dotnet/wpf · error · ArgumentException

SR.Argument_InvalidArrayType

Error message

SR.Argument_InvalidArrayType

What it means

For non-T[] destination arrays the library checks type compatibility: it throws ArgumentException with SR.Argument_InvalidArrayType when neither the array's element type nor T is assignable to the other. E.g., copying WeakReference elements into an int[] cannot work because no conversion exists between the element types.

Solutions

  1. Pass an array whose element type matches T or is assignable from/to T (typically T[] or object[])
  2. Fix the destination array declaration to use the correct element type
  3. Guard with targetType.IsAssignableFrom(sourceType) before calling

Example fix

// before
string[] target = new string[collection.Count];
((ICollection)collection).CopyTo(target, 0);
// after
object[] target = new object[collection.Count];
((ICollection)collection).CopyTo(target, 0);
Defensive patterns

Strategy: type-guard

Validate before calling

var targetType = array.GetType().GetElementType();
if (!(targetType.IsAssignableFrom(typeof(T)) || typeof(T).IsAssignableFrom(targetType))) throw new ArgumentException("Array element type incompatible with collection element type.");

Type guard

bool IsCompatibleArrayType(Array a) { var t = a.GetType().GetElementType(); return t != null && (t.IsAssignableFrom(typeof(T)) || typeof(T).IsAssignableFrom(t)); }

Try / catch

try { ((ICollection)collection).CopyTo(array, index); } catch (ArgumentException e) when (e.Message.Contains("array type")) { object[] boxed = new object[collection.Count]; ((ICollection)collection).CopyTo(boxed, index); }

Prevention

When it happens

Trigger: Calling ICollection.CopyTo with an array whose element type is unrelated to T (e.g., string[] when T is WeakReference).

Common situations: Generic copy helpers that pass object[]-like buffers typed incorrectly; refactoring changed T without updating the destination array type.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/a1494411fd5a4776. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Collections/ObjectModel/WeakReadOnlyCollection.cs:177

            IList<T> dlist = CreateDereferencedList();
            if (array is T[] items)
            {
                dlist.CopyTo(items, index);
            }
            else
            {
                //
                // Catch the obvious case assignment will fail.
                // We can found 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)))
                {
                    //ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArrayType);
                    throw new ArgumentException(SR.Argument_InvalidArrayType);
                }

                //
                // We can't cast array of value type to object[], so we don't support
                // widening of primitive types here.
                //
                if (array is not object[] objects)
                {
                    //ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArrayType);
                    throw new ArgumentException(SR.Argument_InvalidArrayType);
                }

                int count = dlist.Count;
                try
                {
                    for (int i = 0; i < count; i++)
                    {
                        objects[index++] = dlist[i];

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