dotnet/wpf · error · ArgumentException

SR.Format(SR.CannotConvertType, typeof(FamilyTypeface)…

Error message

SR.Format(SR.CannotConvertType, typeof(FamilyTypeface), elementType)

What it means

The explicit ICollection.CopyTo(Array, int) of FamilyMapCollection requires the destination element type to accept FontFamilyMap elements; if elementType.IsAssignableFrom(typeof(FamilyTypeface)) fails (note: the collection's element type check used in this code path), it throws ArgumentException with CannotConvertType naming the source and target types.

Solutions

  1. Use a FontFamilyMap[] (or a base type/interface array the element is assignable to) as the destination.
  2. Check array.GetType().GetElementType().IsAssignableFrom(typeof(FontFamilyMap)) before copying.
  3. Use ToArray()/Cast<T>() instead of the non-generic ICollection path when types are uncertain.

Example fix

// before
Array dest = Array.CreateInstance(typeof(int), familyMaps.Count);
((ICollection)familyMaps).CopyTo(dest, 0);
// after
var dest = new FontFamilyMap[familyMaps.Count];
((ICollection)familyMaps).CopyTo(dest, 0);
Defensive patterns

Strategy: type-guard

Validate before calling

bool ok = array.GetType().GetElementType().IsAssignableFrom(typeof(FontFamilyMap));

Type guard

static bool CanHoldFamilyMaps(Array a) => a.GetType().GetElementType()?.IsAssignableFrom(typeof(FontFamilyMap)) == true;

Try / catch

try { ((ICollection)col).CopyTo(array, index); }
catch (ArgumentException) { /* use correctly typed array */ }

Prevention

When it happens

Trigger: Calling ((ICollection)familyMapCollection).CopyTo with a non-assignable array type such as object[]-typed Array instances created via Array.CreateInstance, or arrays of an unrelated type like FontFamily[] or int[].

Common situations: Reflection-based serialization writing into Array.CreateInstance(typeof(...), n) buffers, or mismatched generic/legacy collection bridges.

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/c219071ae426f5bd. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/FamilyMapCollection.cs:95

                throw new ArgumentException(SR.Format(SR.Collection_CopyTo_IndexGreaterThanOrEqualToArrayLength, "index", "array"));

            if (_count > array.Length - index)
                throw new ArgumentException(SR.Format(SR.Collection_CopyTo_NumberOfElementsExceedsArrayLength, index, "array"));

            if (_count != 0)
                Array.Copy(_items, 0, array, index, _count);
        }

        void SC.ICollection.CopyTo(Array array, int index)
        {
            ArgumentNullException.ThrowIfNull(array);

            if (array.Rank != 1)
                throw new ArgumentException(SR.Collection_CopyTo_ArrayCannotBeMultidimensional);

            Type elementType = array.GetType().GetElementType();
            if (!elementType.IsAssignableFrom(typeof(FamilyTypeface)))
                throw new ArgumentException(SR.Format(SR.CannotConvertType, typeof(FamilyTypeface), elementType));

            if (index >= array.Length)
                throw new ArgumentException(SR.Format(SR.Collection_CopyTo_IndexGreaterThanOrEqualToArrayLength, "index", "array"));

            if (_count > array.Length - index)
                throw new ArgumentException(SR.Format(SR.Collection_CopyTo_NumberOfElementsExceedsArrayLength, index, "array"));

            if (_count != 0)
                Array.Copy(_items, 0, array, index, _count);
        }

        bool SC.ICollection.IsSynchronized
        {
            get { return false; }
        }

        object SC.ICollection.SyncRoot
        {

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