.Net笔记:System.IO之Stream的使用详解

Stream在msdn的定义：提供字节序列的一般性视图（provides a generic view of a sequence of bytes）。这个解释太抽象了，不容易理解；从stream的字面意思“河，水流”更容易理解些，stream是一个抽象类，它定义了类似“水流”的事物的一些统一行为，包括这个“水流”是否可以抽水出来（读取流内容）；是否可以往这个“水流”中注水（向流中写入内容）；以及这个“水流”有多长；如何关闭“水流”，如何向“水流”中注水，如何从“水流”中抽水等“水流”共有的行为。
常用的Stream的子类有：
1） MemoryStream 存储在内存中的字节流
2） FileStream  存储在文件系统的字节流
3） NetworkStream 通过网络设备读写的字节流
4） BufferedStream 为其他流提供缓冲的流
Stream提供了读写流的方法是以字节的形式从流中读取内容。而我们经常会用到从字节流中读取文本或者写入文本，微软提供了StreamReader和StreamWriter类帮我们实现在流上读写字符串的功能。
下面看下如何操作Stream，即如何从流中读取字节序列，如何向流中写字节
1. 使用Stream.Read方法从流中读取字节，如下示例注释：


代码如下:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
namespace UseStream
{
    class Program
    {
        //示例如何从流中读取字节流
        static void Main(string[] args)
        {
            var bytes = new byte[] {(byte)1,(byte)2,(byte)3,(byte)4,(byte)5,(byte)6,(byte)7,(byte)8};
            using (var memStream = new MemoryStream(bytes))
            {
                int offset = 0;
                int readOnce = 4;
                do
                {
                    byte[] byteTemp = new byte[readOnce];
                    // 使用Read方法从流中读取字节
                    //第一个参数byte[]存储从流中读出的内容
                    //第二个参数为存储到byte[]数组的开始索引，
                    //第三个int参数为一次最多读取的字节数
                    //返回值是此次读取到的字节数，此值小于等于第三个参数
                    int readCn = memStream.Read(byteTemp, 0, readOnce);
                    for (int i = 0; i < readCn; i++)
                    {
                        Console.WriteLine(byteTemp[i].ToString());
                    }

offset += readCn;

//当实际读取到的字节数小于设定的读取数时表示到流的末尾了
                    if (readCn < readOnce) break;
                } while (true);
            }
            Console.Read();
        }
    }
}

2. 使用Stream.BeginRead方法读取FileStream的流内容
注意：BeginRead在一些流中的实现和Read完全相同，比如MemoryStream；而在FileStream和NetwordStream中BeginRead就是实实在在的异步操作了。
如下示例代码和注释：


代码如下:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Threading;
namespace UseBeginRead
{
    class Program
    {
        //定义异步读取状态类
        class AsyncState
        {
            public FileStream FS { get; set; }

public byte[] Buffer { get; set; }

public ManualResetEvent EvtHandle { get; set; }
        }
        static  int bufferSize = 512;
        static void Main(string[] args)
        {
            string filePath = "d:\\test.txt";
            //以只读方式打开文件流
            using (var fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read))
            {
                var buffer = new byte[bufferSize];

//构造BeginRead需要传递的状态
                var asyncState = new AsyncState { FS = fileStream, Buffer = buffer ,EvtHandle = new ManualResetEvent(false)};

//异步读取
                IAsyncResult asyncResult = fileStream.BeginRead(buffer, 0, bufferSize, new AsyncCallback(AsyncReadCallback), asyncState);

//阻塞当前线程直到读取完毕发出信号
                asyncState.EvtHandle.WaitOne();
                Console.WriteLine();
                Console.WriteLine("read complete");
                Console.Read();
            }
        }
        //异步读取回调处理方法
        public static void AsyncReadCallback(IAsyncResult asyncResult)
        {
            var asyncState = (AsyncState)asyncResult.AsyncState;
            int readCn = asyncState.FS.EndRead(asyncResult);
            //判断是否读到内容
            if (readCn > 0)
            {
                byte[] buffer;
                if (readCn == bufferSize) buffer = asyncState.Buffer;
                else
                {
                    buffer = new byte[readCn];
                    Array.Copy(asyncState.Buffer, 0, buffer, 0, readCn);
                }

//输出读取内容值
                string readContent = Encoding.UTF8.GetString(buffer);
                Console.Write(readContent);
            }

if (readCn < bufferSize)
            {
                asyncState.EvtHandle.Set();
            }
            else {
                Array.Clear(asyncState.Buffer, 0, bufferSize);
                //再次执行异步读取操作
                asyncState.FS.BeginRead(asyncState.Buffer, 0, bufferSize, new AsyncCallback(AsyncReadCallback), asyncState);
            }
        }
    }
}

3. 使用Stream.Write方法向流中写字节数组
在使用Write方法时，需要先使用Stream的CanWrite方法判断流是否可写，如下示例定义了一个MemoryStream对象，然后向内存流中写入一个字节数组


代码如下:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
namespace UseStreamWrite
{
    class Program
    {
        static void Main(string[] args)
        {
            using (var ms = new MemoryStream())
            {
                int count = 20;
                var buffer = new byte[count];
                for (int i = 0; i < count; i++)
                {
                    buffer[i] = (byte)i;
                }

//将流当前位置设置到流的起点
                ms.Seek(0, SeekOrigin.Begin);

Console.WriteLine("ms position is " + ms.Position);

//注意在调用Stream的Write方法之前要用CanWrite判断Stream是否可写
                if (ms.CanWrite)
                {
                    ms.Write(buffer, 0, count);
                }

//正确写入的话，流的位置会移动到写入开始位置加上写入的字节数
                Console.WriteLine("ms position is " + ms.Position);
            }

Console.Read();
        }
    }
}

4. 使用Stream.BeginWrite方法异步写；异步写可以提高程序性能，这是因为磁盘或者网络IO的速度远小于cpu的速度，异步写可以减少cpu的等待时间。
如下使用FileStream异步写文件的操作示例


代码如下:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Threading;
namespace UseStreamBeginWrite
{
    class Program
    {
        /// <summary>
        /// 异步回调需要的参数封装类
        /// </summary>
        class AsyncState {
            public int WriteCountOnce { get; set; }
            public int Offset { get; set; }
            public byte[] Buffer { get; set; }
            public ManualResetEvent WaitHandle { get; set; }
            public FileStream FS { get; set; }
        }
        static void Main(string[] args)
        {
            //准备一个1K的字节数组
            byte[] toWriteBytes = new byte[1 << 10];
            for (int i = 0; i < toWriteBytes.Length; i++)
            {
                toWriteBytes[i] = (byte)(i % byte.MaxValue);
            }
            string filePath = "d:\\test.txt";
            //FileStream实例
            using (var fileStream = new FileStream(filePath, FileMode.Create, FileAccess.ReadWrite, FileShare.Read))
            {
                int offset = 0;
                //每次写入32字节
                int writeCountOnce = 1 << 5;
                //构造回调函数需要的状态
                AsyncState state = new AsyncState{
                    WriteCountOnce = writeCountOnce,
                    Offset = offset,
                    Buffer = toWriteBytes,
                    WaitHandle = new ManualResetEvent(false),
                    FS = fileStream
                };

//做异步写操作
                fileStream.BeginWrite(toWriteBytes, offset, writeCountOnce, WriteCallback, state);

//等待写完毕或者出错发出的继续信号
                state.WaitHandle.WaitOne();
            }
            Console.WriteLine("Done");
            Console.Read();
        }
        /// <summary>
        /// 异步写的回调函数
        /// </summary>
        /// <param name="asyncResult">写状态</param>
        static void WriteCallback(IAsyncResult asyncResult)
        {
            AsyncState state = (AsyncState)asyncResult.AsyncState;

try
            {
                state.FS.EndWrite(asyncResult);
            }
            catch (Exception ex)
            {
                Console.WriteLine("EndWrite Error:" + ex.Message);
                state.WaitHandle.Set();
                return;
            }

Console.WriteLine("write to " + state.FS.Position);
            //判断是否写完，未写完继续异步写
            if (state.Offset + state.WriteCountOnce < state.Buffer.Length)
            {
                state.Offset += state.WriteCountOnce;
                Console.WriteLine("call BeginWrite again");
                state.FS.BeginWrite(state.Buffer, state.Offset, state.WriteCountOnce, WriteCallback, state);
            }
            else {
                //写完发出完成信号
                state.WaitHandle.Set();
            }
        }
    }
}