devlooped/moq · error · ArgumentException

Resources.ConstructorArgsForDelegate

Error message

Resources.ConstructorArgsForDelegate

What it means

Moq throws this ArgumentException when constructor arguments are supplied to a Mock<T> whose T is a delegate type. Delegates have no class constructors to invoke, so ctor arguments cannot be applied to delegate mocks.

Solutions

1. Create the delegate mock without arguments: new Mock<MyDelegateType>() and set up the invocation via Setup(d => d(It.IsAny<...>())).
2. If you actually need a concrete delegate instance, just use a lambda instead of mocking with ctor args.
3. Guard generic factory code so constructor args are only passed for non-delegate, non-interface classes.

Example fix

// before
var mock = new Mock<Func<int, string>>(42);
// after
var mock = new Mock<Func<int, string>>();

if (typeof(T).IsDelegateType() && ctorArgs is { Length: > 0 })
    throw new ArgumentException("Constructor arguments are not valid for delegate mocks.");
var mock = new Mock<T>();

bool IsDelegateMock<T>() => typeof(T).IsSubclassOf(typeof(Delegate));

try { return new Mock<T>(args); } catch (ArgumentException) { return new Mock<T>(); }

When it happens

Trigger: new Mock<MyDelegateType>(someArg) or any Mock<T> ctor overload accepting constructor arguments when typeof(T).IsDelegateType(), raised in Mock<T>.CheckParameters (src/Moq/Mock`1.cs:226).

Common situations: Mocking Func/Action/custom delegate types (e.g. for HttpClient delegates or event handlers) while accidentally passing constructor arguments copied from a class-mock snippet.

Related errors

• Resources.ConstructorArgsForInterface
• Resources.CallBaseCannotBeUsedWithDelegateMocks
• Resources.AsMustBeInterface
• Unsupported expression
• Could not determine the correct positions for all argument…

AI-assisted analysis of devlooped/moq@89a5be629c (2026-09-16). Data as JSON: /api/errors/c04bd6cc6c0f784f. Report an issue: GitHub.

Appendix: source

Thrown at src/Moq/Mock`1.cs:226

        {
            var serialNumber = Interlocked.Increment(ref serialNumberCounter);

            var name = new StringBuilder();
            name.Append("Mock<").AppendNameOf(typeof(T)).Append(':').Append(serialNumber).Append('>');
            return name.ToString();
        }

        void CheckParameters()
        {
            if (this.constructorArguments.Length > 0)
            {
                if (typeof(T).IsInterface)
                {
                    throw new ArgumentException(Resources.ConstructorArgsForInterface);
                }
                if (typeof(T).IsDelegateType())
                {
                    throw new ArgumentException(Resources.ConstructorArgsForDelegate);
                }
            }
        }

        #endregion

        #region Properties

        /// <inheritdoc/>
        public override MockBehavior Behavior => this.behavior;

        /// <inheritdoc/>
        public override bool CallBase
        {
            get => this.callBase;
            set
            {
                if (value && this.MockedType.IsDelegateType())

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