Do I need to Dispose() or Close() an EventWaitHandle?

Question

If I am using EventWaitHandle (or AutoResetEvent , ManualResetEvent ) to synchronise between threads then do I need to call the Close() or Dispose() methods on that event handle when I am done with it?

EventWaitHandle inherits from WaitHandle , which implements IDisposable . And FxCop complains if I don't implement IDisposable on any class that contains an EventWaitHandle . So this suggests that I do need to call it.

However none of these MSDN usage examples call Dispose() or Close() :

http://msdn.microsoft.com/en-us/library/system.threading.eventwaithandle(VS.80).aspx http://msdn.microsoft.com/en-us/library/system.threading.manualresetevent(VS.80).aspx http://msdn.microsoft.com/en-us/library/system.threading.autoresetevent(VS.80).aspx

Is this just an example of Microsoft ignoring their own advice?

Answer 1

The disposable resource of an EventWaitHandle is actually a SafeHandle (wrapped in a SafeWaitHandle ). SafeHandle implements a finalizer, which eventually makes sure the necessary resource is release, so it should be safe to let the garbage collector / finalizer thread handle it in this case .

However, it is always a good idea to explicitly call Dispose() when the resource is no longer needed.

The threading chapter in C# 3.0 in a Nutshell states

This practice is (arguably) acceptable with wait handles because they have a light OS burden (asynchronous delegates rely on exactly this mechanism to release their IAsyncResult 's wait handle).

Answer 2

You need to dispose them explicitly. Close() is more appropriate for them as it does call Dispose().

Answer 3

Class definitions from MSDN:

public class EventWaitHandle : WaitHandle
public abstract class WaitHandle : MarshalByRefObject, IDisposable

So yes you must as WaitHandle is IDisposable. FxCop would find this as a rule violation if you didn't.

Answer 4

Close method disposes it internally.

Answer 5

MSDN says :

Always call Close or Dispose() before you release your last reference to the WaitHandle. Otherwise, the resources it is using will not be freed.

So, as they inherit WaitHandle , in my opinion, you should dispose the EventWaitHandle (or AutoResetEvent , ManualResetEvent ) too