Question

I am trying to catch exceptions for my form client not being able to establish a connection to a server with this in the Connect callback:

try
{
    client.EndConnect(async);
}
catch (Exception e)
{
    client.Close();

    return;
}

This works fine but this behavior is encapsulated in to a class so I want to call throw; instead of return; so that the client class can handle it instead, like so:

try
{
    client.Connect(host, port);
}
catch
{
    Console.WriteLine("Could not connect to: " + host + ":" + port.ToString());
}

So why not just call throw; then? Well, for some reason if I call throw;, throw new Exception();, or basically anything other than return; the program failsfast. I’m really not sure what’s causing this. I tried removing client.Close(); to see if it was the problem but nothing. If I don’t call return; the program just immediately exits with no error.

Anyone know what’s going on here?

Edit: I do not understand why I am getting downvoted so much. I showed how I am attempting to catch these exceptions and am asking why they are not working properly. I think the problem may be (not sure, just came up with this) because within the asynchronous callback, because it is a new thread in the ThreadPool, calling throw; does not do anything because, because it is not synchronous, there is nothing to throw back to and the application dies. Even with this knowledge, I am not sure how to solve this problem unless I put some sort of try-catch on the entire program.

I suppose a solution could be just sticking with return; because there is nothing to throw back to (due to the asynchronous callback nature of the method) and instead raise an event indicating a failure of connection. Regardless, many thanks for the downvotes and helping me solve this problem. Oh wait…

Answer 1

What’s happening is that the EndConnect is not happening on the same thread as your BeginConnect. When EndConnect throws an exception, it is caught by the worker thread’s unhandled exception handler, which fails fast (the other option is that it gets ignored and you never find out that your code isn’t working).

You have to come up with a way to tell your main form thread that the connect failed.