using System;

public class Program

{

    public static void Main()

    {

        string str="remove the sub-string";

        string  MainString="This is the main string and I want to remove the sub-string";

        string resultatntString = MainString.Replace(str, " ");

        Console.WriteLine("Using Replace :{0}",resultatntString);

        Console.WriteLine("Using Substring :{0}",MainString.Substring(0,MainString.IndexOf(str)));

        Console.WriteLine("Using Remove :{0}",MainString.Remove(MainString.IndexOf(str)));

    }

}