C# - String 在字符串中删除字符

private string delstr(string theoldstr,string startstr,string endstr)
        {
            string teststr=theoldstr;            
            int StartPosition=teststr.IndexOf(startstr);
            int EndPosition=teststr.IndexOf(endstr);
            int DelLength=EndPosition-StartPosition+endstr.Length;            
            teststr=teststr.Remove(StartPosition,DelLength);
            if(teststr.IndexOf(startstr)>=0 && teststr.IndexOf(endstr)>=0)
            {
             return    delstr(teststr,startstr,endstr);
            }
            else
            {
                return teststr;
            }

        }