dotnet/wpf · error · ArgumentException

SR.CannotConvertType

Error message

SR.CannotConvertType

What it means

The explicit ICollection.CopyTo throws ArgumentException when the destination Array's element type cannot accept a System.Collections.DictionaryEntry — i.e. elementType.IsAssignableFrom(typeof(DictionaryEntry)) is false (after the array passed the multidimensional check).

Solutions

  1. Use DictionaryEntry[] (or object[]) as the destination for the non-generic CopyTo.
  2. Use the strongly typed CopyTo(KeyValuePair<XmlLanguage,string>[], int) overload instead.
  3. Project entries with LINQ (dict.Select(kv => new DictionaryEntry(kv.Key, kv.Value)).ToArray()) into the type you need.

Example fix

// before
var arr = new string[dict.Count];
((ICollection)dict).CopyTo(arr, 0); // string not assignable from DictionaryEntry
// after
var arr = new DictionaryEntry[dict.Count];
((ICollection)dict).CopyTo(arr, 0);
Defensive patterns

Strategy: type-guard

Validate before calling

var elementType = array.GetType().GetElementType();
if (!elementType.IsAssignableFrom(typeof(DictionaryEntry))) array = new DictionaryEntry[((ICollection)dict).Count];
((ICollection)dict).CopyTo(array, index);

Type guard

static bool CanHoldDictionaryEntries(Array a) => a.Rank == 1 && a.GetType().GetElementType().IsAssignableFrom(typeof(DictionaryEntry));

Try / catch

try { ((ICollection)dict).CopyTo(array, index); }
catch (ArgumentException ex) when (ex.Message.Contains("CannotConvertType")) { /* switch to DictionaryEntry[] */ }

Prevention

When it happens

Trigger: Calling (ICollection)dict.CopyTo with an array of an incompatible element type, e.g. new string[dict.Count] or new int[dict.Count], routed through the generic Array branch.

Common situations: Copy-pasting copy code that used object[] or DictionaryEntry[] into code with a different array type; interop APIs that hand back typed buffers (string[]); generics that inferred the wrong element 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/8f0596a539958db5. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/CultureSpecificStringDictionary.cs:170

            SC.DictionaryEntry[] typedArray = array as SC.DictionaryEntry[];
            if (typedArray != null)
            {
                // it's an array of the exact type
                foreach (KeyValuePair<XmlLanguage, string> item in _innerDictionary)
                {
                    typedArray[index++] = new SC.DictionaryEntry(item.Key, item.Value);
                }
            }
            else
            {
                // it's an array of some other type, e.g., object[]; make sure it's one dimensional
                if (array.Rank != 1)
                    throw new ArgumentException(SR.Collection_CopyTo_ArrayCannotBeMultidimensional);

                // make sure the element type is compatible
                Type elementType = array.GetType().GetElementType();
                if (!elementType.IsAssignableFrom(typeof(SC.DictionaryEntry)))
                    throw new ArgumentException(SR.Format(SR.CannotConvertType, typeof(SC.DictionaryEntry), elementType));

                foreach (KeyValuePair<XmlLanguage, string> item in _innerDictionary)
                {
                    array.SetValue(new SC.DictionaryEntry(item.Key, item.Value), index++);
                }
            }
        }

        #endregion

        #region IDictionary members

        /// <summary>
        /// Adds a language and associated string to the collection.
        /// </summary>
        public void Add(XmlLanguage key, string value)
        {
            _innerDictionary.Add(key, ValidateValue(value));

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