dotnet/wpf · error · ArgumentException

SR.Format(SR.Collection_CopyTo_NumberOfElementsExceedsArrayL…

Error message

SR.Format(SR.Collection_CopyTo_NumberOfElementsExceedsArrayLength, index, "array")

What it means

CopyItems (invoked from FamilyTypefaceCollection.CopyTo) throws this ArgumentException when the destination array starting at 'index' is too small to hold all _count FamilyTypeface elements in the collection. It mirrors standard ICollection.CopyTo contract enforcement, throwing before Array.Copy runs so the destination array is not partially modified.

Solutions

Size the destination array to collection.Count (e.g. new FamilyTypeface[ collection.Count ]) and pass index 0.
When using an offset, ensure array.Length >= collection.Count + index before calling CopyTo.
Or use LINQ ToArray()/ToList() instead of manual CopyTo.

Example fix

// before
var arr = new FamilyTypeface[3];
collection.CopyTo(arr, 0);
// after
var arr = new FamilyTypeface[collection.Count];
collection.CopyTo(arr, 0);

Defensive patterns

Strategy: validation

Validate before calling

if (array == null) throw new ArgumentNullException(nameof(array));
if (array.Rank != 1) throw new ArgumentException("array must be 1-D");
if (index >= array.Length) throw new ArgumentOutOfRangeException(nameof(index));
if (collection.Count > array.Length - index)
    throw new ArgumentException("array too small at index");

Try / catch

try { collection.CopyTo(array, index); }
catch (ArgumentException ex) { /* resize buffer and retry */ }

Prevention

Always allocate the destination array with collection.Count elements.
Never reuse buffers sized from a previous snapshot without re-checking Count.
Prefer ToArray()/ToList() over manual CopyTo.

When it happens

Trigger: Calling FamilyTypefaceCollection.CopyTo(array, index) where _count > array.Length - index, i.e. array has fewer than count+index elements (e.g. copying 5 typefaces into a 3-element array, or into a 10-element array at index 8).

Common situations: Sizing the target array from a stale or wrong count, reusing a buffer sized for a different collection, or passing an offset that leaves insufficient room.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/7cffda21d37bd77e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/FamilyTypefaceCollection.cs:379

            return familyTypeface;
        }

        private void CopyItems(Array array, int index)
        {
            ArgumentNullException.ThrowIfNull(array);

            if (array.Rank != 1)
                throw new ArgumentException(SR.Collection_CopyTo_ArrayCannotBeMultidimensional);

            Type elementType = array.GetType().GetElementType();
            if (!elementType.IsAssignableFrom(typeof(FamilyTypeface)))
                throw new ArgumentException(SR.Format(SR.CannotConvertType, typeof(FamilyTypeface[]), elementType));

            if (index >= array.Length)
                throw new ArgumentException(SR.Format(SR.Collection_CopyTo_IndexGreaterThanOrEqualToArrayLength, "index", "array"));

            if (_count > array.Length - index)
                throw new ArgumentException(SR.Format(SR.Collection_CopyTo_NumberOfElementsExceedsArrayLength, index, "array"));

            if (_count != 0)
            {
                InitializeItemsFromInnerList();
                Array.Copy(_items, 0, array, index, _count);
            }
        }

        private class Enumerator : IEnumerator<FamilyTypeface>, SC.IEnumerator
        {
            private FamilyTypefaceCollection _list;
            private int _index;
            private FamilyTypeface _current;

            internal Enumerator(FamilyTypefaceCollection list)
            {
                _list = list;
                _index = -1;

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