stream was not readable.

StreamWriter使用时的报错情况：

stream was not readable.

错误原因：

没有指定StreamWriter的写入文件

正确代码示例1：

byte[] businessDataByte = dh.downFile(dataActionUrl);
logger.InfoFormat("{0}", businessDataByte.Length);

StreamWriter sw = new StreamWriter("a.txt"));
sw.Write(Encoding.UTF8.GetString(businessDataByte));
sw.Flush();
sw.Close();

示例2：

using System;
using System.IO;
using System.Text;

namespace ConsoleApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            string fileName = "test.txt";
            string textToAdd = "Example text in file";

            using (FileStream fs = new FileStream(fileName, FileMode.CreateNew))
            {
                using (StreamWriter writer = new StreamWriter(fs))
                {
                    writer.Write(textToAdd);
                }
            }            
        }
    }
}