C# tostring("0000000")

 public string ConverNo(string str)
        {
            string result = "";

            char[] chars = new char[7];
            for (int i = 0; i < chars.Length; i++)
            chars[i] = '0';
            char[] chars1 = str.ToArray(); //字符串生成数组

            string str0 = "0000000";
            result =  str0.Substring(0,str0.Length-str.Length);
            result = result += str;

            return "";
        }