C#.NET: "Multi Threaded Apartment" and "Multithreading"

Question

I am learning multi threading concepts (in general and targeted to C#.NET). Reading different articles, still could not fully understand few basic concepts.

I post this question. "Hans Passant" explained it well but I was not able to understand some of its part. So I started googling.

I read this question which have no answers.

Is Multithreading and MTA same?

Suppose I write a WinForm application which is STA (as mentioned above its Main() method), still I can create multiple threads in my application. I can safely say my application is "multi-threaded". Does that also mean my application is MTA?

While talking about STA/MTA, most of the articles (like this) talk about COM/DCOM/Automation/ActiveX. Does that mean DotNet have nothing to do with STA/MTA?

Answer 1

No. MTA is a property of a single thread, just like STA. You now make the exact opposite promise, you declare that the thread does absolutely nothing to keep external code thread-safe. So no need to have a dispatcher and you can block as much and as long as you like.

This has consequences of course and they can be quite unpleasant. It is deadly if the UI thread of your program is in the MTA since it uses so many external components that are fundamentally thread-unsafe. The clipboard won't work, drag+drop doesn't work, OpenFileDialog typically just hangs your program, WebBrowser won't fire its events.

Some components check for this and raise an exception but this check isn't consistently implemented. WPF is notable, while apartment state normally matters only to unmanaged code, WPF borrowed the concept and raises "The calling thread must be STA, because many UI components require this." Which is a bit misleading, what it really means is that the thread must have a dispatcher to allow its controls to work. But otherwise consistent with the STA promise.

It can work when the component uses COM and the author has provided a proxy. The COM infrastructure now steps in to make the component thread-safe, it creates a new thread that is STA to give it a safe home. And every method call is automatically marshaled so it runs on that thread, thus providing thread-safety. The exact equivalent of Dispatcher.Invoke() but done entirely automatic. The consequence however is that this is slow, a simple property access that normally takes a few nanoseconds can now take multiple microseconds.

You'd be lucky if the component supports MTA as well as STA. This is not common, only somebody like Microsoft goes the extra thousand miles to keep their libraries thread-safe.

I should perhaps emphasize that the concepts of apartments is entirely missing in the .NET Framework. Other than the basics of stating the apartment type, necessary since .NET programs often need to interop with unmanaged code. So writing a Winforms app with worker threads is just fine, and those worker threads are always in the MTA, you do however get to deal with thread-safety yourself and nothing is automatic.

This is generally well-understood, just about everybody knows how to use the lock keyword, the Task and BackgroundWorker classes and knows that the Control.Begin/Invoke() method is required to update UI from a worker thread. With an InvalidOperationException to remind you when you get it wrong. Leaving it up to the programmer instead of the system taking care of thread-safety does make it harder to use threads. But gives you lots of opportunities to do it better than the system can. Which was necessary, this system-provided thread-safety got a serious black eye when Java punched it in the face during the middleware wars of the late 90s.

Answer 2

There are some questions but first let's start by this:

An Apartment is a context where a COM object is initialized and executed, and it can be a either single thread (STA), normally used for not thread-safe objects, or multi thread.

the term apartment, which describes the constructs in which COM objects are created

From: https://msdn.microsoft.com/en-us/library/ms809971.aspx

So Multithreading and MTA are not the same, but MTA is Multithreaded. We can say that STA and MTA are related to COM objects. You can read more here: https://msdn.microsoft.com/en-us/library/ms693344(v=vs.85).aspx

So, for your second question, if your WinForm application is "multi-threaded" does not mean it is "MTA".

Finally, the MTA/STA concepts are older than .Net technology, but we cannot say that they have nothing related to, because .Net supports COM technology in both STA and MTA.

I expect my answer help you to undestand the difference between Apartment and Threading.

More interesting reading here:Could you explain STA and MTA?