Moq + Unit Testing - System.Reflection.TargetParameterCountException: Parameter count mismatch

Question

I'm tring to use a lambda with a multiple-params function but Moq throws this exception at runtime when I attempt to call the mock.Object.Convert(value, null, null, null); line.

System.Reflection.TargetParameterCountException: Parameter count mismatch

The code is:

var mock = new Mock<IValueConverter>();  mock.Setup(conv => conv.Convert(It.IsAny<Object>(), It.IsAny<Type>(),     It.IsAny<Object>(), It.IsAny<CultureInfo>())).Returns((Int32 num) => num + 5);  var value = 5; var expected = 10; var actual = mock.Object.Convert(value, null, null, null); 

What is the proper way to implement it?

Answer 1

It's your Returns clause. You have a 4 parameter method that you're setting up, but you're only using a 1 parameter lambda. I ran the following without issue:

[TestMethod] public void IValueConverter() {     var myStub = new Mock<IValueConverter>();     myStub.Setup(conv => conv.Convert(It.IsAny<object>(), It.IsAny<Type>(), It.IsAny<object>(), It.IsAny<CultureInfo>())).         Returns((object one, Type two, object three, CultureInfo four) => (int)one + 5);      var value = 5;     var expected = 10;      var actual = myStub.Object.Convert(value, null, null, null);      Assert.AreEqual<int>(expected, (int) actual); } 

No exceptions, test passed.

Answer 2

Not an answer for OP but perhaps for future googlers:

I had a Callback that didn't match the signature of the method being setup

Mock     .Setup(r => r.GetNextCustomerNumber(It.IsAny<int>()))     .Returns(AccountCounter++)     .Callback<string, int>(badStringParam, leadingDigit =>     {         // Doing stuff here, note that the 'GetNextCustomerNumber' signature is a single int          // but the callback unreasonably expects an additional string parameter.     }); 

This was the result of some refactoring and the refactoring tool of course couldn't realise that the Callback signature was incorrect