JamesNK/Newtonsoft.Json · error · ArgumentException

Unexpected member type '{0}' for member '{1}'.

Error message

Unexpected member type '{0}' for member '{1}'.

What it means

Thrown by ReflectionObject.Create's switch default case when a resolved member's MemberType is not Field, Property, or Method. ReflectionObject only wires up fields, properties, and zero-or-one-argument methods; events, nested types, constructors, and TypeInfo fall into this default branch.

Source

Thrown at Src/Newtonsoft.Json/Utilities/ReflectionObject.cs:170

                    case MemberTypes.Method:
                        MethodInfo method = (MethodInfo)member;
                        if (method.IsPublic)
                        {
                            ParameterInfo[] parameters = method.GetParameters();
                            if (parameters.Length == 0 && method.ReturnType != typeof(void))
                            {
                                MethodCall<object, object?> call = delegateFactory.CreateMethodCall<object>(method);
                                reflectionMember.Getter = target => call(target);
                            }
                            else if (parameters.Length == 1 && method.ReturnType == typeof(void))
                            {
                                MethodCall<object, object?> call = delegateFactory.CreateMethodCall<object>(method);
                                reflectionMember.Setter = (target, arg) => call(target, arg);
                            }
                        }
                        break;
                    default:
                        throw new ArgumentException("Unexpected member type '{0}' for member '{1}'.".FormatWith(CultureInfo.InvariantCulture, member.MemberType(), member.Name));
                }

                reflectionMember.MemberType = ReflectionUtils.GetMemberUnderlyingType(member);

                d.Members[memberName] = reflectionMember;
            }

            return d;
        }
    }
}

View on GitHub (pinned to 4f73e74372)

Solutions

1. Use a member name that refers to a field, property, or suitable method (parameterless with a return, or single-param void).
2. Avoid passing names of events or nested types to ReflectionObject.Create.
3. Inspect member.MemberType before calling Create to filter to supported kinds.

Example fix

// before
var ro = ReflectionObject.Create(typeof(Foo), "Clicked");
// 'Clicked' is an event -> Unexpected member type 'Event'

// after
var ro = ReflectionObject.Create(typeof(Foo), "Value"); // Value is a property

MemberTypes supported = MemberTypes.Field | MemberTypes.Property | MemberTypes.Method;
MemberInfo m = type.GetMember(name, BindingFlags.Instance | BindingFlags.Public).Single();
if ((m.MemberType & supported) == 0) throw new ArgumentException($"{m.MemberType} not supported");

static bool IsSupportedMember(MemberInfo m) => m.MemberType is MemberTypes.Field or MemberTypes.Property or MemberTypes.Method;

When it happens

Trigger: Calling ReflectionObject.Create with a member name that resolves to an EventInfo, a nested type, or a constructor rather than a field/property/method.

Common situations: A type that has an event or nested type whose name shadows the intended field/property, or exploration/tooling code that passes arbitrary member names found via reflection.

Related errors

• Expected a single member with the name '{0}'.
• MemberInfo must be of type FieldInfo, PropertyInfo, EventInf
• MemberInfo '{0}' is not of type FieldInfo or PropertyInfo
• MemberInfo '{0}' must be of type FieldInfo or PropertyInfo
• type

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/9cc19589cd24e5cc. Report an issue: GitHub.