AvaloniaUI/Avalonia · error · ArgumentException

Invalid array type.

Error message

Invalid array type.

What it means

Centralized ThrowHelper.ThrowArgumentException_Argument_InvalidArrayType throws ArgumentException "Invalid array type." It is reached from the non-generic ICollection.CopyTo on both PooledList<T> and PooledStack<T> inside a `catch (ArrayTypeMismatchException)` block: Array.Copy throws ArrayTypeMismatchException when the destination array's element type is not compatible with T, and this helper rewraps it as a clearer ArgumentException.

Source

Thrown at src/Avalonia.Base/Collections/Pooled/ThrowHelper.cs:282

            throw new NotSupportedException();
        }

        [DoesNotReturn]
        internal static void ThrowAggregateException(List<Exception> exceptions)
        {
            throw new AggregateException(exceptions);
        }

        [DoesNotReturn]
        internal static void ThrowOutOfMemoryException()
        {
            throw new OutOfMemoryException();
        }

        [DoesNotReturn]
        internal static void ThrowArgumentException_Argument_InvalidArrayType()
        {
            throw new ArgumentException("Invalid array type.");
        }

        [DoesNotReturn]
        internal static void ThrowInvalidOperationException_InvalidOperation_EnumNotStarted()
        {
            throw new InvalidOperationException("Enumeration has not started.");
        }

        [DoesNotReturn]
        internal static void ThrowInvalidOperationException_InvalidOperation_EnumEnded()
        {
            throw new InvalidOperationException("Enumeration has ended.");
        }

        [DoesNotReturn]
        internal static void ThrowInvalidOperationException_EnumCurrent(int index)
        {
            throw GetInvalidOperationException_EnumCurrent(index);

View on GitHub (pinned to 11c5427268)

Solutions

1. Use a correctly typed array: `var arr = new T[list.Count]; ((ICollection)list).CopyTo(arr, 0);`.
2. Prefer the generic CopyTo(T[], int) overload, which is type-checked at compile time.
3. When using the non-generic interface, validate `array.GetType().GetElementType()` is compatible with T.

Example fix

// before
object[] arr = new object[list.Count];
((ICollection)list).CopyTo(arr, 0); // type mismatch

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

var arr = new T[list.Count];
list.CopyTo(arr, 0); // compile-time type-safe, avoids the non-generic path

static bool IsCompatibleArray<T>(Array a)
    => a is not null && a.Rank == 1
       && typeof(T).IsAssignableFrom(a.GetType().GetElementType());

When it happens

Trigger: Calling `((ICollection)list).CopyTo(wrongTypeArray, index)` where the array element type is not assignment-compatible with T — e.g. copying a PooledList<string> into an int[], or a PooledList<Derived> into a Base[] that the runtime rejects (covariance edge cases).

Common situations: Non-generic code passing an object[]/wrongly-typed array through ICollection.CopyTo; reflection or dynamic typing producing a mismatched array type; covariance surprises with reference arrays.

Related errors

• Destination span is shorter than the list to be copied.
• array
• Stack was empty.
• Destination too short.
• Overlap alignment mismatch.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/25a194f79eff8c51. Report an issue: GitHub.