//找到文件夹
System.IO.DirectoryInfo dd = new System.IO.DirectoryInfo("C://qq");
//得到该文件夹下所有的文件
System.IO.FileInfo[] file = dd.GetFiles();
//得到某个文件的路径
string filePath = file[1].DirectoryName + "\\" + file[1].Name;
//用streamread读这个文件
System.IO.StreamReader sr = new StreamReader(filePath, Encoding.Default, true);

int index;
//实例化一个内存流
System.IO.MemoryStream tempStream = new MemoryStream();
//将流转换为字节数组
while ((index = sr.BaseStream.ReadByte()) != -1)
{
tempStream.WriteByte(((byte)index));
}
byte[] array = tempStream.ToArray();
tempStream.Close();
//将得到的字节数组转换为base64位编码
string result = Convert.ToBase64String(array);

System.IO.File.WriteAllText("C://zz.txt", result);

//将64位编码转换为字节数组
byte[] cc = Convert.FromBase64String(result);

//用filestream创造一个文件
FileStream fs = new FileStream("C://a.jpg", FileMode.Create);

BinaryWriter bw = new BinaryWriter(fs);

bw.Write(cc);