websocket + TLS under .net Framework 4.6.2

[MikeUwe]

Just want to help others as it took me several days to figure out the problem why TSL worked fine with MQTT protocoll but did not work via websockets.

In case of .net Framework 4.6.2 and older it is required to set DontEnableSchUseStrongCrypto=false
for example in C# code via System.AppContext.SetSwitch("Switch.System.Net.DontEnableSchUseStrongCrypto", false);
This is default in .net Framework 4.7 and newer: see https://learn.microsoft.com/en-us/dotnet/framework/network-programming/tls

I had to add an Logger to catch the exceptions information and forward it into Visual Studio Debug console so to understand what is going wrong here.

Adding the logger is as simple as implementing a small class and add it to the MqttFactory constructor.
Here a simple logger for debugging (not intented to use for real production code of course as it would lower the performance):

sealed class MyLogger : IMqttNetLogger
{
      public bool IsEnabled { get; set; } = true;

      public void Publish( MqttNetLogLevel logLevel, string source, string message, object[] parameters, Exception exception )
      {
          if (logLevel == MqttNetLogLevel.Error)
          {
              if (exception != null)
                  Debug.WriteLine(source + ": " + exception.ToString() );
              else
                  Debug.WriteLine(source + ": " + message);
          }
          else
          {
              if (parameters != null)
                  Debug.WriteLine( source + ": " + String.Format(message, parameters) );
              else
                  Debug.WriteLine( source + ": " + message );
          }
      }
}