C# 金钱添加逗号0000

        private void Form1_Load(object sender, EventArgs e)
        {
            decimal dd = (decimal)11234567890.01;
            string ddstr = dd.ToString();
            test(ddstr);
 
        }
 
        public void test(string num_str)
        {
            if (num_str.IndexOf(".") != -)
            {
                //小数
                string[] strlist = num_str.Split(".".ToCharArray(), StringSplitOptions.RemoveEmptyEntries);//返回值不包含空字符串元素,
 
                var 整数 = strlist[];
                var 小数 = strlist[];
                var arry_int = 整数.ToArray();
                StringBuilder sb = new StringBuilder();
                var t = Math.Floor((double)整数.Length / );
 
                if (arry_int.Length %  == )
                {
                    for (int i = ; i < arry_int.Length; i++)
                    {
                        if (i %  ==  && i != )
                        {
                            sb.Append(",");
                            sb.Append(arry_int[i]);
                            continue;
                        }
                        sb.Append(arry_int[i]);
                    }
                }
                else
                {
                    var t0 = arry_int.Length - (t * );
                    for (int i = ; i < arry_int.Length; i++)
                    {
                        if ((i-t0) %  ==  && i != )
                        {
                            sb.Append(",");
                            sb.Append(arry_int[i]);
                            continue;
                        }
                        sb.Append(arry_int[i]);
                    }
 
                }
 
                var result = sb.ToString();
 
            }
        }