dotnet/wpf · error · ArgumentException

SR.Animation_ChildMustBeKeyFrame

Error message

SR.Animation_ChildMustBeKeyFrame

What it means

ColorAnimationUsingKeyFrames.AddChild throws ArgumentException when the child object passed to it is not (and cannot be converted to) a ColorKeyFrame. Key-frame animations only accept key frame children; anything else is rejected. The SR resource key Animation_ChildMustBeKeyFrame carries this message, with the parameter name 'child' attached.

Solutions

1. Pass only ColorKeyFrame-derived objects (DiscreteColorKeyFrame, LinearColorKeyFrame, SplineColorKeyFrame, EasingColorKeyFrame) as children
2. Wrap raw values in a key frame, e.g. new DiscreteColorKeyFrame(Colors.Red, KeyTime.FromTimeSpan(...))
3. Fix the XAML so each child of ColorAnimationUsingKeyFrames is a key frame element
4. Use KeyFrames.Add explicitly with the correct key frame type

Example fix

// before
animation.AddChild(Colors.Red); // ArgumentException
// after
animation.AddChild(new DiscreteColorKeyFrame(Colors.Red, KeyTime.FromTimeSpan(TimeSpan.FromSeconds(1))));

if (child is ColorKeyFrame keyFrame)
    animation.AddChild(keyFrame);
else
    throw new ArgumentException($"Child must be a ColorKeyFrame, got {child?.GetType().Name}");

static bool IsColorKeyFrame(object child) => child is ColorKeyFrame;

try { animation.AddChild(child); }
catch (ArgumentException ex) when (ex.ParamName == "child") { /* convert or reject the child */ }

When it happens

Trigger: Calling AddChild(object) on a ColorAnimationUsingKeyFrames with a value that is not a ColorKeyFrame — e.g. passing a Color, a string, or another animation type as XAML/content child.

Common situations: XAML authoring mistakes where a child element of the key-frame animation is not a DiscreteColorKeyFrame/LinearColorKeyFrame/SplineColorKeyFrame; generated/designer code inserting a wrong child type.

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
• Animation_ChildMustBeKeyFrame
• ColorKeyFrameCollection
• DecimalKeyFrameCollection
• DoubleKeyFrameCollection

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/0bbfa73c3b09ced1. Report an issue: GitHub.

Appendix: source

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

            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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