dotnet/wpf · error · ArgumentNullException

PointKeyFrameCollection

Error message

PointKeyFrameCollection[{0}]

What it means

PointKeyFrameCollection's this[index] setter throws ArgumentNullException with the message "PointKeyFrameCollection[{0}]" (index interpolated) when a null PointKeyFrame is stored into the collection. Unlike most ArgumentNullExceptions the message is the formatted parameter name, so the exception's message looks like a collection indexer label. The collection enforces that every slot holds a real key frame.

Solutions

1. Assign a valid PointKeyFrame instance instead of null.
2. Use RemoveAt(index) to remove an entry rather than setting the slot to null.
3. Null-check the key frame variable before indexing into the collection.
4. Catch ArgumentNullException around collection writes if items come from external data.

Example fix

// before
keyFrames[0] = null;
// after
keyFrames.RemoveAt(0);
// or
keyFrames[0] = new LinearPointKeyFrame(new Point(10,10), KeyTime.FromTimeSpan(TimeSpan.FromSeconds(1)));

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

static bool IsStorableKeyFrame(PointKeyFrame f) => f != null;

try { collection[i] = frame; }
catch (ArgumentNullException) { collection.RemoveAt(i); }

When it happens

Trigger: Assigning null via the indexer: myKeyFrameCollection[i] = null; or via databinding/tools that write null into a collection item slot.

Common situations: Programmatically building key frame animations where a key frame variable was never created; templated/generic code that clears items by assigning null instead of calling RemoveAt; deserialization producing null entries.

Related errors

• Int32KeyFrameCollection
• Int64KeyFrameCollection
• RectKeyFrameCollection
• ColorKeyFrameCollection
• DecimalKeyFrameCollection

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/3d6e1c4096553027. Report an issue: GitHub.

Appendix: source

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

            }
        }

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

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