Are c# events handled serially or in parallel?

Question

For example, I have a class that fires an event, and 1000 subscribers to that event. Is a single thread used to fire each subscriber delegate one-by-one? Or does .Net use a thread pool to process some or all of the subscriptions in parallel?

Answer 1

As Tigran said, event invocation is serial. Even more if one of the subscribers throws an exception at some point the rest of them will not be triggered.

The easiest way to trigger an event in parallel will be

    public event Action Event;

    public void Trigger()
    {
        if (Event != null)
        {
            var delegates = Event.GetInvocationList();
            Parallel.ForEach(delegates, d => d.DynamicInvoke());
        }
    }

This implementation will suffer from same problem in case of an exception.

Answer 2

As is, event are simple serial invocation. If you want you can run it in async way, but this is an implementation detail.

In short: there is no any built-in parallelization or async of standard .NET events, it's up to you to implement it.