dotnet/wpf · error · ArgumentNullException

MatrixKeyFrameCollection

Error message

MatrixKeyFrameCollection[{0}]

What it means

The MatrixKeyFrameCollection indexer setter throws ArgumentNullException when a null key frame is assigned. The exception's message ( paramName) is the formatted string "MatrixKeyFrameCollection[index]", identifying the collection position rather than a parameter name.

Solutions

1. Guard the value for null before assigning into the collection
2. Only add non-null MatrixKeyFrame instances (use Add, which also validates)
3. If a slot must be 'empty', remove the frame instead of assigning null

Example fix

// before
keyFrames[0] = GetFrameMaybeNull();
// after
var frame = GetFrameMaybeNull();
if (frame == null) throw new InvalidOperationException("frame required");
keyFrames[0] = frame;

if (frame == null) throw new ArgumentException($"MatrixKeyFrameCollection[{index}] requires a non-null frame");
collection[index] = frame;

bool IsValidFrame(MatrixKeyFrame f) => f != null;

try { collection[index] = frame; }
catch (ArgumentNullException ex) when (ex.ParamName?.StartsWith("MatrixKeyFrameCollection[") == true) { /* supply a valid frame or remove the slot */ }

When it happens

Trigger: Executing collection[index] = null on a MatrixKeyFrameCollection.

Common situations: Building key-frame animations from data where individual frames may be null; reflection or designer code assigning frames by index.

Related errors

• DoubleKeyFrameCollection
• Int16KeyFrameCollection
• ObjectKeyFrameCollection
• Point3DKeyFrameCollection
• [[instance.TypeName]]KeyFrameCollection

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/f5e30572a8510acb. Report an issue: GitHub.

Appendix: source

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

            }
        }

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

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