dotnet/wpf · error · ArgumentException

SR.Animation_ChildMustBeKeyFrame

Error message

SR.Animation_ChildMustBeKeyFrame

What it means

RectAnimationUsingKeyFrames.AddChild throws ArgumentException (SR.Animation_ChildMustBeKeyFrame) when a child added in XAML or code is not a RectKeyFrame. Key-frame animations only accept key frame children; anything else (another Timeline, a string, etc.) cannot be added to the KeyFrames collection.

Solutions

  1. Add only RectKeyFrame-derived children: LinearRectKeyFrame, DiscreteRectKeyFrame, SplineRectKeyFrame, EasingRectKeyFrame
  2. Fix XAML so children use the correct key frame type matching the animation's value type
  3. If adding programmatically, call KeyFrames.Add(keyFrame) directly after verifying the type

Example fix

// before
<RectAnimationUsingKeyFrames>
  <LinearDoubleKeyFrame Value="100" KeyTime="0:0:1" /> <!-- wrong type -->
</RectAnimationUsingKeyFrames>
// after
<RectAnimationUsingKeyFrames>
  <LinearRectKeyFrame Value="new Rect(0,0,100,100)" KeyTime="0:0:1" />
</RectAnimationUsingKeyFrames>
Defensive patterns

Strategy: type-guard

Validate before calling

if (child is RectKeyFrame kf) keyFrames.KeyFrames.Add(kf); else throw new ArgumentException(nameof(child));

Type guard

bool IsRectKeyFrame(object o) => o is RectKeyFrame;

Try / catch

try { keyFrameAnimation.AddChild(child); } catch (ArgumentException) { /* child was not a RectKeyFrame; use correct key frame type */ }

Prevention

When it happens

Trigger: Calling AddChild(child) with an object that is not a RectKeyFrame — e.g. adding a DoubleAnimation or arbitrary object as a child of RectAnimationUsingKeyFrames in XAML or via the IAddChild path.

Common situations: XAML authoring mistakes: nesting the wrong animation type inside a key-frame animation, or copy-pasting children between animations of different target types (e.g. DoubleKeyFrame inside a Rect key-frame animation).

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/6884b4122d1d42cb. Report an issue: GitHub.

Appendix: source

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

            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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