使用StreamReader將stream讀取出來，然後轉為string。

using System;
using System.IO;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            byte[] b = { 0, 1, 2, 3, 4 };
            Stream stream = new MemoryStream(b);
            using (StreamReader reader = new StreamReader(stream))
            {
               string str = reader.ReadToEnd(); //將stream轉為string
            }
        }
    }
}