.NET Asynchronous stream read/write

Question

Hi, folks. I would really appreciate to have your help on the following issue:

I have been trying to solve this "Concurrent Programming" exam exercise (in C#):

Knowing that Stream class contains int Read(byte[] buffer, int offset, int size) and void Write(byte[] buffer, int offset, int size) methods, implement in C# the NetToFile method that copies all data received from NetworkStream net instance to the FileStream file 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 net read operation returns value 0. To simplify, it is not necessary to support controlled cancel of the operation.

void NetToFile(NetworkStream net, FileStream file);

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:

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 > 0);
}

The question I have is that, in the question statement, is said:

To do the transfer, use asynchronous reads and synchronous writes, avoiding one thread to be blocked during read operations

I'm not really sure if my solution accomplishes what is wanted in this exercise, because I'm using AsyncWaitHandle.WaitOne() to wait until the asynchronous read completes.

On the other side, I'm not really figuring out what is meant to be a "non-blocking" solution in this scenario, as the FileStream write is meant to be made synchronously... and to do that, I have to wait until NetworkStream read completes to proceed with the FileStream writing, isn't it?

Can you, please, help me out with this? Thanks in advance for your colaboration.

---

[ EDIT 1 ] Using callback solution

Ok, if I understood what Mitchel Sellers and willvv replied (thank you guys), I've been counseled to use a callback method to turn this into a "non-blocking" solution. Here is my code, then...

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
    file.BeginRead(buffer,0,file.Length,OnEndRead,fs);
}

//asynchronous callback
static void OnEndRead(IAsyncResult ar) {
    //NetworkStream retrieve
    NetworkStream ns = (NetworkStream) ar.IAsyncState;
    //get number of bytes read
    int nBytesRead = net.EndRead(ar);

    //write content to file
    //... and know, how do I write to FileStream instance without
    //having it's referente??
    //fs.Write(buffer,0,nBytesRead);
}

As you may have noticed, I'm stuck on the callback method, as I don't have a reference to the FileStream instance where I want to invoke the "Write(...)" method.

Additionally, this is not a thread-safe solution, as the byte[] field is exposed and may be shared among concurrent NetToFile invocations. I don't know how to solve this problem without exposing this byte[] field in the outer-scope... and I'm almost sure it may not be exposed this way.

I don't want to use a lambda or anonymous method solution, because that's not in the curriculum of "Concurrent Programing" course.

Answer 1

To give you a little bit of a hint here, you are going to need to use a callback to handle the write operation, that way you are not blocking the thread while the read is occurring.

Answer 2

Hi

You're right, what you're doing is basically synchronous reading, because you use the WaitOne() method and it just stops the execution until the data is ready, that's basically the same as doing it using Read() instead of BeginRead() and EndRead().

What you have to do, is use the callback argument in the BeginRead() method, with it, you define a callback method (or a lambda expression), this method will be invoked when the information has been read (in the callback method you have to check for the end of the stream, and write to the output stream), this way you won't be blocking the main thread (you won't need the WaitOne() nor the EndRead().

Hope this helps.

Answer 3

You are going to need to use the callback from the NetStream read to handle this. And frankly it might be easier to wrap the copying logic into its own class so that you can maintain the instance of the active Streams.

This is how I'd approach it (not tested):

public class Assignment1
{
    public static void NetToFile(NetworkStream net, FileStream file) 
    {
        var copier = new AsyncStreamCopier(net, file);
        copier.Start();
    }

    public static void NetToFile_Option2(NetworkStream net, FileStream file) 
    {
        var completedEvent = new ManualResetEvent(false);

        // copy as usual but listen for completion
        var copier = new AsyncStreamCopier(net, file);
        copier.Completed += (s, e) => completedEvent.Set();
        copier.Start();

        completedEvent.WaitOne();
    }

    /// <summary>
    /// The Async Copier class reads the input Stream Async and writes Synchronously
    /// </summary>
    public class AsyncStreamCopier
    {
        public event EventHandler Completed;

        private readonly Stream input;
        private readonly Stream output;

        private byte[] buffer = new byte[4096];

        public AsyncStreamCopier(Stream input, Stream output)
        {
            this.input = input;
            this.output = output;
        }

        public void Start()
        {
            GetNextChunk();
        }

        private void GetNextChunk()
        {
            input.BeginRead(buffer, 0, buffer.Length, InputReadComplete, null);
        }

        private void InputReadComplete(IAsyncResult ar)
        {
            // input read asynchronously completed
            int bytesRead = input.EndRead(ar);

            if (bytesRead == 0)
            {
                RaiseCompleted();
                return;
            }

            // write synchronously
            output.Write(buffer, 0, bytesRead);

            // get next
            GetNextChunk();
        }

        private void RaiseCompleted()
        {
            if (Completed != null)
            {
                Completed(this, EventArgs.Empty);
            }
        }
    }
}