What causes "Member {0}.{1} does not exist." in devlooped/moq?

Correct the member name string, using nameof where possible

devlooped/moq · error · ArgumentException

Member . does not exist.

Error message

Member {0}.{1} does not exist.

What it means

ProtectedMock.ProtectedAs<T>... string-name based APIs (Setup/SetupGet/SetupSet/Verify with a member name string) look up the member on the mocked type T; ThrowIfMemberMissing throws ArgumentException with Resources.MemberMissing ('Member {0}.{1} does not exist.') when the reflection lookup returns null. It means the named member simply is not present on T with the requested accessibility.

Solutions

Correct the member name string, using nameof where possible
Check the member's actual declared accessibility and name via reflection or the class source
Switch to mock.Protected().As<TAnalog>() or strong-typed Mock<T> setups to get compile-time checking
Update the name after upgrading the mocked library version

Example fix

// before
mock.Protected().Setup<int>("GetValu");
// after
mock.Protected().Setup<int>("GetValue"); // member actually named GetValue on T

Defensive patterns

Strategy: validation

Validate before calling

static bool MemberExists(Type target, string name) =>
    target.GetMember(name, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance).Length > 0;

Try / catch

try { mock.Protected().Setup<int>("GetValue"); }
catch (ArgumentException ex) when (ex.Message.Contains("does not exist")) { /* fix member-name string */ }

Prevention

Use nameof or constants for member-name strings to survive renames
Double-check spelling and accessibility before writing string-based protected setups
Prefer Protected().As<TAnalog>() for compile-time safety
After library upgrades, grep tests for renamed member strings

When it happens

Trigger: mock.Protected().Setup(x => x.MethodName(...)) / SetupGet("PropName") / SetupSet("PropName", value) where no method, property, or field with that exact name exists (non-public instance, or public — depending on the lookup overload) on T.

Common situations: Typo in the member-name string; member renamed in a newer library version (string-based API misses compile-time safety); member is actually public and resolved on a different path; member exists only on a base type with unexpected accessibility.

Understand the failure class

Background: "Not found" and "does not exist" errors: why "Task not found", "No such folder", and "Can't find" fire when a lookup comes back empty — this error's family across 14 libraries.

Related errors

AI-assisted analysis of devlooped/moq@89a5be629c (2026-09-16). Data as JSON: /api/errors/452d4cee8791d644. Report an issue: GitHub.

Appendix: source

Thrown at src/Moq/Protected/ProtectedMock.cs:371

        static Expression<Action<T>> GetSetterExpression(PropertyInfo property, Expression value)
        {
            var param = Expression.Parameter(typeof(T), "mock");

            return Expression.Lambda<Action<T>>(
                Expression.Call(param, property.GetSetMethod(true), value),
                param);
        }

#if NULLABLE_REFERENCE_TYPES
        static void ThrowIfMemberMissing(string memberName, [NotNull] MemberInfo? member)
#else
        static void ThrowIfMemberMissing(string memberName, MemberInfo? member)
#endif
        {
            if (member == null)
            {
                throw new ArgumentException(string.Format(
                    CultureInfo.CurrentCulture,
                    Resources.MemberMissing,
                    typeof(T).Name,
                    memberName));
            }
        }

#if NULLABLE_REFERENCE_TYPES
        static void ThrowIfMethodMissing(string methodName, [NotNull] MethodInfo? method, object[] args)
#else
        static void ThrowIfMethodMissing(string methodName, MethodInfo? method, object[] args)
#endif
        {
            if (method == null)
            {
                List<string> extractedTypeNames = new List<string>();
                foreach (object o in args)
                {

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