Why does this runtime dynamic binding fail?

Question

Why does the following test fail?

[TestClass]
public class DynamicTests
{
    public class ListOfIntsTotaller
    {
        public float Total(List<int> list) { return list.Sum(); }
    }
    public static class TotalFormatter
    {
        public static string GetTotal(IEnumerable list, dynamic listTotaller)
        {
            //  Get a string representation of a sum
            return listTotaller.Total(list).ToString();
        }
    }
    [TestMethod]
    public void TestDynamic()
    {
        var list = new List<int> { 1, 3 };
        var totaller = new ListOfIntsTotaller();
        Assert.AreEqual("4", totaller.Total(list).ToString()); // passes 
        Assert.AreEqual("4", TotalFormatter.GetTotal(list, totaller)); // fails
    }
}

With the following error:

Test method MyTests.DynamicTests.TestDynamic threw exception:
Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: The best overloaded method match for 'MyTests.DynamicTests.ListOfIntsTotaller.Total(System.Collections.Generic.List<int>)'
has some invalid arguments

Shouldn't the binder be smart enough to match list to its underlying type of List<int> and thus successfully bind to the GetTotal method?

Answer 1

The problem is that list in the GetTotal method is not a List<int>.

The dynamic call is determined based on the type of the variable that you use, not the actual type of the object that it's pointing to. The method Total takes a List<int>, not an IEnumerable.