线程需要使用Invoke方法来访问UI控件，但BackGroundworker则不需要。 为什么？

Question

BackgroundWorker can access UI controls directly, but Thread 's cannot, Why?

Isn't BackgroundWorker a thread? If it isn't, what is it? Also, why is direct accessing restricted? Is restricting direct access Microsoft's choosen way for doing something or must it be done this way?

Answer 1

According to MSDN, "The BackgroundWorker class allows you to run an operation on a separate, dedicated thread."

BackgroundWorker acutally uses Form.Invoke() to switch control to the main UI thread to report progress. In the fact BackgroundWorker is just a helper class that you could easily write yourself, it uses a second thread to do the work but then it switches to the main UI thread to access GUI.

Also from MSDN: "You can listen for events that report the progress of your operation and signal when your operation is completed." This means that the background operation runs on a separate thread and you can't access GUI from that thread. But you can access GUI controls from progress event because it is raised on the main UI thread. Whenever you call BackgroundWorker.ReportProgress() on the worker thread, it raises BackgroundWorker.ProgressChanged event on the main thread.

Also, you ask why GUI cannot be accessed from other threads. It is because whole Windows GUI subsystem is not thread safe. As far as I know this is pretty normal in other platforms (outside Windows) as well, so it definitely isn't Microsoft specific. It is probably from historical reasons and probably has something to do with code efficiency. (A thread safe GUI would be slower.) But I can't provide any strong information source as a reference.

Answer 2

A background worker will throw a cross-thread exception just as a new thread would. A backgroundworker is just a fancy way to start a new thread.

If you are using the ProgressChanged event then you do not need to invoke it because there is some .NET magic behind the scenes doing it for you.

More information on background workers: http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker(v=vs.100).aspx