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PO.NET Asynchronous stream read/write
 

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    <p>I have been trying to solve this "Concurrent Programming" exam exercise (in C#):</p> <blockquote> <p>Knowing that <code>Stream</code> class contains <code>int Read(byte[] buffer, int offset, int size)</code> and <code>void Write(byte[] buffer, int offset, int size)</code> methods, implement in C# the <code>NetToFile</code> method that copies all data received from <code>NetworkStream net</code> instance to the <code>FileStream file</code> instance. To do the transfer, use asynchronous reads and synchronous writes, avoiding one thread to be blocked during read operations. The transfer ends when the <code>net</code> read operation returns value 0. To simplify, it is not necessary to support controlled cancel of the operation.</p> </blockquote> <pre><code>void NetToFile(NetworkStream net, FileStream file); </code></pre> <p>I've been trying to solve this exercise, but I'm struggling with a question related with the question itself. But first, here is my code:</p> <pre><code>public static void NetToFile(NetworkStream net, FileStream file) { byte[] buffer = new byte[4096]; // buffer with 4 kB dimension int offset = 0; // read/write offset int nBytesRead = 0; // number of bytes read on each cycle IAsyncResult ar; do { // read partial content of net (asynchronously) ar = net.BeginRead(buffer,offset,buffer.Length,null,null); // wait until read is completed ar.AsyncWaitHandle.WaitOne(); // get number of bytes read on each cycle nBytesRead = net.EndRead(ar); // write partial content to file (synchronously) fs.Write(buffer,offset,nBytesRead); // update offset offset += nBytesRead; } while( nBytesRead &gt; 0); } </code></pre> <p>The question I have is that, in the question statement, is said:</p> <blockquote> <p>To do the transfer, use asynchronous reads and synchronous writes, avoiding one thread to be blocked during read operations</p> </blockquote> <p>I'm not really sure if my solution accomplishes what is wanted in this exercise, because I'm using <code>AsyncWaitHandle.WaitOne()</code> to wait until the asynchronous read completes.</p> <p>On the other side, I'm not really figuring out what is meant to be a "non-blocking" solution in this scenario, as the <code>FileStream</code> write is meant to be made synchronously... and to do that, I have to wait until <code>NetworkStream</code> read completes to proceed with the <code>FileStream</code> writing, isn't it?</p> <p>Can you, please, help me out with this?</p> <hr> <p>[ EDIT 1 ] <strong>Using <em>callback</em> solution</strong></p> <p>Ok, if I understood what <a href="https://stackoverflow.com/users/13279/mitchel-sellers">Mitchel Sellers</a> and <a href="https://stackoverflow.com/users/84167/willvv">willvv</a> replied, I've been counseled to use a callback method to turn this into a "non-blocking" solution. Here is my code, then:</p> <pre><code>byte[] buffer; // buffer public static void NetToFile(NetworkStream net, FileStream file) { // buffer with same dimension as file stream data buffer = new byte[file.Length]; //start asynchronous read net.BeginRead(buffer,0,buffer.Length,OnEndRead,net); } //asynchronous callback static void OnEndRead(IAsyncResult ar) { //NetworkStream retrieve NetworkStream net = (NetworkStream) ar.IAsyncState; //get number of bytes read int nBytesRead = net.EndRead(ar); //write content to file //... and now, how do I write to FileStream instance without //having its reference?? //fs.Write(buffer,0,nBytesRead); } </code></pre> <p>As you may have noticed, I'm stuck on the callback method, as I don't have a reference to the <code>FileStream</code> instance where I want to invoke the "Write(...)" method.</p> <p>Additionally, this is not a thread-safe solution, as the <code>byte[]</code> field is exposed and may be shared among concurrent <code>NetToFile</code> invocations. I don't know how to solve this problem without exposing this <code>byte[]</code> field in the outer-scope... and I'm almost sure it may not be exposed this way.</p> <p>I don't want to use a lambda or anonymous method solution, because that's not in the curriculum of "Concurrent Programing" course.</p>
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