dotnet/wpf · error · MissingMethodException

SR.NoDefaultConstructor

Error message

SR.NoDefaultConstructor

What it means

During XamlTypeInvoker.CreateInstance, EnsureConstructorDelegate throws MissingMethodException('NoDefaultConstructor') when the underlying CLR type has no public parameterless constructor. The message deliberately mirrors Activator.CreateInstance's MissingMethodException since the invoker only supports default-constructor activation for XAML object elements.

Solutions

  1. Add a public parameterless constructor to the type.
  2. Supply constructor arguments via x:Arguments in the XAML or pass non-empty arguments to CreateInstance so a matching ctor path is used.
  3. Catch MissingMethodException and substitute a factory or a different instantiation strategy.
  4. Add a TypeConverter on the type so the XAML reader can construct it from a string instead of invoking a ctor.

Example fix

// before
class Widget { public Widget(int size) { ... } }
// after
class Widget { public Widget() : this(1) { } public Widget(int size) { ... } }
Defensive patterns

Strategy: try-catch

Validate before calling

bool ok = xamlType.UnderlyingType?.GetConstructor(Type.EmptyTypes) is not null;

Type guard

static bool HasDefaultCtor(XamlType t) => t.UnderlyingType?.GetConstructor(Type.EmptyTypes) is not null;

Try / catch

try { obj = invoker.CreateInstance(null); } catch (MissingMethodException ex) { /* no public parameterless ctor */ }

Prevention

When it happens

Trigger: Calling XamlTypeInvoker.CreateInstance (with null or empty arguments path reaching DefaultCtorXamlActivator) on a XamlType whose UnderlyingType only defines parameterized constructors — e.g. a POCO with just MyType(int x) — or whose parameterless ctor is non-public.

Common situations: XAML object elements for types lacking a default ctor without a x:TypeArguments/constructor-argument directive; model classes with required-argument ctors used in XAML resources; types that had a default ctor removed in a refactor (version-upgrade break); non-public ctors after making a class stricter.

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

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/44e2b240ee56a40d. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Schema/XamlTypeInvoker.cs:314

                if (s_securityFailureWithCtorDelegate == ThreeValuedBool.NotSet)
                {
                    s_securityFailureWithCtorDelegate =
                        ThreeValuedBool.False;
                }

                if (s_securityFailureWithCtorDelegate == ThreeValuedBool.True)
                {
                    return false;
                }

                Type underlyingType = type._xamlType.UnderlyingType.UnderlyingSystemType;
                // Look up public ctors only, for equivalence with Activator.CreateInstance
                ConstructorInfo tConstInfo = underlyingType.GetConstructor(Type.EmptyTypes);
                if (tConstInfo is null)
                {
                    // Throwing MissingMethodException for equivalence with Activator.CreateInstance
                    throw new MissingMethodException(SR.Format(SR.NoDefaultConstructor, underlyingType.FullName));
                }

                if ((tConstInfo.IsSecurityCritical && !tConstInfo.IsSecuritySafeCritical) ||
                    (tConstInfo.Attributes & MethodAttributes.HasSecurity) == MethodAttributes.HasSecurity ||
                    (underlyingType.Attributes & TypeAttributes.HasSecurity) == TypeAttributes.HasSecurity)
                {
                    // We don't want to bypass security checks for a critical or demanding ctor,
                    // so just treat it as if it were non-public
                    type._isPublic = ThreeValuedBool.False;
                    return false;
                }

                IntPtr constPtr = tConstInfo.MethodHandle.GetFunctionPointer();
                // This requires Reflection Permission
                Action<object> ctorDelegate = ctorDelegate =
                    (Action<object>)s_actionCtor.Invoke(new object[] { null, constPtr });
                type._constructorDelegate = ctorDelegate;
                return true;

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