你如何从MemoryStream中获取字符串？

MemoryStream memoryStream = new MemoryStream();
StreamWriter writer = new StreamWriter(memoryStream, Encoding.ASCII);
writer.Write("Hello, world!");
writer.Flush();
memoryStream.Position = 0;

StreamReader reader = new StreamReader(memoryStream, Encoding.ASCII);
string result = reader.ReadToEnd();

MemoryStream memoryStream = new MemoryStream();
StreamWriter writer = new StreamWriter(memoryStream, Encoding.UTF8);
writer.Write("你好，世界！");
writer.Flush();
memoryStream.Position = 0;

StreamReader reader = new StreamReader(memoryStream, Encoding.UTF8);
string result = reader.ReadToEnd();

MemoryStream memoryStream = new MemoryStream();
StreamWriter writer = new StreamWriter(memoryStream, Encoding.Unicode);
writer.Write("你好，世界！");
writer.Flush();
memoryStream.Position = 0;

StreamReader reader = new StreamReader(memoryStream, Encoding.Unicode);
string result = reader.ReadToEnd();