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