What.cs

using System;

class A
{
    public string ToUpper() => "A";
}

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("s".ToUpper()); // prints "S"

        What("s");
        What(new A());
    }

    static void What(dynamic x)
    {
        Console.WriteLine(x.ToUpper());
    }
}