dotnet/wpf · error · XamlObjectReaderException

SR.ObjectReaderNoMatchingConstructor

Error message

SR.ObjectReaderNoMatchingConstructor

What it means

XamlObjectReader throws this when a value's type has constructor-argument properties (x:Arguments recorded) but none of the available constructors accept arguments matching what was recorded, and the type also lacks a default-constructor fallback path. It means XAML cannot reconstruct the object with any constructor.

Solutions

1. Ensure the type exposes a constructor whose parameter list matches the recorded x:Arguments in count and type
2. Reorder or re-type x:Arguments to match an actual constructor
3. Add an explicit constructor accepting the argument types used by the graph
4. Rebuild the object graph after assembly signature changes

Example fix

// before: x:Arguments supplies <s:String>, <s:Int32> but ctor is Config(int, string)
// after: swap argument order
<s:Arguments><s:Int32>1</s:Int32><s:String>x</s:String></s:Arguments>

bool MatchesSomeCtor(Type t, object[] args) => t.GetConstructors().Any(c => { var ps = c.GetParameters(); return ps.Length == args.Length && ps.Zip(args, (p, a) => p.ParameterType.IsInstanceOfType(a ?? (object)"")).All(x => x); });

try { using var r = new XamlObjectReader(obj); ... } catch (XamlObjectReaderException ex) when (ex.Message.Contains("NoMatchingConstructor")) { /* realign x:Arguments */ }

When it happens

Trigger: new XamlObjectReader(obj) where a reference type has ctor-argument members but no constructor whose parameters match the recorded argument set (count or types mismatch).

Common situations: Constructor signatures changed between assembly versions while object graphs contain instances built with the old signature; x:Arguments supplied in the wrong order or with incompatible types; custom markup extensions emitting wrong-typed arguments.

Related errors

• SR.ObjectReaderNoDefaultConstructor
• ArgumentNullException(nameof(arrayType))
• ArgumentNullException(nameof(assemblyNames))
• ArgumentNullException(nameof(contentType))
• ArgumentNullException(nameof(loaderType))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/bb9b01248e7eef0a. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/XamlObjectReader.cs:1704

                        {
                            if (!valueXamlType.IsCollection && !valueXamlType.IsDictionary) // always need to serialize items of a collection or dictionary
                            {
                                isComplete = true;
                            }
                        }

                        break;
                    }
                }

                if (member is null && !valueXamlType.UnderlyingType.IsValueType)
                {
                    if (ctorArgProps.Count == 0)
                    {
                        throw new XamlObjectReaderException(SR.Format(SR.ObjectReaderNoDefaultConstructor, value.GetType()));
                    }

                    throw new XamlObjectReaderException(SR.Format(SR.ObjectReaderNoMatchingConstructor, value.GetType()));
                }
            }

            private static void CheckTypeCanRoundtrip(ObjectMarkupInfo objInfo)
            {
                var xamlType = objInfo.XamlNode.XamlType;
                if (!xamlType.IsConstructible)
                {
                    foreach (var property in objInfo.Properties)
                    {
                        if (((MemberMarkupInfo)property).IsFactoryMethod && !xamlType.UnderlyingType.IsNested)
                        {
                            // this is the case when the class has no public constructor we can use but contains a factory method
                            // and the class is not nested

                            return;
                        }
                    }

View on GitHub (pinned to 81131a70a4)