using System;

public class Program

{

    public static void Main()

    {

        var result = "ABC123".Substring("ABC".Length);

        Console.WriteLine(result);

        Console.WriteLine("---");

​

        Test(SubstringAfter_Remove);

        Test(SubstringAfter_Substring);

        Test(SubstringAfter_SubstringWithIndexOf);

    }

​

    public static void Test(Func<string, string, string> f)

    {

        var array = 

            new string[] { "ABC123", "ABC13245", "ABC123456", "ABC1" };

​

        var sw = new System.Diagnostics.Stopwatch();

        sw.Start();

        foreach(var s in array) {

            Console.WriteLine(f.Invoke(s, "ABC"));

        }

        sw.Stop();

        Console.WriteLine(f.Method.Name + " : " + sw.ElapsedTicks + " ticks.");

        Console.WriteLine("---");

    }

​

    public static string SubstringAfter_Remove(string s, string after)

    {

        return s.Remove(0, after.Length);

    }

​

    public static string SubstringAfter_Substring(string s, string after)

    {

        return s.Substring(after.Length);       

    }

​

    public static string SubstringAfter_SubstringWithIndexOf(string s, string after)

    {

        return s.Substring(s.IndexOf(after) + after.Length);        

    }    

}