What happens when a .NET thread throws an exception?

Question

We have an interface IPoller for which we have various implementations. We have a process that will take an IPoller and start it in a separate thread. I'm trying to come up with a generic way of providing exception handling for any IPollers which don't do it themselves.

My original thinking was to create an implementation of IPoller that would accept an IPoller and just provide some logging functionality. The question I ran into though is how would I provide this error handling? If I have IPoller.Start() which is the target for the Thread is that where the exception will occur? Or is there something on the thread itself I can hook into?

Answer 1

You should catch the exception at the method you use at the top of the thread, and do the logging from there.

An unhandled exception (at the top of a thread) will (in 2.0 onwards) kill your process. Not good.

i.e. whatever method you pass to Thread.Start (etc) should have a try/catch, and do something useful in the catch (logging, perhaps graceful shutdown, etc).

To achieve this, you could use:

• static logging methods
• captured variables into the delegate (as an anonymous method)
• expose your method on an instance that already knows about the logger

Answer 2

Something like:

Thread thread = new Thread(delegate() {
    try
    {
        MyIPoller.Start();
    }
    catch(ThreadAbortException)
    {
    }
    catch(Exception ex)
    {
        //handle
    }
    finally
    {
    }
});

This will ensure the exception doesn't make it to the top of the thread.

Answer 3

Take a look at AppDomain.UnhandledException, it will help you at least log those exceptions that you are not handling, and in some cases close down "nicely":

This event provides notification of uncaught exceptions. It allows the application to log information about the exception before the system default handler reports the exception to the user and terminates the application. If sufficient information about the state of the application is available, other actions may be undertaken — such as saving program data for later recovery. Caution is advised, because program data can become corrupted when exceptions are not handled.

Answer 4

In .NET 4.0 you should use Tasks instead of threads. Here's a nice article on exception handling in Task Parallel Library