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

CopyTo catches the InvalidCastException raised while copying elements into the destination array and rethrows ArgumentException with SR.Collection_BadDestArray, wrapping the original exception. This means the destination array's element type cannot hold a TextDecoration.

Solutions

Allocate the destination array as TextDecoration[] or object[]
Verify array.GetType().GetElementType().IsAssignableFrom(typeof(TextDecoration)) before CopyTo
Use Cast<TextDecoration>().ToArray() (LINQ) instead of CopyTo for flexible conversion

Example fix

// before
var arr = new string[collection.Count];
((ICollection)collection).CopyTo(arr, 0);
// after
var arr = new TextDecoration[collection.Count];
collection.CopyTo(arr, 0);

Defensive patterns

Strategy: type-guard

Validate before calling

if (array == null) throw new ArgumentNullException(nameof(array));
var elemType = array.GetType().GetElementType();
if (elemType == null || !elemType.IsAssignableFrom(typeof(TextDecoration))) throw new ArgumentException($"Array element type {elemType} cannot hold TextDecoration");

Type guard

bool IsCompatibleDestArray(Array a) => a is TextDecoration[] || a is object[] || (a != null && a.GetType().GetElementType()?.IsAssignableFrom(typeof(TextDecoration)) == true);

Prevention

Prefer strongly typed TextDecoration[] destinations
Avoid ICollection.CopyTo with arbitrary array types
Use LINQ ToArray/Cast for conversion instead of CopyTo

When it happens

Trigger: Calling ICollection.CopyTo with an array typed to something incompatible, e.g. string[] or non-covariant element types, so Array.SetValue throws InvalidCastException.

Common situations: Copying into object[] is fine, but copying into string[]/int[] fails; reflection-based or serialization code that allocates arrays from the wrong element type; IEnumerable.CopyTo generic helpers.

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

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/c7694728c4504f4d. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Generated/TextDecorationCollection.cs:408

            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();

View on GitHub (pinned to 81131a70a4)