dotnet/wpf · error · XamlXmlWriterException

XamlXmlWriterException(SR.ExpandPositionalParametersinTypeWi…

Error message

XamlXmlWriterException(SR.ExpandPositionalParametersinTypeWithNoDefaultConstructor)

What it means

When XamlXmlWriter expands positional parameters of a markup extension into property-element form, it must instantiate the extension type to look up constructor argument names. If the containing type's ConstructionRequiresArguments is true (no default constructor), the expansion cannot proceed, so it throws XamlXmlWriterException ExpandPositionalParametersinTypeWithNoDefaultConstructor.

Solutions

1. Add a public parameterless constructor to the markup extension type
2. Write the extension using explicit named property syntax instead of positional parameters so expansion is unnecessary
3. Adjust the type's XAML schema/type mapping so ConstructionRequiresArguments is false (e.g. via XamlDeferLoadAttribute or constructor configuration)

Example fix

// before
class MyExt { public MyExt(string a) {...} } // no default ctor, positional args -> throws
// after
class MyExt {
  public MyExt() {...}
  public MyExt(string a) {...}
}

if (extType.ConstructionRequiresArguments) throw new InvalidOperationException($"{extType.UnderlyingType} needs a default constructor for positional-parameter expansion");

bool CanExpandPositionalParams(XamlType t) => t == null || !t.ConstructionRequiresArguments;

try { writer.WriteNode(reader); }
catch (XamlXmlWriterException ex) when (ex.Message.Contains("positional")) { /* fall back to named-property syntax */ }

When it happens

Trigger: Writing a markup-extension object that uses positional constructor arguments (x:Arguments style) where the extension's XamlType has ConstructionRequiresArguments == true, triggering ExpandPositionalParameters state.

Common situations: Custom markup extensions without a parameterless constructor being serialized; types changed to remove their default constructor in a newer library version; serializers encountering {Extension arg=value} syntax requiring expansion.

Related errors

• ArgumentNullException(nameof(arrayType))
• ArgumentNullException(nameof(member))
• brokenRule (SR.UnexpectedToken-based parser rule error)
• IAmbientProvider
• IXamlSchemaContextProvider

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/3f8644e50f2264e4. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/XamlXmlWriter.cs:1073

                }
                else
                {
                    WriteMemberAsElement(writer);
                    writer.WriteDeferredNamespaces(XamlNodeType.StartMember);
                }

                if (property == XamlLanguage.PositionalParameters)
                {
                    // the writer is not in a state where it can write markup extensions in curly form
                    // so it expands the positional parameters as properties.

                    writer.namespaceScopes.Pop();

                    // but in order to expand parameters, the markup extension needs to have a default constructor

                    if (containingType is not null && containingType.ConstructionRequiresArguments)
                    {
                        throw new XamlXmlWriterException(SR.ExpandPositionalParametersinTypeWithNoDefaultConstructor);
                    }

                    writer.ppStateInfo.ReturnState = State;
                    writer.currentState = ExpandPositionalParameters.State;
                }
                else
                {
                    writer.currentState = InMember.State;
                }
            }

            public override void WriteEndObject(XamlXmlWriter writer)
            {
                Debug.Assert(writer.namespaceScopes.Count > 0);
                Frame frame = writer.namespaceScopes.Pop();
                if (frame.AllocatingNodeType != XamlNodeType.StartObject &&
                    frame.AllocatingNodeType != XamlNodeType.GetObject)
                {

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