dotnet/wpf · error · ArgumentNullException

ColorKeyFrameCollection

Error message

ColorKeyFrameCollection[{0}]

What it means

ColorKeyFrameCollection's public indexer setter throws ArgumentNullException when assigning null at a given index. The parameter name is built as "ColorKeyFrameCollection[{index}]" so the message points at the offending slot. The collection requires every element to be a non-null ColorKeyFrame.

Solutions

1. Assign a valid ColorKeyFrame instance instead of null
2. Use RemoveAt(index) to delete a slot rather than setting null
3. Validate the key frame for null before the assignment
4. Fix binding/source data so it never yields null key frames

Example fix

// before
keyFrameCollection[0] = null; // ArgumentNullException
// after
if (newFrame != null) keyFrameCollection[0] = newFrame; else keyFrameCollection.RemoveAt(0);

if (frame == null) keyFrames.RemoveAt(index); else keyFrames[index] = frame;

static bool IsValidSlotAssignment(ColorKeyFrame frame) => frame != null;

try { keyFrames[i] = frame; }
catch (ArgumentNullException ex) when (ex.ParamName == $"ColorKeyFrameCollection[{i}]") { /* remove slot or supply default frame */ }

When it happens

Trigger: Executing keyFrames[i] = null on a ColorKeyFrameCollection, or API/XAML paths that end up assigning a null key frame into an existing slot.

Common situations: Data-binding a null item into the collection; clearing logic that assigns null instead of removing; deserialization producing null entries.

Related errors

• DecimalKeyFrameCollection
• Animation_ChildMustBeKeyFrame
• DoubleKeyFrameCollection
• [[instance.TypeName]]KeyFrameCollection
• Int16KeyFrameCollection

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/3dcefe69a932d486. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Animation/Generated/ColorKeyFrameCollection.cs:466

            }
        }

        /// <summary>
        /// Gets or sets the ColorKeyFrame at a given index.
        /// </summary>
        public ColorKeyFrame this[int index]
        {
            get
            {
                ReadPreamble();

                return _keyFrames[index];
            }
            set
            {
                if (value == null)
                {
                    throw new ArgumentNullException(String.Format(CultureInfo.InvariantCulture, "ColorKeyFrameCollection[{0}]", index));
                }

                WritePreamble();

                if (value != _keyFrames[index])
                {
                    OnFreezablePropertyChanged(_keyFrames[index], value);
                    _keyFrames[index] = value;

                    Debug.Assert(_keyFrames[index] != null);

                    WritePostscript();
                }
            }
        }

        #endregion
    }

View on GitHub (pinned to 81131a70a4)