static void Main(string[] args)
  {
   string temp = "Hello World!";
   Console.WriteLine("Original String:{0}", temp);
   
   byte[] tempBytes = System.Text.Encoding.ASCII.GetBytes(temp);
   Console.WriteLine("Bytes Array:{0}", BitConverter.ToString(tempBytes));

   BigInteger integer = new BigInteger(tempBytes);
   Console.WriteLine("BigInteger:{0}", integer);

   string res = System.Text.Encoding.ASCII.GetString(tempBytes);
   Console.WriteLine("Convert Back String:{0}", res);
   Console.ReadKey();
  }

Original String:Hello World!
Bytes Array:48-65-6C-6C-6F-20-57-6F-72-6C-64-21
BigInteger:10334410032597741434076685640
Convert Back String:Hello World!

string temp = "你好，世界！";

Original String:你好，世界！
Bytes Array:3F-3F-3F-3F-3F-3F
BigInteger:69540876599103
Convert Back String:??????

static void Main(string[] args)
  {
   string temp = "你好，世界！";
   Console.WriteLine("Original String:{0}", temp);
   
   byte[] tempBytes = System.Text.Encoding.UTF8.GetBytes(temp);
   Console.WriteLine("Bytes Array:{0}", BitConverter.ToString(tempBytes));

   BigInteger integer = new BigInteger(tempBytes);
   Console.WriteLine("BigInteger:{0}", integer);

   string res = System.Text.Encoding.UTF8.GetString(tempBytes);
   Console.WriteLine("Convert Back String:{0}", res);
   Console.ReadKey();
  }

Original String:你好，世界！
Bytes Array:E4-BD-A0-E5-A5-BD-EF-BC-8C-E4-B8-96-E7-95-8C-EF-BC-81
BigInteger:-10998968812899434720462615123889939386679836
Convert Back String:你好，世界！

Original String:Hello World!
Bytes Array:48-65-6C-6C-6F-20-57-6F-72-6C-64-21
BigInteger:10334410032597741434076685640
Convert Back String:Hello World!

Original String:Hello World!
Bytes Array:48-00-65-00-6C-00-6C-00-6F-00-20-00-57-00-6F-00-72-00-6C-00-64-00-21-00
BigInteger:3160918205608148134863399242437668999277801104545742920
Convert Back String:Hello World!

Original String:你好，世界！
Bytes Array:60-4F-7D-59-0C-FF-16-4E-4C-75-01-FF
BigInteger:-307722159543719876182061216
Convert Back String:你好，世界！