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

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?

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.

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