What causes "RectKeyFrameCollection[{0}]" in dotnet/wpf?

Check for null before assigning: only set collection[index] when the key frame is non-null

dotnet/wpf · error · ArgumentNullException

RectKeyFrameCollection

Error message

RectKeyFrameCollection[{0}]

What it means

RectKeyFrameCollection's this[int] setter throws ArgumentNullException whose (misused) message parameter is the formatted string "RectKeyFrameCollection[{index}]" when a null RectKeyFrame is assigned at the given index. The collection forbids null key frames; the formatted string is used as the paramName, identifying the failing index.

Solutions

Check for null before assigning: only set collection[index] when the key frame is non-null
Create a valid RectKeyFrame instance instead of assigning null
Use Add() and validate elements before insertion

Example fix

// before
keyFrames[0] = GetKeyFrame(); // may return null
// after
var kf = GetKeyFrame();
if (kf != null) keyFrames[0] = kf;

Defensive patterns

Strategy: type-guard

Validate before calling

if (kf == null) throw new InvalidOperationException("Cannot assign null key frame at index " + index); keyFrames[index] = kf;

Type guard

bool IsValidKeyFrame(RectKeyFrame kf) => kf != null;

Try / catch

try { keyFrames[index] = kf; } catch (ArgumentNullException ex) { /* kf was null at ex.ParamName index */ }

Prevention

Null-check key frames before indexed assignment
Ensure factory methods never return null key frames
Prefer Add() with validation over direct index sets

When it happens

Trigger: Executing collection[index] = null on a RectKeyFrameCollection, e.g. myCollection[0] = null;

Common situations: Programmatic collection manipulation where a key frame variable is null (failed lookup/creation) before assignment; generic collection-filling code that doesn't check for null elements.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/5a5a50d0b2f52f9e. Report an issue: GitHub.

Appendix: source

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

            }
        }

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

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