AvaloniaUI/Avalonia · error · ArgumentException

Multi-dimensional arrays are not supported.

Error message

Multi-dimensional arrays are not supported.

What it means

ArgumentException thrown by AvaloniaList.ICollection.CopyTo when the destination array has Rank != 1 (i.e. it is multi-dimensional, like T[,]). The non-generic CopyTo only supports single-dimensional arrays.

Source

Thrown at src/Avalonia.Base/Collections/AvaloniaList.cs:666

        }

        /// <inheritdoc/>
        void IList.RemoveAt(int index)
        {
            RemoveAt(index);
        }

        /// <inheritdoc/>
        void ICollection.CopyTo(Array array, int index)
        {
            if (array == null)
            {
                throw new ArgumentNullException(nameof(array));
            }

            if (array.Rank != 1)
            {
                throw new ArgumentException("Multi-dimensional arrays are not supported.");
            }

            if (array.GetLowerBound(0) != 0)
            {
                throw new ArgumentException("Non-zero lower bounds are not supported.");
            }

            if (index < 0)
            {
                throw new ArgumentException("Invalid index.");
            }

            if (array.Length - index < Count)
            {
                throw new ArgumentException("The target array is too small.");
            }

            if (array is T[] tArray)

View on GitHub (pinned to 11c5427268)

Solutions

1. Allocate a single-dimensional array (new T[list.Count]) for the destination.
2. If you need a 2D layout, copy into a 1D array then index manually into the 2D structure.
3. Validate array.Rank == 1 before invoking CopyTo.

Example fix

// before
var dest = new T[10, 10];
((ICollection)list).CopyTo(dest, 0);

// after
var dest = new T[list.Count];
((ICollection)list).CopyTo(dest, 0);

if (array.Rank != 1) throw new ArgumentException("Use a 1D array.", nameof(array));

static bool IsSingleDimensional(Array a) => a.Rank == 1;

When it happens

Trigger: Passing a multi-dimensional array such as 'new T[rows, cols]' to ((ICollection)list).CopyTo(array, index).

Common situations: Interop code or serializers that allocate a 2D array and attempt to flatten a list into it; matrix/grid helpers misusing CopyTo.

Related errors

• Non-zero lower bounds are not supported.
• array
• Invalid index.
• The target array is too small.
• Invalid array type

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/9c6558093c412af3. Report an issue: GitHub.