dotnet/wpf · error · ArgumentException

SR.Animation_ChildMustBeKeyFrame

Error message

SR.Animation_ChildMustBeKeyFrame

What it means

PointAnimationUsingKeyFrames.AddChild accepts only PointKeyFrame objects as XAML children; any other (or null) child causes this ArgumentException. WPF's key-frame animations are content-typed so the parser routes children through AddChild, which enforces the key-frame-only rule. The message literally states the child must be a key frame.

Solutions

1. Use only PointKeyFrame-derived children: LinearPointKeyFrame, DiscretePointKeyFrame, SplinePointKeyFrame (or PointKeyFrame base elements).
2. If converting from DoubleAnimationUsingKeyFrames, replace each Double*KeyFrame element with the corresponding Point*KeyFrame.
3. Verify the XAML namespace maps to PresentationCore/WPF, not another framework's key frame types.
4. If adding in code, call KeyFrames.Add(new LinearPointKeyFrame(...)) instead of AddChild.

Example fix

// before (XAML)
<PointAnimationUsingKeyFrames>
  <LinearDoubleKeyFrame Value="10" KeyTime="0:0:1" />
</PointAnimationUsingKeyFrames>
// after
<PointAnimationUsingKeyFrames>
  <LinearPointKeyFrame Value="10,10" KeyTime="0:0:1" />
</PointAnimationUsingKeyFrames>

if (child is PointKeyFrame kf) pointAnimationUsingKeyFrames.KeyFrames.Add(kf); else throw new ArgumentException($"Child must be a PointKeyFrame, got {child?.GetType().Name}");

static bool IsPointKeyFrame(object o) => o is PointKeyFrame;

try { xamlReader.Load(xaml); }
catch (ArgumentException ex) when (ex.Message.Contains("key frame") || ex.Message.Contains("KeyFrame")) { /* fix child element type in XAML */ }

When it happens

Trigger: Placing an element other than a PointKeyFrame (e.g. a raw Point, DoubleKeyFrame, or wrong-type key frame such as DoubleAnimationKeyFrame) inside a PointAnimationUsingKeyFrames element in XAML, or calling AddChild(null).

Common situations: Copy-pasting a DoubleAnimationUsingKeyFrames block and forgetting to change the key frame types to PointKeyFrame/LinearPointKeyFrame/DiscretePointKeyFrame/SplinePointKeyFrame; typo'd XAML namespace or element name inside the key frames collection.

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

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

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/9dbd60556def4cb2. Report an issue: GitHub.

Appendix: source

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

            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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