What's the difference between QueueUserWorkIte

问题:

Following on from my BeginInvoke()/EndInvoke() question, are there major differences in performance/anything else between Delegate.BeginInvoke() and using QueueUserWorkItem() to invoke a delegate asynchronously?

回答1:

http://blogs.msdn.com/cbrumme/archive/2003/07/14/51495.aspx

says:

"One surprising fact is that this is also why Delegate.BeginInvoke / EndInvoke are so slow compared to equivalent techniques like ThreadPool.QueueUserWorkItem (or UnsafeQueueUserWorkItem if you understand the security implications and want to be really efficient). The codepath for BeginInvoke / EndInvoke quickly turns into the common Message processing code of the general remoting pathway."

回答2:

The main thing I can think of with QueueUserWorkItem is that you have to use the WaitCallback delegate type, which looks tricky if you already have a SomeRandomDelegate instance and some args. The good news is that you can fix this with a closure:

ThreadPool.QueueUserWorkItem(
    delegate { someDelegate(arg1, arg2); }
);

This pattern also ensures you get proper strong typing at compile time (unlike passing an object state arg to QueueUserWorkItem and casting it in the target method). This pattern can also be used when calling methods directly:

ThreadPool.QueueUserWorkItem(
    delegate { SomeMethod(arg1, arg2); }
);

Obviously, without an EndInvoke equivalent, you also can't get a return value back out unless you call a method / raise an event / etc at the end of your method... on a related note, you need to be careful with exception handling.

回答3:

The EndInvoke() has a useful but rarely mentioned behavior - it rethrows all unhandled exceptions that the delegate generated in the context of the original thread so you can move the exception processing logic into the main code.

Also, if your delegate has out/ref parameters, they will be added to the EndInvoke() signature allowing you to get them when method finishes execution.

回答4:

If you call ThreadPool.QueueUserWorkItem, exceptions raised in the work item will be unhandled on the background thread (unless you explicitly catch them). In .Net 2 and above this will terminate your AppDomain.

If you call delegate.BeginInvoke() then exceptions are queued to be re-thrown when EndInvoke() is called. If you never call EndInvoke(), then the exceptions are essentially 'leaked' memory (as is any other state not released by the async operation).

回答5:

There should not be any big difference, I also think that the generated BeginInvoke/EndInvoke for a delegate uses the thread pool to execute.

回答6:

There shouldn't be any performance difference, as both Delegate.BeginInvoke and ThreadPool.QueueUserWorkItem will execute on a thread pool thread.

The biggest difference is that if you call BeginInvoke, you're obliged to call EndInvoke at some point. In contrast, ThreadPool.QueueUserWorkItem is "fire and forget". That has benefits and drawbacks. The benefit being that you can forget about it. The drawback being that you have no way of knowing, unless you add your own synchronization/notification mechanism, when the task has completed.