Dynamic parameter type and RuntimeBinderException

I have once found that it may be quite surprising behavior of your program when using dynamic typing. Let's look at the following C# code that turns a string into a list of strings. No dynamic typing so far, all static:

private static IList<string> Do(string p)
{
    return new List<string> { p };
}

And then a program that calls that method, passing a dynamic variable in it, and removes one element from the result list:

class Program
{
    static void Main()
    {
        dynamic str = "hello world";
        var myList = Do(str);
        myList.RemoveAt(0);
    }

    private static IList<string> Do(string p)
    {
        return new List<string> { p };
    }
}

The code above compiles and runs just as expected. Note that a variable str is of type dynamic. It happens that you are getting a dynamic from someone else's code, right? The method Do has a completely static typing.

Now we change myList.RemoveAt(0); to be myList.MyRemoveAt(0);. Like this:

static void Main()
{
    dynamic str = "hello world";
    var myList = Do(str);
    myList.MyRemoveAt(0); // changed to non-existing method
}

Obviously IList does not have a method MyRemoveAt but the code still compiles. And only at runtime there will be caught 'Microsoft.CSharp.RuntimeBinder.RuntimeBinderException' occurred in System.Core.dll, Additional information: 'System.Collections.Generic.List<string>' does not contain a definition for 'MyRemoveAt'.

That all happens because of the late binding and the variable myList is treated as a dynamic. More about this at MSDN article in Overload Resolution with Arguments of Type Dynamic.

So it may be very dangerous of using dynamics without proper care, since the error will only pop-up at runtime. Note that in the code above we still can get a compile error if we have IList<string> myList instead of implicit var.