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

Ensure the DoubleKeyFrame instance is non-null before assigning into the collection

dotnet/wpf · error · ArgumentNullException

DoubleKeyFrameCollection

Error message

DoubleKeyFrameCollection[{0}]

What it means

The DoubleKeyFrameCollection indexer setter throws ArgumentNullException whose message is "DoubleKeyFrameCollection[{index}]" when a null DoubleKeyFrame is assigned at the given index. Null key frames are not permitted in the collection.

Solutions

Ensure the DoubleKeyFrame instance is non-null before assigning into the collection
Filter nulls from source lists before building the collection
If a placeholder is needed, use a key frame with a valid value instead of null

Example fix

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

Defensive patterns

Strategy: type-guard

Validate before calling

if (frame == null) throw new ArgumentNullException(nameof(frame));
collection[index] = frame;

Type guard

static bool IsValidFrame(DoubleKeyFrame f) => f != null;

Try / catch

try { collection[index] = frame; } catch (ArgumentNullException ex) when (ex.Message.StartsWith("DoubleKeyFrameCollection[")) { /* supply a non-null frame */ }

Prevention

Guard factory methods that may return null frames before indexing into the collection
Filter nulls out of source data when programmatically building key-frame collections
Use non-null placeholder key frames instead of null slots

When it happens

Trigger: Executing collection[index] = null on a DoubleKeyFrameCollection, or XAML/serialization paths inserting a null key frame at an index.

Common situations: Programmatic population of key frames where a null slipped through (e.g. a factory method returned null); data-driven construction of animations from a list containing nulls.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/d3aaaef8c14480a0. Report an issue: GitHub.

Appendix: source

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

            }
        }

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

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