dotnet/wpf · error · ArgumentException

SR.Animation_ChildMustBeKeyFrame

Error message

SR.Animation_ChildMustBeKeyFrame

What it means

ArgumentException thrown by Point3DAnimationUsingKeyFrames.AddChild (the IAddChild path used for XAML markup children): the child object is not a Point3DKeyFrame, and key-frame animations only accept key frames of their specific type as direct children.

Solutions

1. Use Point3D key frames such as LinearPoint3DKeyFrame or SplinePoint3DKeyFrame as children
2. Type-check the child (is Point3DKeyFrame) before calling AddChild
3. Add frames via KeyFrames.Add in code

Example fix

// before
anim.AddChild(new Point3D(1,2,3));
// after
anim.KeyFrames.Add(new LinearPoint3DKeyFrame(new Point3D(1,2,3), KeyTime.FromTimeSpan(TimeSpan.FromSeconds(1))));

if (child is Point3DKeyFrame kf) animation.KeyFrames.Add(kf);

bool IsPoint3DKeyFrame(object c) => c is Point3DKeyFrame;

try { animation.AddChild(child); }
catch (ArgumentException ex) when (ex.ParamName == nameof(child)) { /* use a Point3DKeyFrame child instead */ }

When it happens

Trigger: Calling AddChild with null or a non-Point3DKeyFrame object, or XAML nesting an unsupported element inside <Point3DAnimationUsingKeyFrames>.

Common situations: Wrong key-frame type in XAML (e.g. DoubleKeyFrame), plain values as children, or code passing arbitrary objects to AddChild.

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

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

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/ccbb4c50c4337d5b. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Animation/Generated/Point3DAnimationUsingKeyFrames.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)
        {
            Point3DKeyFrame keyFrame = child as Point3DKeyFrame;

            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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