using System;

public class Program

{

    public static void Main()

    {

        var foo = new Foo();

        // This.

        Console.WriteLine(foo);

        // Is not the same as this.

        Console.WriteLine(foo.ToString());

    }

    private class Foo

    {

        private readonly Guid value = Guid.NewGuid();

        public override string ToString()

        {

            return string.Format("Parameterless ToString() => {0}", value);

        }

        public string ToString(string format = "N")

        {

            return string.Format("Parameter: ToString({1}) => {0}", value.ToString(format), format);

        }

    }

}