What is the exitContext used for on a WaitHandle.WaitOne method

I can’t claim credit (hence wiki) but there is a good answer here.

From the link, answer to the question, by Sasha Goldshtein

The CLR has support for a notion called contexts. A context is a
logical grouping of objects. When a method call is made on an object
that is inside your context, nothing in particular happens except for
the method itself. When a method call is made on an object that is
outside your context, then “something” might happen before the method
itself is executed. All .NET classes derived from ContextBoundObject
will be associated with a context at runtime.

An example of that “something” that can happen before the method is
called would be enforcing synchronization. An object that derives
from ContextBoundObject and specifies the [Synchronization] attribute
will get automatic synchronization services from the runtime. This
means that only one thread will ever be able to execute within the
object’s context.

The exitContext parameter of the WaitOne method specifies whether to
leave the synchronization context before issuing the wait. This
enables reentrancy. Here’s a scenario where this is necessary:

Code Snippet

[Synchronization]
public class MyCounter : ContextBoundObject {
 
  private int _expectedCounterVal;
  private int _currentCounterVal; 
  private ManualResetEvent _event = new ManualResetEvent(false);

  public void WaitUntilCounterIs(int counterVal) {  
    _expectedCounterVal = counterVal;
    _event.WaitOne(TimeSpan.FromDays(1), true);
  }

  public void IncrementCounter() {
    if (++_currentCounterVal >= _expectedCounterVal) {    
      _event.Set();
    }
  }
}

In this case, if the WaitUntilCounterIs method is issued without the
exitContext=true parameter of the WaitOne method, then no other thread
will be able to ever call the IncrementCounter method on the object,
resulting in a deadlock. However, once exitContext=true is specified,
other threads can enter the IncrementCounter method and signal the
event at some point, even though the WaitUntilCounterIs method has not
yet returned.