What causes "DoubleKeyFrameCollection[{0}]" in HandyOrg/HandyControl?

Do not assign null to collection slots; remove unwanted frames with Remove/RemoveAt instead.

HandyOrg/HandyControl · warning · ArgumentNullException

DoubleKeyFrameCollection

Error message

DoubleKeyFrameCollection[{0}]

What it means

The GeometryKeyFrameCollection indexer setter rejects null values but incorrectly throws ArgumentNullException with a formatted message string ("DoubleKeyFrameCollection[{index}]") instead of (paramName, message). The exception occurs before WritePreamble, so the collection state is unchanged; the odd parameter usage is a copy-paste from WPF's DoubleKeyFrameCollection.

Solutions

Do not assign null to collection slots; remove unwanted frames with Remove/RemoveAt instead.
Replace a frame by constructing a valid GeometryKeyFrame instance rather than nulling the index.
Clear the whole collection with Clear() and re-add frames if many need replacing.

Example fix

// before
keyFrames[0] = null;
// after
keyFrames.RemoveAt(0); // or keyFrames[0] = new GeometryKeyFrame { Value = ... };

Defensive patterns

Strategy: validation

Validate before calling

if (value != null)
    keyFrames[index] = value;
else
    keyFrames.RemoveAt(index);

Type guard

static bool IsValidKeyFrame(HandyControl.Media.Animation.GeometryKeyFrame frame) => frame != null;

Try / catch

try { keyFrames[i] = frame; } catch (ArgumentNullException) { // frame was null; remove the slot instead
    keyFrames.RemoveAt(i); }

Prevention

Never assign null into GeometryKeyFrameCollection; use Remove/RemoveAt to drop frames.
Filter null frames out before populating the collection from dynamic data.
Use Clear() plus re-add for bulk replacement instead of nulling slots.

When it happens

Trigger: Assigning null into the collection: keyFrames[i] = null; on a GeometryKeyFrameCollection used by GeometryAnimationUsingKeyFrames.

Common situations: Dynamically rebuilding key frames in code where a slot is cleared with null; generic collection-filling code that assigns null placeholders.

Related errors

AI-assisted analysis of HandyOrg/HandyControl@2c0875ebd6 (2026-09-14). Data as JSON: /api/errors/d0f833a927203e2d. Report an issue: GitHub.

Appendix: source

Thrown at src/Shared/HandyControl_Shared/Media/Animation/GeometryKeyFrameCollection.cs:288

    object IList.this[int index]
    {
        get => this[index];
        set => this[index] = (GeometryKeyFrame) value;
    }

    public GeometryKeyFrame 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;

                WritePostscript();
            }
        }
    }

    public bool IsReadOnly
    {
        get
        {

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