Please tell me that I did not just read the words single-threaded apartment! Isn’t that a legacy COM thing?
Yes, apartments are a COM mechanism. And like previous Win32-based user interface frameworks (including Windows Forms), WPF requires the main thread to live in a single threaded apartment. This is mainly the case to enable seamless interoperability with non-WPF technologies (the topic of Chapter 19, “Interoperability with Non-WPF Technologies”).
But even without the interoperability requirement, the STA model—in which developers don’t need to worry about correctly handling calls from arbitrary threads—is valuable for making programming with WPF easier. When an object is created on an STA thread, it can be called only on that same thread.
WPF enforces that many of its APIs (on DispatcherObject-derived classes) are called from the correct thread by throwing an exception if the call comes from any other thread. That way, there’s no chance of accidentally calling such members from the wrong thread and only seeing intermittent failures (which can be incredibly hard to debug). At the same time, WPF provides an easy mechanism for multiple threads to communicate with the UI thread, as discussed in a later sidebar.
But even without the interoperability requirement, the STA model—in which developers don’t need to worry about correctly handling calls from arbitrary threads—is valuable for making programming with WPF easier. When an object is created on an STA thread, it can be called only on that same thread.
WPF enforces that many of its APIs (on DispatcherObject-derived classes) are called from the correct thread by throwing an exception if the call comes from any other thread. That way, there’s no chance of accidentally calling such members from the wrong thread and only seeing intermittent failures (which can be incredibly hard to debug). At the same time, WPF provides an easy mechanism for multiple threads to communicate with the UI thread, as discussed in a later sidebar.
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