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

Assign a valid CharKeyFrame (e.g. new DiscreteCharKeyFrame('a', KeyTime.FromTimeSpan(TimeSpan.FromSeconds(1)))) instead of null

dotnet/wpf · error · ArgumentNullException

CharKeyFrameCollection

Error message

CharKeyFrameCollection[{0}]

What it means

The indexer setter of CharKeyFrameCollection throws ArgumentNullException (message "CharKeyFrameCollection[{0}]") when null is assigned into the collection at an index. Every slot must hold a concrete CharKeyFrame instance.

Solutions

Assign a valid CharKeyFrame (e.g. new DiscreteCharKeyFrame('a', KeyTime.FromTimeSpan(TimeSpan.FromSeconds(1)))) instead of null
Null-check before inserting and skip or substitute a default key frame
Verify XAML resource keys resolve to CharKeyFrame instances

Example fix

// before
frames[0] = GetCharKeyFrame(); // may return null
// after
var kf = GetCharKeyFrame();
frames[0] = kf ?? new DiscreteCharKeyFrame('\0', KeyTime.FromTimeSpan(TimeSpan.Zero));

Defensive patterns

Strategy: validation

Validate before calling

if (frame == null) throw new ArgumentException("Key frame cannot be null", nameof(frame));
charKeyFrameCollection[index] = frame;

Type guard

static bool IsValidKeyFrame(CharKeyFrame f) => f != null;

Try / catch

try { collection[index] = frame; }
catch (ArgumentNullException ex) { /* message names the index; supply a valid CharKeyFrame */ }

Prevention

Never assign null into key-frame collections; use typed Add methods
Null-check key frames from factories/resources before insertion
Use collection initializers to catch nulls at authoring time

When it happens

Trigger: charKeyFrameCollection[i] = null; or any generated/binding code path inserting a null CharKeyFrame into the collection.

Common situations: Unresolved key-frame resources in XAML, or dynamically computed key frames that can evaluate to null.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/71d3b259f927c61e. Report an issue: GitHub.

Appendix: source

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

            }
        }

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

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