AvaloniaUI/Avalonia · error · ArgumentNullException

array

Error message

array

What it means

ArgumentNullException thrown by the non-generic ICollection.CopyTo implementation on AvaloniaList when the destination array is null. The method copies the list's elements into the supplied System.Array.

Source

Thrown at src/Avalonia.Base/Collections/AvaloniaList.cs:661

        /// <inheritdoc/>
        void IList.Remove(object? value)
        {
            Remove((T)value!);
        }

        /// <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)

View on GitHub (pinned to 11c5427268)

Solutions

1. Allocate a correctly-sized destination array before calling CopyTo (e.g. new T[list.Count]).
2. Prefer the strongly-typed list.CopyTo(T[] array, int index) overload which has its own checks.
3. Null-check the array and skip/throw a meaningful error upstream.

Example fix

// before
((ICollection)list).CopyTo(null, 0);

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

_ = array ?? throw new ArgumentNullException(nameof(array));

When it happens

Trigger: Calling ((ICollection)list).CopyTo(null, index) or ((Array)null, ...) via the non-generic ICollection interface; frameworks/collection adapters passing a null destination array.

Common situations: Interop with code that allocates the destination array conditionally and forwards null on failure; an IList/ICollection adapter that does not allocate before calling CopyTo.

Related errors

• items
• Multi-dimensional arrays are not supported.
• Non-zero lower bounds are not supported.
• Invalid index.
• The target array is too small.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/d3b3127c20da5e38. Report an issue: GitHub.