dotnet/wpf · error · InvalidOperationException

SR.Format(SR.MarkupExtensionTypeNameBad, _typeName)

Error message

SR.Format(SR.MarkupExtensionTypeNameBad, _typeName)

What it means

TypeExtension.ProvideValue throws InvalidOperationException when the IXamlTypeResolver.Resolve(_typeName) call returns null, i.e. the type-name string could not be resolved to any known type. The extension surfaces the offending _typeName in the message via SR.MarkupExtensionTypeNameBad.

Solutions

1. Fix the TypeName string to match a resolvable type name, including the correct XML namespace prefix.
2. Add or correct the xmlns/clr-namespace mapping at the top of the XAML file.
3. Verify the type's assembly is referenced and the type is public.
4. Check for renames after refactoring and update the XAML reference.

Example fix

// before
<Window xmlns:local="clr-namespace:App.Views">
  <Button Tag="{x:Type local:MyControll}" /> <!-- typo -->
// after
<Window xmlns:local="clr-namespace:App.Views">
  <Button Tag="{x:Type local:MyControl}" />

// before writing {x:Type prefix:Name} in XAML, verify:
// 1) xmlns:prefix mapping exists and points to the right clr-namespace/assembly
// 2) the type name is spelled correctly and is public

try { value = ext.ProvideValue(provider); }
catch (InvalidOperationException ex) { throw new XamlLoadException($"Cannot resolve type '{ext.TypeName}'. Check xmlns mappings and assembly references.", ex); }

When it happens

Trigger: Setting TypeName to a string that the XAML type resolver cannot resolve — misspelled type name, missing xmlns prefix mapping, type not in referenced assemblies — then calling ProvideValue during a XAML parse.

Common situations: Typos in x:Type markup usage like {x:Type local:MyControll}; missing clr-namespace/xmlns declaration in the XAML file; renamed or moved type after a refactor; assembly not referenced by the project.

Understand the failure class

Background: 'Could not be found', 'does not exist', 'not found in database': the resource-not-found family when an ID, slug, key, or URI lookup comes back empty — this error's family across 20 libraries.

Related errors

• ArgumentNullException(nameof(localName))
• ArgumentNullException(nameof(xmlNamespace))
• IAmbientProvider
• IXamlSchemaContextProvider
• NotSupportedException

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/04ad44a6983e57d9. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Windows/Markup/TypeExtension.cs:78

            // Validate the initialization.
            if (_typeName is null)
            {
                throw new InvalidOperationException(SR.MarkupExtensionTypeName);
            }

            // Get the IXamlTypeResolver from the service provider
            ArgumentNullException.ThrowIfNull(serviceProvider);

            if (serviceProvider.GetService(typeof(IXamlTypeResolver)) is not IXamlTypeResolver xamlTypeResolver)
            {
                throw new InvalidOperationException(SR.Format(SR.MarkupExtensionNoContext, GetType().Name, nameof(IXamlTypeResolver)));
            }

            // Get the type
            _type = xamlTypeResolver.Resolve(_typeName);
            if (_type is null)
            {
                throw new InvalidOperationException(SR.Format(SR.MarkupExtensionTypeNameBad, _typeName));
            }

            return _type;
        }

        /// <summary>
        /// The typename represented by this markup extension. The name has the format
        /// Prefix:Typename, where Prefix is the xml namespace prefix for this type.
        /// </summary>
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
        public string TypeName
        {
            get => _typeName;
            set
            {
                // Reset the type so ProvideValue does not use the existing type.
                _typeName = value ?? throw new ArgumentNullException(nameof(value));
                _type = null;

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