dotnet/wpf · error · ArgumentNullException

BooleanKeyFrameCollection

Error message

BooleanKeyFrameCollection[{index}]

What it means

The indexer setter of BooleanKeyFrameCollection throws ArgumentNullException when a caller assigns null into the collection at a given index. The exception's message is built as "BooleanKeyFrameCollection[{index}]" and passed as the paramName-style argument, identifying the position that received null. Key frame collections require a concrete BooleanKeyFrame instance at every slot.

Solutions

1. Assign a valid non-null BooleanKeyFrame instance (e.g. new DiscreteBooleanKeyFrame(true, KeyTime.FromTimeSpan(TimeSpan.FromSeconds(1)))) instead of null
2. Check the value for null before inserting and skip or substitute a default key frame
3. If coming from XAML binding, verify the bound resource/key actually resolves to a BooleanKeyFrame

Example fix

// before
frames[0] = GetKeyFrame(); // may return null
// after
var kf = GetKeyFrame();
frames[0] = kf ?? new DiscreteBooleanKeyFrame(false, KeyTime.FromTimeSpan(TimeSpan.Zero));

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

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

try { collection[index] = frame; }
catch (ArgumentNullException ex) { /* ex.ParamName contains "BooleanKeyFrameCollection[i]"; supply a valid key frame or skip */ }

When it happens

Trigger: Assigning null via the indexer: booleanKeyFrameCollection[i] = null; or any data-bound/generated code path that writes a null key frame into the collection.

Common situations: XAML or code that builds a key-frame animation where a resource fails to resolve to a BooleanKeyFrame, or computed values that yield null before being inserted.

Related errors

• ByteKeyFrameCollection
• CharKeyFrameCollection
• anchorLocator.Parts
• Animation_AnimationTimelineTypeMismatch
• Animation_CalculatedValueIsInvalidForProperty

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/51c64fac075ae38d. Report an issue: GitHub.

Appendix: source

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

            }
        }

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

                return _keyFrames[index];
            }
            set
            {
                if (value == null)
                {
                    throw new ArgumentNullException(String.Format(CultureInfo.InvariantCulture, "BooleanKeyFrameCollection[{0}]", index));
                }

                WritePreamble();

                if (value != _keyFrames[index])
                {
                    OnFreezablePropertyChanged(_keyFrames[index], value);
                    _keyFrames[index] = value;

                    Debug.Assert(_keyFrames[index] != null);

                    WritePostscript();
                }
            }
        }

        #endregion
    }

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