dotnet/wpf · error · ArgumentException

SR.Format(SR.Collection_BadDestArray, this.GetType().Name)

Error message

SR.Format(SR.Collection_BadDestArray, this.GetType().Name)

What it means

VectorCollection.CopyTo writes each element into the destination array with Array.SetValue, which throws InvalidCastException when an element cannot be cast to the array's element type (e.g., copying into a Point[] or string[] instead of Vector[]). CopyTo catches that and rethrows ArgumentException(SR.Format(SR.Collection_BadDestArray, this.GetType().Name)) with the original exception as InnerException, matching BCL collection conventions.

Solutions

1. Copy into an array whose element type is exactly Vector (or a compatible base type such as object)
2. Check the destination array's element type before calling CopyTo (typeof(Vector).IsAssignableFrom(array.GetType().GetElementType()))
3. Read the InnerException of the ArgumentException to confirm the original InvalidCastException during diagnostics

Example fix

// before
var dest = new Point[collection.Count];
collection.CopyTo(dest, 0);
// after
var dest = new Vector[collection.Count];
collection.CopyTo(dest, 0);

var elemType = dest?.GetType().GetElementType();
if (elemType == null || !typeof(Vector).IsAssignableFrom(elemType))
    throw new ArgumentException($"Destination array element type must be Vector (got {elemType?.Name}).");

static bool IsVectorArray(Array a) => a != null && a.Rank == 1 && typeof(Vector).IsAssignableFrom(a.GetType().GetElementType());

try { collection.CopyTo(dest, index); }
catch (ArgumentException ex) when (ex.InnerException is InvalidCastException)
{ /* dest element type incompatible; allocate Vector[] and retry */ }

When it happens

Trigger: Calling ((ICollection)vectorCollection).CopyTo(array, index) where array's element type is not assignable from Vector (e.g., a Point[], object[] is fine, double[] is not), so array.SetValue fails with InvalidCastException inside the try block.

Common situations: Assuming VectorCollection copies to Point arrays like some WPF collections do; generic array-conversion helpers with wrong T; refactors where the element type changed between collection and destination buffer.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

• Collection_CopyTo_ArrayCannotBeMultidimensional
• Collection_CopyTo_ArrayCannotBeMultidimensional
• Collection_CopyTo_IndexGreaterThanOrEqualToArrayLength
• Collection_CopyTo_IndexGreaterThanOrEqualToArrayLength
• Collection_CopyTo_NumberOfElementsExceedsArrayLength

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/88a8171f9ee4f879. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Generated/VectorCollection.cs:382

            if (array.Rank != 1)
            {
                throw new ArgumentException(SR.Collection_BadRank);
            }

            // Elsewhere in the collection we throw an AE when the type is
            // bad so we do it here as well to be consistent
            try
            {
                int count = _collection.Count;
                for (int i = 0; i < count; i++)
                {
                    array.SetValue(_collection[i], index + i);
                }
            }
            catch (InvalidCastException e)
            {
                throw new ArgumentException(SR.Format(SR.Collection_BadDestArray, this.GetType().Name), e);
            }
        }

        bool ICollection.IsSynchronized
        {
            get
            {
                ReadPreamble();

                return IsFrozen || Dispatcher != null;
            }
        }

        object ICollection.SyncRoot
        {
            get
            {
                ReadPreamble();

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