UdpClient.ReceiveAsync correct early termination

Question

Good day. I work with UdpClient and have wrapper upon it.

For reading I have asynchronous method:

private async Task Recei         


        

Answer 1

The only way to cancel a pending receive is to disconnect/stop/dispose as you did. This is correct. You need to catch and ignore that exception.

It is an unfortunate design problem with the .NET Framework that this is the only way to do it.

Note, that WithCancellation does not cancel the IO. The Receive is still running. That's why WithCancellation must be followed by disposing of the socket to make sure that there are no further pending IOs.

Answer 2

And so, after nearly a week of suffering, I have found the reason and solution.

At first, I looked at the UdpClient source code. The ReceiveAsync method:

[HostProtection(ExternalThreading = true)]
public Task<UdpReceiveResult> ReceiveAsync()
{
    return Task<UdpReceiveResult>.Factory.FromAsync((callback, state) => BeginReceive(callback, state), (ar)=>
        {
            IPEndPoint remoteEP = null;
            Byte[] buffer = EndReceive(ar, ref remoteEP);
            return new UdpReceiveResult(buffer, remoteEP);

        }, null);
}

At second, I found this post with perfect answer:How to abort socket's BeginReceive()?, in which said:

To cancel a pending call to the BeginConnect() method, close the Socket. When the Close() method is called while an asynchronous operation is in progress, the callback provided to the BeginConnect() method is called. A subsequent call to the EndConnect(IAsyncResult) method will throw an ObjectDisposedException to indicate that the operation has been cancelled.

And, as we can see, the original ReceiveAsync method return us the ObjectDisposedException, because IOOperation has not been completed after Close invoking.

To overcome this problem I have done like this:

New ReceiveAsync realization:

/// <summary>
/// Асинхронный запрос на ожидание приёма данных с возможностью досрочного выхода
/// (для выхода из ожидания вызовите метод Disconnect())
/// </summary>
/// <param name="client">Рабочий экземпляр класса UdpClient</param>
/// <param name="breakToken">Признак досрочного завершения</param>
/// <returns>Если breakToken произошёл до вызова данного метода или в режиме ожидания
/// ответа, вернёт пустой UdpReceiveResult; при удачном получении ответа-результат
/// асинхронной операции чтения</returns>
public Task<UdpReceiveResult> ReceiveAsync(UdpClient client, CancellationToken breakToken)
    => breakToken.IsCancellationRequested
        ? Task<UdpReceiveResult>.Run(() => new UdpReceiveResult())
        : Task<UdpReceiveResult>.Factory.FromAsync(
            (callback, state) => client.BeginReceive(callback, state),
            (ar) =>
                {
                    /// Предотвращение <exception cref="ObjectDisposedException"/>
                    if (breakToken.IsCancellationRequested)
                        return new UdpReceiveResult();

                    IPEndPoint remoteEP = null;
                    var buffer = client.EndReceive(ar, ref remoteEP);
                    return new UdpReceiveResult(buffer, remoteEP);
                },
            null);

New Dispose realization:

protected virtual void Dispose(bool disposing)
{
    if (disposing)
    {
        this.cancelReceive?.Cancel();
        this.client?.Close();
        this.cancelReceive?.Dispose();
    }
}

I very much hope, that my decision will deprive someone else of the pain that I experienced.