What causes "The target array is too small." in AvaloniaUI/Avalonia?

Allocate the destination as new T[list.Count] and use index 0.

AvaloniaUI/Avalonia · error · ArgumentException

The target array is too small.

Error message

The target array is too small.

What it means

ArgumentException thrown by AvaloniaList.ICollection.CopyTo when the destination array does not have enough remaining space from the starting index to hold all Count elements of the list.

Source

Thrown at src/Avalonia.Base/Collections/AvaloniaList.cs:681

            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)
            {
                _inner.CopyTo(tArray, index);
            }
            else
            {
                //
                // Catch the obvious case assignment will fail.
                // We can't find all possible problems by doing the check though.
                // For example, if the element type of the Array is derived from T,
                // we can't figure out if we can successfully copy the element beforehand.
                //
                Type targetType = array.GetType().GetElementType()!;
                Type sourceType = typeof(T);
                if (!(targetType.IsAssignableFrom(sourceType) || sourceType.IsAssignableFrom(targetType)))
                {

View on GitHub (pinned to 11c5427268)

Solutions

Allocate the destination as new T[list.Count] and use index 0.
Ensure array.Length - index >= list.Count before calling CopyTo.
Recompute the array size from the current Count immediately before copying.

Example fix

// before
var arr = new T[5];
((ICollection)list).CopyTo(arr, 3); // list.Count == 4

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

Defensive patterns

Strategy: validation

Validate before calling

if (array.Length - index < list.Count) throw new ArgumentException("Destination too small.");

Prevention

Size the destination as new T[list.Count] and use index 0.
Recompute size from current Count right before copying.
Verify array.Length - index >= Count at the call site.

When it happens

Trigger: Calling CopyTo with an index/array combination where array.Length - index < list.Count; e.g. a 5-element array starting at index 3 for a 4-item list.

Common situations: Under-allocating the destination array (using an old count), reusing a shared buffer that is too small, or an index that is not 0 leaving insufficient tail room.

Related errors

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/b6ee01e584dc06e2. Report an issue: GitHub.