dotnet/wpf · error · ArgumentException

SR.IncorrectGetterParamNum

Error message

SR.IncorrectGetterParamNum

What it means

ValidateGetter rejects getter MethodInfo objects that do not have exactly one parameter and a non-void return type, throwing ArgumentException with SR.IncorrectGetterParamNum. A valid attachable getter must take the owning object and return the value.

Solutions

1. Change the getter to accept exactly one parameter (the object) and return the property value.
2. Pass a different, correct MethodInfo.
3. If only a setter is valid, omit the getter by passing null instead of an invalid one.

Example fix

// before
public static void GetMyProp(object o) { ... }  // void return
// after
public static string GetMyProp(object o) { return ...; }

if (getter is not null && (getter.GetParameters().Length != 1 || getter.ReturnType == typeof(void))) throw new ArgumentException("Getter must take 1 parameter and return non-void");

bool IsValidGetter(MethodInfo m) => m is null || (m.GetParameters().Length == 1 && m.ReturnType != typeof(void));

When it happens

Trigger: Constructing an attachable-property XamlMember with a getter MethodInfo whose GetParameters().Length != 1 or whose ReturnType == typeof(void).

Common situations: Hand-written or code-generated Get<Property> methods with wrong arity or void return, often after signature refactors.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• SR.IncorrectSetterParamNum
• Attached property setter and attached event adder methods…
• SR.CanOnlyHaveOneChild
• SR.CanOnlyHaveOneChild
• SR.Format(SR.ParserPrefixNSProperty, nsPrefix, nameString)

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/2d1ce1baa68b628d. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/XamlMember.cs:849

                {
                    result = LookupIsWritePublic();
                    _reflector.SetFlag(BoolMemberBits.WritePublic, result.Value);
                }

                return result.Value;
            }
        }

        private static void ValidateGetter(MethodInfo method, string argumentName)
        {
            if (method is null)
            {
                return;
            }

            if ((method.GetParameters().Length != 1) || (method.ReturnType == typeof(void)))
            {
                throw new ArgumentException(SR.IncorrectGetterParamNum, argumentName);
            }
        }

        private static void ValidateSetter(MethodInfo method, string argumentName)
        {
            if ((method is not null) && (method.GetParameters().Length != 2))
            {
                throw new ArgumentException(SR.IncorrectSetterParamNum, argumentName);
            }
        }

        // This property needs to be checked before any attribute lookups on _reflector. It's not
        // only informational, it also ensures that the right state is initialized.
        private bool AreAttributesAvailable
        {
            get
            {
                EnsureReflector();

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