Use Int32KeyFrame-derived children (e.g. Int32KeyFrame subclasses like DiscreteInt32KeyFrame, LinearInt32KeyFrame) inside the animation.
Fix XAML markup so the child element matches the animation's key frame type.
If adding programmatically, call KeyFrames.Add with a typed Int32KeyFrame instead of AddChild.

Example fix

// before
<Int32AnimationUsingKeyFrames>
  <Int32Animation ... />
</Int32AnimationUsingKeyFrames>
// after
<Int32AnimationUsingKeyFrames>
  <LinearInt32KeyFrame KeyTime="0:0:1" Value="100" />
</Int32AnimationUsingKeyFrames>

Defensive patterns

Strategy: validation

Validate before calling

if (child is not Int32KeyFrame)
    throw new ArgumentException("Children of Int32AnimationUsingKeyFrames must be Int32KeyFrame instances", nameof(child));

Type guard

bool IsInt32KeyFrame(object o) => o is Int32KeyFrame;

Try / catch

try { animation.AddChild(child); }
catch (ArgumentException ex) when (ex.ParamName == "child") {
    // replace with the correct typed key frame
}

Prevention

Keep key frame child types matched to the animation's value type
Avoid hand-editing generated XAML animation markup without type checking
Prefer KeyFrames.Add over AddChild in code so the compiler enforces the type

When it happens

Trigger: Declaring a non-key-frame child element inside <Int32AnimationUsingKeyFrames> in XAML, or calling AddChild with an object that is not an Int32KeyFrame.

Common situations: XAML typo or wrong child element (e.g. putting a plain Int32Animation or a DoubleKeyFrame inside an Int32 key-frame animation); copy-paste between differently-typed animations.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/63d3a4b374a4bf06. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Animation/Generated/Int32AnimationUsingKeyFrames.cs:227

            WritePostscript();
        }

        /// <summary>
        /// Implemented to allow KeyFrames to be direct children
        /// of KeyFrameAnimations in markup.
        /// </summary>
        [EditorBrowsable(EditorBrowsableState.Advanced)]
        protected virtual void AddChild(object child)
        {
            Int32KeyFrame keyFrame = child as Int32KeyFrame;

            if (keyFrame != null)
            {
                KeyFrames.Add(keyFrame);
            }
            else
            {        
                throw new ArgumentException(SR.Animation_ChildMustBeKeyFrame, nameof(child));
            }
        }

        /// <summary>
        /// Adds a text string as a child of this KeyFrameAnimation.
        /// </summary>
        /// <param name="childText">
        /// The text to add.
        /// </param>
        /// <remarks>
        /// A KeyFrameAnimation does not accept text as a child, so this method will
        /// raise an InvalididOperationException unless a derived class has
        /// overridden the behavior to add text.
        /// </remarks>
        /// <exception cref="ArgumentNullException">The childText parameter is
        /// null.</exception>
        void IAddChild.AddText(string childText)
        {

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