dotnet/wpf · error · XamlParseException

SR.Format(SR.TypeNotFound, xamlType.GetQualifiedName())

Error message

SR.Format(SR.TypeNotFound, xamlType.GetQualifiedName())

What it means

ObjectWriterContext.ServiceProvider_Resolve resolves a qualified XAML type name to a CLR Type. When neither the direct namespace lookup (ServiceProvider_ResolveXamlType) nor the fallback XamlTypeName.Parse/GetXamlType path yields a type with an underlying CLR type, it throws XamlParseException with SR.TypeNotFound naming the qualified name.

Solutions

1. Check the XAML prefix declaration (xmlns:local="clr-namespace:Ns;assembly=Asm") for typos in namespace and assembly name.
2. Add a project reference to the assembly containing the type so it is loaded into the AppDomain.
3. Verify the type is public and its name (including nested-type '+' syntax) is spelled correctly.
4. Confirm the referenced assembly version actually contains the type (rebuild or update package).

Example fix

<!-- before: assembly name wrong or missing -->
xmlns:local="clr-namespace:MyApp.Controls;assembly=WrongAsm"
<!-- after -->
xmlns:local="clr-namespace:MyApp.Controls;assembly=MyApp"

// Before loading XAML, verify the referenced type resolves
var asm = Assembly.LoadFrom("MyApp.dll");
if (asm.GetType("MyApp.Controls.MyControl") is null)
    throw new InvalidOperationException("Referenced XAML type missing from assembly");

static bool TypeResolves(SchemaContext ctx, string prefix, string localName)
    => ctx.GetXamlType(new XamlTypeName(prefix, localName))?.UnderlyingType is not null;

try { Resolve(typeName); }
catch (XamlParseException ex) when (ex.Message.Contains("was not found"))
{ /* log qualified name; hint at missing xmlns/assembly reference */ }

When it happens

Trigger: XAML markup (or IXamlSchemaContextProvider-based resolution) references a type whose xmlns-prefix mapping does not resolve to a loaded assembly containing that type — e.g. during x:Type resolution or type-converted property values.

Common situations: Missing assembly reference in the project; wrong clr-namespace/assembly spelling in the XAML; type exists in a different assembly version than referenced; type is internal/non-public so it is not visible from the local assembly.

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))
• SR.Format(SR.MarkupExtensionTypeNameBad, _typeName)
• throw new ArgumentNullException( nameof(element));
• throw new ArgumentNullException( nameof(typeName));

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/112e126395608345. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Context/ObjectWriterContext.cs:161

        }

        #region ServiceProvider Interfaces

        // This class doesn't implement the IServiceProvider.  That is done
        // with worker classes ValueConverterContext or MarkupConverterContext.
        // The worker class implements IServiceProvider but uses the real
        // context for the implementation of the actual services.

        internal Type ServiceProvider_Resolve(string qName)
        {
            // As soon as we have the necessary setting on ObjectWriter, we need to start passing
            // the local assembly into the context; currently, this will only return publics.
            XamlType xamlType = ServiceProvider_ResolveXamlType(qName);
            if (xamlType is null || xamlType.UnderlyingType is null)
            {
                XamlTypeName name = XamlTypeName.Parse(qName, _serviceProviderContext);
                xamlType = GetXamlType(name, true, true);
                throw new XamlParseException(SR.Format(SR.TypeNotFound, xamlType.GetQualifiedName()));
            }

            return xamlType.UnderlyingType;
        }

        internal XamlType ServiceProvider_ResolveXamlType(string qName)
        {
            return ResolveXamlType(qName, true);
        }

        internal AmbientPropertyValue ServiceProvider_GetFirstAmbientValue(IEnumerable<XamlType> ceilingTypes, XamlMember[] properties)
        {
            List<AmbientPropertyValue> valueList = FindAmbientValues(ceilingTypes, searchLiveStackOnly: false, types: null, properties, true);
            return (valueList.Count == 0) ? null : valueList[0];
        }

        internal object ServiceProvider_GetFirstAmbientValue(XamlType[] types)
        {

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