c# 与java base64 不一致解决方案

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Base64
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        
        private void button1_Click(object sender, EventArgs e)
        {
            Dictionary<string, string> dic = new Dictionary<string, string>();
            dic.Add("Cid", "123456"); dic.Add("type", "int");
            JsonStr jsonstr=new JsonStr(dic,"http://localhost");
            string result = JavaBase64(JsonConvert.SerializeObject(jsonstr));
        }


        #region 实体类
        /// <summary>
        /// 实体类
        /// </summary>
        class JsonStr
        {
            public string Method = "POST";
            public Dictionary<string, string> param;
            public string url = "";
            public JsonStr(Dictionary<string, string> _param, string _url)
            {
                param = _param; url = _url;
            }
        } 
        #endregion


        #region 转换为单字节  java base64
        /// <summary>
        /// 转换为单字节  java base64
        /// </summary>
        /// <param name="str"></param>
        /// <returns></returns>
        public static string JavaBase64(string str)
        {
            byte[] by = Encoding.UTF8.GetBytes(str);
            sbyte[] sby = new sbyte[by.Length];
            for (int i = 0; i < by.Length; i++)
            {
                if (by[i] > 127)
                    sby[i] = (sbyte)(by[i] - 256);
                else
                    sby[i] = (sbyte)by[i];
            }
            byte[] newby = (byte[])(object)sby;
            return Convert.ToBase64String(newby);
        } 
        #endregion



    }
}