private string GetFileMd5Code(string filePath)
        {
            StringBuilder builder = new StringBuilder();
            using(var md5=new MD5CryptoServiceProvider())
            {
                File.Copy(filePath,filePath+"e");//复制一份，防止占用
                using(FileStream fs=new FileStream(filePath+"e",FileMode.Open))
                {
                    byte[] bt=md5.ComputeHash(fs);
                     for (int i = 0; i < bt.Length; i++)
                     {
                         builder.Append(bt[i].ToString("x2"));
                    }
                }
                File.Delete(filePath+"e");//删除复制的文件，这里没处理异常等....
            }
            return builder.ToString();
        }