What causes "SR.OnlySupportedOnDictionaries" in dotnet/wpf?

Check xamlType.IsDictionary before calling AddToDictionary and route collections to AddToCollection instead.

dotnet/wpf · error · NotSupportedException

SR.OnlySupportedOnDictionaries

Error message

SR.OnlySupportedOnDictionaries

What it means

XamlTypeInvoker.AddToDictionary throws NotSupportedException('OnlySupportedOnDictionaries') when the instance's XamlType is not marked IsDictionary. The invoker can only add keyed items to dictionary-shaped types; anything else is rejected before looking up an Add(key, item) method. Unlike the XamlSchemaException cases, this is a plain NotSupportedException indicating the caller used the wrong API for the type.

Solutions

Check xamlType.IsDictionary before calling AddToDictionary and route collections to AddToCollection instead.
Implement IDictionary (or use Dictionary<TKey,TValue>) on the underlying type so IsDictionary is true.
Catch NotSupportedException and branch to the correct invoker method based on the XamlType shape.

Example fix

// before
invoker.AddToDictionary(instance, key, item);
// after
if (xamlType.IsDictionary) { invoker.AddToDictionary(instance, key, item); } else { invoker.AddToCollection(instance, item); }

Defensive patterns

Strategy: validation

Validate before calling

if (!xamlType.IsDictionary) throw new InvalidOperationException("AddToDictionary requires a dictionary XamlType");

Type guard

static bool CanAddToDictionary(object instance, XamlType t) => instance is IDictionary || t.IsDictionary;

Try / catch

try { invoker.AddToDictionary(instance, key, item); } catch (NotSupportedException) { /* not a dictionary type */ }

Prevention

Route keyed items only when xamlType.IsDictionary is true; use AddToCollection otherwise.
Have dictionary-like custom types implement IDictionary.
Check the XamlType shape before choosing an invoker method.
Write schema tests covering every keyed XAML property type.

When it happens

Trigger: Calling XamlTypeInvoker.AddToDictionary(instance, key, item) where instance is non-null, not an IDictionary, the invoker is known (not the Unknown invoker), and _xamlType.IsDictionary is false — i.e. invoking dictionary semantics on a plain collection or scalar-backed XamlType.

Common situations: Authoring a custom XAML writer/parser that routes x:Key items through AddToDictionary without first checking XamlType.IsDictionary; misclassifying a key-value custom type as a dictionary in schema code; copy-pasting AddToDictionary where AddToCollection was intended.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/eb10953ff6f558fb. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Schema/XamlTypeInvoker.cs:103

                throw new XamlSchemaException(SR.Format(SR.NoAddMethodFound, _xamlType, itemType));
            }

            addMethod.Invoke(instance, new object[] { item });
        }

        public virtual void AddToDictionary(object instance, object key, object item)
        {
            ArgumentNullException.ThrowIfNull(instance);
            if (instance is IDictionary dictionary)
            {
                dictionary.Add(key, item);
                return;
            }

            ThrowIfUnknown();
            if (!_xamlType.IsDictionary)
            {
                throw new NotSupportedException(SR.OnlySupportedOnDictionaries);
            }

            XamlType itemType;
            if (item is not null)
            {
                itemType = _xamlType.SchemaContext.GetXamlType(item.GetType());
            }
            else
            {
                itemType = _xamlType.ItemType;
            }

            MethodInfo addMethod = GetAddMethod(itemType);
            if (addMethod is null)
            {
                throw new XamlSchemaException(SR.Format(SR.NoAddMethodFound, _xamlType, itemType));
            }

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