MqttClient hangs on ConnectAsync

Describe the bug

IMqttClient instance hangs on ConnectAsync when trying to connect to a MQTT server while internet connection is off.

Which project is your bug related to?

• Client

Basic Information

MQTTnet version: 3.0.11 Xamarin.Forms version: 4.7.0.1179 Platform: Android Device: Samsung SM-G937F, Android 10 - API 29

To Reproduce

Steps to reproduce the behavior:

1. Make sure that internet connection is off on a mobile device
2. Start application
3. Try to connect to a mqtt server
4. Application will hang on ConnectAsync method

Expected behavior

• Client should cancel the operation and throw exception after cancellation is requested

Code example

var serverIpAddress = "176.222.222.222";
var serverPort = 1883;
try
{
    if (!Uri.TryCreate($"http://{serverIpAddress}:{serverPort}", UriKind.Absolute, out var uri) ||
        !IPAddress.TryParse(uri.Host, out _))
    {
        return;
    }

    var clientOptions = new MqttClientOptionsBuilder()
        .WithClientId(Guid.NewGuid().ToString())
        .WithCredentials(string.Empty, string.Empty)
        .WithTcpServer(uri.Host, uri.Port)
        .Build();

    var client = _mqttFactory.CreateMqttClient();

    // Timeout is one second
    using (var cancellationToken = new CancellationTokenSource(TimeSpan.FromSeconds(1)))
    {
        // Client hangs on a line below
        await client.ConnectAsync(clientOptions, cancellationToken.Token).ConfigureAwait(false);
    }
}
catch (Exception e)
{
    // ignored
}

ps. Let me know if you need example application/code 😃

[chkr1011]

Do you have more details or a full solution which has this issue? I also use MQTTnet in a Xamarin application and I cannot reproduce the issue.

What exactly do you mean with “internet connection is off”? Is Wifi AND Cellular just deactivated? Or is no WiFi in range etc?

[chkr1011]

I assume this issue is fixed. If the issue still exists in the latest version 4 please reopen the ticket and try connecting with apps like MQTTnetApp to see if the library is the root cause of this issue.