dotnet/wpf · error · ArgumentException

SR.Collection_CopyTo_IndexGreaterThanOrEqualToArrayLength

Error message

SR.Collection_CopyTo_IndexGreaterThanOrEqualToArrayLength

What it means

PartialArray<T>.CopyTo throws ArgumentException wrapping Collection_CopyTo_IndexGreaterThanOrEqualToArrayLength when arrayIndex >= destination array.Length. The starting offset must point inside the destination array.

Solutions

  1. Validate arrayIndex < array.Length before calling CopyTo.
  2. Recompute the offset from the current destination array each call.
  3. Ensure the destination array is allocated with length >= Count + arrayIndex.

Example fix

// before
partialArray.CopyTo(buffer, buffer.Length);
// after
if (buffer.Length > 0 && partialArray.Count <= buffer.Length)
    partialArray.CopyTo(buffer, 0);
Defensive patterns

Strategy: validation

Validate before calling

if (arrayIndex < 0 || arrayIndex >= array.Length) throw new ArgumentOutOfRangeException(nameof(arrayIndex));

Type guard

bool IsValidCopyOffset(Array dest, int arrayIndex) => arrayIndex >= 0 && arrayIndex < dest.Length;

Try / catch

try { coll.CopyTo(dest, arrayIndex); }
catch (ArgumentException e) when (e.ParamName == "arrayIndex") { /* fix offset and retry */ }

Prevention

When it happens

Trigger: Calling CopyTo(array, arrayIndex) where arrayIndex equals or exceeds array.Length, e.g. CopyTo(arr, arr.Length) or a stale offset after the destination was reallocated smaller.

Common situations: Cursor-based copy loops that increment arrayIndex without rechecking the destination length, or reuse of an offset computed for a different, larger array.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b4fdfad83935f376. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/PartialArray.cs:137

        }

        public void CopyTo(T[] array, int arrayIndex)
        {
            // parameter validations
            ArgumentNullException.ThrowIfNull(array);

            if (array.Rank != 1)
            {
                throw new ArgumentException(
                    SR.Collection_CopyTo_ArrayCannotBeMultidimensional, 
                    nameof(array));                
            }

            ArgumentOutOfRangeException.ThrowIfNegative(arrayIndex);

            if (arrayIndex >= array.Length)
            {
                throw new ArgumentException(
                    SR.Format(
                        SR.Collection_CopyTo_IndexGreaterThanOrEqualToArrayLength, 
                        "arrayIndex", 
                        "array"),
                        nameof(arrayIndex));
            }

            if ((array.Length - Count - arrayIndex) < 0)
            {
                throw new ArgumentException(
                    SR.Format(
                        SR.Collection_CopyTo_NumberOfElementsExceedsArrayLength,
                        "arrayIndex",
                        "array"));
            }           
            

            // do the copying here

View on GitHub (pinned to 81131a70a4)