使用Net.Sockets.TcpListener和Net.Sockets.TcpClient进行图片传输时怎样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已经为我们封装了全部Socket关于tcp部分，操作也更为简单，面向数据流。使用TcpClient的GetStream方法获取数据流后能够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，使得程序猿在设计程序时更为便捷简单。</span>

但假设你使用过这两个对象进行传输数据的时候，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，你无法获取它的详细长度。

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，假设实际发送的数据超出了这个长度，你可能无法接收到所有完整的数据，而假设发送的数据少于缓冲区的大小，那么非常显然你的内存会比别人消耗的更快。更为严重的时。假设你要发送一副图片，图片容量可能依据内容的不同而各有千秋，容量也不止256Byte这么小，该怎样精确控制缓冲区呢？

事实上我们能够非常easy的解决问题，就像非常多文件格式所做的事情一样，我们能够在Stream的前几个字节写入实际有效数据的长度。然后依据这个长度来分配内存。再读取内容，我所做的client与server之间传递屏幕的源码例如以下：

'----------------client----------------

<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, 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

'------------server----------------

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