using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = "111aa222aa333aa444aa555";

            string findStr = "aa";//删除找到的第一个aa
            int startIndex = s.IndexOf(findStr);
            if (-1 != startIndex)
            {
                s = s.Substring(0, startIndex) + s.Substring(startIndex + findStr.Length);
                Console.WriteLine(s);
            }

            Console.ReadKey();
        }
    }
}

运行效果