How to set the timeout for a TcpClient?

问题:

I have a TcpClient which I use to send data to a listener on a remote computer. The remote computer will sometimes be on and sometimes off. Because of this, the TcpClient will fail to connect often. I want the TcpClient to timeout after one second, so it doesn't take much time when it can't connect to the remote computer. Currently, I use this code for the TcpClient:

try
{
    TcpClient client = new TcpClient("remotehost", this.Port);
    client.SendTimeout = 1000;

    Byte[] data = System.Text.Encoding.Unicode.GetBytes(this.Message);
    NetworkStream stream = client.GetStream();
    stream.Write(data, 0, data.Length);
    data = new Byte[512];
    Int32 bytes = stream.Read(data, 0, data.Length);
    this.Response = System.Text.Encoding.Unicode.GetString(data, 0, bytes);

    stream.Close();
    client.Close();    

    FireSentEvent();  //Notifies of success
}
catch (Exception ex)
{
    FireFailedEvent(ex); //Notifies of failure
}

This works well enough for handling the task. It sends it if it can, and catches the exception if it can't connect to the remote computer. However, when it can't connect, it takes ten to fifteen seconds to throw the exception. I need it to time out in around one second? How would I change the time out time?

回答1:

You would need to use the async BeginConnect method of TcpClient instead of attempting to connect synchronously, which is what the constructor does. Something like this:

var client = new TcpClient();
var result = client.BeginConnect("remotehost", this.Port, null, null);

var success = result.AsyncWaitHandle.WaitOne(TimeSpan.FromSeconds(1));

if (!success)
{
    throw new Exception("Failed to connect.");
}

// we have connected
client.EndConnect(result);

回答2:

Starting with .NET 4.5, TcpClient has a cool ConnectAsync method that we can use like this, so it's now pretty easy:

var client = new TcpClient();
if (!client.ConnectAsync("remotehost", remotePort).Wait(1000))
{
    // connection failure
}

回答3:

Another alternative using https://stackoverflow.com/a/25684549/3975786:

var timeOut = TimeSpan.FromSeconds(5);     
var cancellationCompletionSource = new TaskCompletionSource<bool>();
try
{
    using (var cts = new CancellationTokenSource(timeOut))
    {
        using (var client = new TcpClient())
        {
            var task = client.ConnectAsync(hostUri, portNumber);

            using (cts.Token.Register(() => cancellationCompletionSource.TrySetResult(true)))
            {
                if (task != await Task.WhenAny(task, cancellationCompletionSource.Task))
                {
                    throw new OperationCanceledException(cts.Token);
                }
            }

            ...

        }
    }
}
catch(OperationCanceledException)
{
    ...
}

回答4:

One thing to take note of is that it is possible for the BeginConnect call to fail before the timeout expires. This may happen if you are attempting a local connection. Here's a modified version of Jon's code...

        var client = new TcpClient();
        var result = client.BeginConnect("remotehost", Port, null, null);

        result.AsyncWaitHandle.WaitOne(TimeSpan.FromSeconds(1));
        if (!client.Connected)
        {
            throw new Exception("Failed to connect.");
        }

        // we have connected
        client.EndConnect(result);

回答5:

The answers above don't cover how to cleanly deal with a connection that has timed out. Calling TcpClient.EndConnect, closing a connection that succeeds but after the timeout, and disposing of the TcpClient.

It may be overkill but this works for me.

    private class State
    {
        public TcpClient Client { get; set; }
        public bool Success { get; set; }
    }

    public TcpClient Connect(string hostName, int port, int timeout)
    {
        var client = new TcpClient();

        //when the connection completes before the timeout it will cause a race
        //we want EndConnect to always treat the connection as successful if it wins
        var state = new State { Client = client, Success = true };

        IAsyncResult ar = client.BeginConnect(hostName, port, EndConnect, state);
        state.Success = ar.AsyncWaitHandle.WaitOne(timeout, false);

        if (!state.Success || !client.Connected)
            throw new Exception("Failed to connect.");

        return client;
    }

    void EndConnect(IAsyncResult ar)
    {
        var state = (State)ar.AsyncState;
        TcpClient client = state.Client;

        try
        {
            client.EndConnect(ar);
        }
        catch { }

        if (client.Connected && state.Success)
            return;

        client.Close();
    }

回答6:

Set ReadTimeout or WriteTimeout property on the NetworkStream for synchronous reads/writes. Updating OP's code:

try
{
    TcpClient client = new TcpClient("remotehost", this.Port);
    Byte[] data = System.Text.Encoding.Unicode.GetBytes(this.Message);
    NetworkStream stream = client.GetStream();
    stream.WriteTimeout = 1000; //  <------- 1 second timeout
    stream.ReadTimeout = 1000; //  <------- 1 second timeout
    stream.Write(data, 0, data.Length);
    data = new Byte[512];
    Int32 bytes = stream.Read(data, 0, data.Length);
    this.Response = System.Text.Encoding.Unicode.GetString(data, 0, bytes);

    stream.Close();
    client.Close();    

    FireSentEvent();  //Notifies of success
}
catch (Exception ex)
{
    // Throws IOException on stream read/write timeout
    FireFailedEvent(ex); //Notifies of failure
}

回答7:

Here is a code improvement based on mcandal solution. Added exception catching for any exception generated from the client.ConnectAsync task (e.g: SocketException when server is unreachable)

var timeOut = TimeSpan.FromSeconds(5);     
var cancellationCompletionSource = new TaskCompletionSource<bool>();

try
{
    using (var cts = new CancellationTokenSource(timeOut))
    {
        using (var client = new TcpClient())
        {
            var task = client.ConnectAsync(hostUri, portNumber);

            using (cts.Token.Register(() => cancellationCompletionSource.TrySetResult(true)))
            {
                if (task != await Task.WhenAny(task, cancellationCompletionSource.Task))
                {
                    throw new OperationCanceledException(cts.Token);
                }

                // throw exception inside 'task' (if any)
                if (task.Exception?.InnerException != null)
                {
                    throw task.Exception.InnerException;
                }
            }

            ...

        }
    }
}
catch (OperationCanceledException operationCanceledEx)
{
    // connection timeout
    ...
}
catch (SocketException socketEx)
{
    ...
}
catch (Exception ex)
{
    ...
}

回答8:

If using async & await and desire to use a time out without blocking, then an alternative and simpler approach from the answer provide by mcandal is to execute the connect on a background thread and await the result. For example:

Task<bool> t = Task.Run(() => client.ConnectAsync(ipAddr, port).Wait(1000));
await t;
if (!t.Result)
{
   Console.WriteLine("Connect timed out");
   return; // Set/return an error code or throw here.
}
// Successful Connection - if we get to here.

See the Task.Wait MSDN article for more info and other examples.

回答9:

As Simon Mourier mentioned, it's possible to use ConnectAsync TcpClient's method with Task in addition and stop operation as soon as possible.
For example:

// ...
client = new TcpClient(); // Initialization of TcpClient
CancellationToken ct = new CancellationToken(); // Required for "*.Task()" method
if (client.ConnectAsync(this.ip, this.port).Wait(1000, ct)) // Connect with timeout as 1 second
{

    // ... transfer

    if (client != null) {
        client.Close(); // Close connection and dipose TcpClient object
        Console.WriteLine("Success");
        ct.ThrowIfCancellationRequested(); // Stop asynchronous operation after successull connection(...and transfer(in needed))
    }
}
else
{
    Console.WriteLine("Connetion timed out");
}
// ...