dotnet/wpf · error · ArgumentException

SR.Format(SR.Collection_CopyTo_NumberOfElementsExceedsArrayL…

Error message

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

What it means

FamilyMapCollection.CopyTo throws ArgumentException when the collection has more elements than the destination array can hold from the given index onward (_count > array.Length - index). This prevents partial/overflow copies per the ICollection.CopyTo contract.

Solutions

Allocate the destination array with length >= index + collection.Count (or exactly collection.Count with index 0).
Re-query collection.Count immediately before CopyTo instead of caching sizes.
Use ToArray() when a correctly sized buffer is all that is needed.

Example fix

// before
var buffer = new FontFamilyMap[4];
familyMaps.CopyTo(buffer, 0);
// after
var buffer = new FontFamilyMap[familyMaps.Count];
familyMaps.CopyTo(buffer, 0);

Defensive patterns

Strategy: validation

Validate before calling

if (array.Length - index < col.Count) array = new FontFamilyMap[col.Count];

Try / catch

try { col.CopyTo(array, index); }
catch (ArgumentException) { array = new FontFamilyMap[col.Count]; col.CopyTo(array, 0); }

Prevention

Re-query Count right before copying
Prefer ToArray() over manual buffers

When it happens

Trigger: Calling CopyTo with an array shorter than the collection (offset by index), e.g. an array allocated for an older, smaller collection or a hardcoded size.

Common situations: Caching a FontFamilyMap[] sized once and reusing it after the composite font gains more family maps, or allocating exactly Count but passing a nonzero index.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/5863ed8dc01da22c. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/FamilyMapCollection.cs:80

        /// Determines whether the FontFamily contains the specified FontFamilyMap.
        /// </summary>
        public bool Contains(FontFamilyMap item)
        {
            return FindItem(item) >= 0;
        }

        /// <summary>
        /// Copies the contents of the list to the specified array.
        /// </summary>
        public void CopyTo(FontFamilyMap[] array, int index)
        {
            ArgumentNullException.ThrowIfNull(array);

            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)
                Array.Copy(_items, 0, array, index, _count);
        }

        void SC.ICollection.CopyTo(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"));

View on GitHub (pinned to 81131a70a4)