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

Use Int64KeyFrame-derived children (DiscreteInt64KeyFrame, LinearInt64KeyFrame, etc.).

dotnet/wpf · error · ArgumentException

SR.Animation_ChildMustBeKeyFrame

Error message

SR.Animation_ChildMustBeKeyFrame

What it means

Int64AnimationUsingKeyFrames.AddChild throws ArgumentException when the supplied child is not a (non-null) Int64KeyFrame. Only key frames of the matching integer type are valid children of this timeline.

Solutions

Use Int64KeyFrame-derived children (DiscreteInt64KeyFrame, LinearInt64KeyFrame, etc.).
Correct the XAML child element type to match the animation.
Prefer KeyFrames.Add with a typed key frame in code.

Example fix

// before
<Int64AnimationUsingKeyFrames>
  <LinearInt32KeyFrame KeyTime="0:0:1" Value="100" />
</Int64AnimationUsingKeyFrames>
// after
<Int64AnimationUsingKeyFrames>
  <LinearInt64KeyFrame KeyTime="0:0:1" Value="100" />
</Int64AnimationUsingKeyFrames>

Defensive patterns

Strategy: validation

Validate before calling

if (child is not Int64KeyFrame)
    throw new ArgumentException("Children of Int64AnimationUsingKeyFrames must be Int64KeyFrame instances", nameof(child));

Type guard

bool IsInt64KeyFrame(object o) => o is Int64KeyFrame;

Try / catch

try { animation.AddChild(child); }
catch (ArgumentException ex) when (ex.ParamName == "child") {
    // swap in the correctly typed Int64KeyFrame
}

Prevention

Match key frame child types to the animation value type (Int64 vs Int32)
Check markup after refactoring animation value types
Use KeyFrames.Add in code for compile-time type safety

When it happens

Trigger: XAML declaring a child that is not an Int64KeyFrame inside <Int64AnimationUsingKeyFrames>, or programmatic AddChild with an incompatible object.

Common situations: Mixing key frame types across animations (e.g. Int32KeyFrame inside an Int64 animation); XAML paste errors.

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/62972c3fe36038a3. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Animation/Generated/Int64AnimationUsingKeyFrames.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)
        {
            Int64KeyFrame keyFrame = child as Int64KeyFrame;

            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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