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

<span style="font-size:18px;">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。</span>

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 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 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,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 String] = [String].Empty

      ' Read the first batch of the Tcpserver response bytes.
      Dim bytes As Int32 = stream.Read(data,data.Length)
      responseData = System.Text.Encoding.ASCII.GetString(data,bytes)
      Console.WriteLine("Received: {0}",responseData)

      ' Close everything.
      stream.Close()
      client.Close()
   Catch e As ArgumentNullException
      Console.WriteLine("ArgumentNullException: {0}",e)
   As SocketException
      Console.WriteLine("SocketException: {0}",sans-serif!important; color:blue">End Try

   Console.WriteLine(ControlChars.Cr + " Press Enter to continue...")
   Console.Read()
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----------------

<pre name="code" class="vb">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,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,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,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