使用Net.Sockets.TcpListener和Net.Sockets.TcpClient进行图片传输时如何精确控制接收缓存数组大小

在dotnet平台net.sockets.tcplistener和net.sockets.tcpclient已经为我们封装了所有socket关于tcp部分，操作也更为简单，面向数据流。使用tcpclient的getstream方法获取数据流后可以方便的对数据流进行读写操作，就如同本地磁盘的文件读写一样，使得程序员在设计程序时更为便捷简单。

但如果你使用过这两个对象进行数据传输的时候，你会发现问题也随之而来——getstream获取的数据流是一个永无止境的stream，你无法获取它的具体长度。

来看一下微软msdn关于这两个对象的例程：

shared sub connect(server as [string], message as [string])
   try
      ' create a tcpclient.
      ' note, for this client to work you need to have a tcpserver 
      ' connected to the same address as specified by the server, port
      ' combination.
      dim port as int32 = 13000
      dim client as new tcpclient(server, port)

      ' translate the passed message into ascii and store it as a byte array.
      dim data as [byte]() = system.text.encoding.ascii.getbytes(message)

      ' get a client stream for reading and writing.
      '  stream stream = client.getstream();
      dim stream as networkstream = client.getstream()

      ' send the message to the connected tcpserver. 
      stream.write(data, 0, data.length)

      console.writeline("sent: {0}", message)

      ' receive the tcpserver.response.
      ' buffer to store the response bytes.
      data = new [byte](256) {}

      ' string to store the response ascii representation.
      dim responsedata as [string] = [string].empty

      ' read the first batch of the tcpserver response bytes.
      dim bytes as int32 = stream.read(data, 0, data.length)
      responsedata = system.text.encoding.ascii.getstring(data, 0, bytes)
      console.writeline("received: {0}", responsedata)

      ' close everything.
      stream.close()
      client.close()
   catch e as argumentnullexception
      console.writeline("argumentnullexception: {0}", e)
   catch e as socketexception
      console.writeline("socketexception: {0}", e)
   end try

   console.writeline(controlchars.cr + " press enter to continue...")
   console.read()
end sub 'connect

你不得不去指定一个固定尺寸的缓冲区来接收数据，如果实际发送的数据超出了这个长度，你可能无法接收到全部完整的数据，而如果发送的数据少于缓冲区的大小，那么很显然你的内存会比别人消耗的更快。更为严重的时，如果你要发送一副图片，图片容量可能根据内容的不同而各有千秋，容量也不止256byte这么小，该如何精确控制缓冲区呢？

其实我们可以很容易的解决这个问题，就像很多文件格式所做的事情一样，我们可以在stream的前几个字节写入实际有效数据的长度，然后根据这个长度来分配内存，再读取内容，我所做的客户端与服务器之间传递屏幕的源代码如下：

"----------------客户端----------------

public class form1


    private sub timer1_tick(byval sender as system.object, byval e as system.eventargs) handles timer1.tick
        dim tcpc as new net.sockets.tcpclient
        dim slens(7) as byte
        try
            tcpc.connect("127.0.0.1", 2099)
            if tcpc.connected then
                for i = 0 to 7
                    slens(i) = tcpc.getstream.readbyte
                next

                dim buf(bitconverter.touint64(slens, 0)) as byte
                me.text = buf.length
                tcpc.getstream.read(buf, 0, buf.length)
                dim mem as new io.memorystream(buf)
                dim bmp as new bitmap(mem)
                me.picturebox1.image = bmp
                tcpc.close()
            end if
        catch ex as exception

        finally
            tcpc.close()
        end try
    end sub

    private sub form1_load(byval sender as system.object, byval e as system.eventargs) handles mybase.load

    end sub
end class






'------------服务器----------------


public class form1

    private sub backgroundworker1_dowork(byval sender as system.object, byval e as system.componentmodel.doworkeventargs) handles backgroundworker1.dowork
        dim tcps as new net.sockets.tcplistener(2099)
        tcps.start()
        while true
            dim slen as uint64
            dim slens(7) as byte
            dim tcpc as net.sockets.tcpclient
            tcpc = tcps.accepttcpclient

            '创建图片
            dim bmp as new bitmap(screen.primaryscreen.bounds.width, screen.primaryscreen.bounds.height)
            bmp.setresolution(1, 1)



            dim gph as graphics = graphics.fromimage(bmp)
            gph.copyfromscreen(new point(0, 0), new point(0, 0), bmp.size)
            gph.flush()

            '存入内存
            dim mem as new io.memorystream
            bmp.save(mem, drawing.imaging.imageformat.tiff)


            '文件长度
            slen = mem.length
            slens = bitconverter.getbytes(slen)

            '发送长度
            for i = 0 to 7
                tcpc.getstream.writebyte(slens(i))
            next
            '发送内容
            tcpc.getstream.write(mem.toarray, 0, mem.length)
            tcpc.close()
        end while
    end sub

    private sub form1_load(byval sender as system.object, byval e as system.eventargs) handles mybase.load
        me.backgroundworker1.runworkerasync()
    end sub
end class