dotnet/wpf · error · XamlXmlWriterException

SR.ConstructorNotFoundForGivenPositionalParameters

Error message

SR.ConstructorNotFoundForGivenPositionalParameters

What it means

Positional-parameter expansion matches the supplied argument count to properties decorated with [ConstructorArgument]. If the number of constructor parameters on the CLR type differs from the number of ConstructorArgument properties, no matching constructor exists and XamlXmlWriterException is thrown.

Solutions

1. Fix the markup extension type so every constructor parameter has a matching [ConstructorArgument] property (same count).
2. Remove stale or extra [ConstructorArgument] attributes from properties.
3. Use a constructor overload whose parameters exactly correspond to the attributed properties.
4. Catch XamlXmlWriterException and emit explicit x:Arguments nodes instead of relying on expansion.

Example fix

// before
public Foo(string a) { ... }
[ConstructorArgument("a")] public string A { get; }
[ConstructorArgument("b")] public string B { get; } // extra CAA, no ctor param b
// after
[ConstructorArgument("a")] public string A { get; } // only params that exist on ctor

var caaProps = GetAllPropertiesWithCAA(type); if (caaProps.Count != ctorParams.Length) throw new InvalidOperationException("ConstructorArgument property count must match constructor parameter count");

try { writer.WriteEndObject(); } catch (XamlXmlWriterException ex) when (ex.Message.Contains("ConstructorNotFound")) { /* emit x:Arguments manually */ }

When it happens

Trigger: Expanding positional parameters where GetParametersInfo(objectXamlType, n) returns a constructor with m parameters but GetAllPropertiesWithCAA finds a different count of [ConstructorArgument]-attributed properties (count mismatch).

Common situations: Markup extension types whose [ConstructorArgument] attributes were added/removed out of sync with the constructor, multiple constructors confusing parameter resolution, third-party extensions with inconsistent metadata.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• ArgumentNullException(nameof(arrayType))
• ArgumentNullException(nameof(assemblyNames))
• ArgumentNullException(nameof(contentType))
• ArgumentNullException(nameof(loaderType))
• ArgumentNullException(nameof(member))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/9669cd6fb00e0576. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/XamlXmlWriter.cs:1923

                XamlType objectXamlType = frame.Type;
                Debug.Assert(objectXamlType is not null);

                Type objectClrType = objectXamlType.UnderlyingType;
                if (objectClrType is null)
                {
                    throw new XamlXmlWriterException(SR.Format(
                        SR.ExpandPositionalParametersWithoutUnderlyingType, objectXamlType.GetQualifiedName()));
                }

                int numOfParameters = writer.ppStateInfo.NodesList.Count;

                ParameterInfo[] constructorParameters = GetParametersInfo(objectXamlType, numOfParameters);
                List<XamlMember> ctorArgProps = GetAllPropertiesWithCAA(objectXamlType);

                // If there aren't the same number of parameters then we throw
                if (constructorParameters.Length != ctorArgProps.Count)
                {
                    throw new XamlXmlWriterException(SR.ConstructorNotFoundForGivenPositionalParameters);
                }

                for (int i = 0; i < constructorParameters.Length; i++)
                {
                    ParameterInfo paraminfo = constructorParameters[i];

                    XamlMember matchingProperty = null;
                    foreach (var potentialProperty in ctorArgProps)
                    {
                        if ((potentialProperty.Type.UnderlyingType == paraminfo.ParameterType) &&
                            (XamlObjectReader.GetConstructorArgument(potentialProperty) == paraminfo.Name))
                        {
                            matchingProperty = potentialProperty;
                            break;
                        }
                    }

                    if (matchingProperty is null)

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