In .NET, what thread will Events be handled in?

After re-reading the question, I think I understand the problem now. You've basically got something like this:

class Producer{    public Producer(ExternalSource src)    {        src.OnData += externalSource_OnData;    }    private void externalSource_OnData(object sender, ExternalSourceDataEventArgs e)    {        // put e.Data onto the queue    }}

And then you've got a consumer thread that pulls stuff off that queue. The problem is that the OnData event is fired by your ExternalSource object - on whatever thread it happens to be running on.

C# events are basically just an easy-to-use collection of delegates and "firing" an event just causes the runtime to loop through all of the delegates and fire them one at a time.

So your OnData event handler is getting called on whatever thread the ExternalSource is running on.

Unless you do the marshaling yourself, an event will execute on whatever thread is invoking it; there's nothing special about the way events are invoked, and your producer thread doesn't have an event handler, your producer thread simply said "hey, when you fire this event, call this function". There's nothing in there that causes the event execution to occur on the attaching thread, nor on its own thread (unless you were to use BeginInvoke rather than invoking the event's delegate normally, but this will just execute it on the ThreadPool).

Raising an event with Invoke is the same as calling a method - it gets executed in the same thread you raised it.

Raising an event with BeginInvoke uses ThreadPool. Here are some minor details