How to check if the WaitHandle was set?

Question

I have a WaitHandle and I would like to know how to check if the WaitHandle has already been set or not.

Note: I can add a bool variable and whenever Set() method is used set the variable to true, but this behaviour must be built in WaitHandle somewhere.

Thanks for help!

My answer was only seconds before SwDevMan's which is much clearer and includes the documentation quote, so I'm deleting it. However, I still wonder "Are you dealing with an auto-reset event that might already have been reset, or that the test code must not reset?" — Ben Voigt, Jul 24 '10 at 18:19

SwDevMan81 · Accepted Answer · 2012-09-20T13:31:14.833

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Try WaitHandle.WaitOne(0)

If millisecondsTimeout is zero, the method does not block. It tests the state of the wait handle and returns immediately.

edited Sep 20 '12 at 13:31

answered Jul 22 '10 at 20:27

SwDevMan81

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+1 Good clear answer. Nice and simple, though I would suggest changing "Try" to "Use". As it stands, you seem unsure of your answer. – Jeff Yates Jul 22 '10 at 20:31
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The only issue is that for some WaitHandles (auto-reset event, semaphore), the ready state will actually be reset by waiting on it. – Ben Voigt Jul 24 '10 at 18:21
@Vivin joy - `millisecondsTimeout` is the parameter name to the `WaitOne` call, please see the link. Its a quote taken from the Remarks section – SwDevMan81 Sep 20 '12 at 13:32
@MartinVseticka Don't be ashamed! I came here with the same question. The answer is perhaps guessable, but by no means self-evident. – Tim Long May 26 '19 at 09:07
It doesn't just check if the wait handle is AutoResetEvent! It actually changes its state to true. – Mohammad Nikravan Sep 27 '21 at 19:25

score 7 · Answer 2 · edited Jun 20 '20 at 09:12

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const int DoNotWait = 0;
                          
ManualResetEvent waitHandle = new ManualResetEvent(false);                   

Console.WriteLine("Is set:{0}", waitHandle.WaitOne(DoNotWait));
         
waitHandle.Set(); 

Console.WriteLine("Is set:{0}", waitHandle.WaitOne(DoNotWait));

Output:

Is set:False

Is set:True

edited Jun 20 '20 at 09:12

Community

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answered Jul 22 '10 at 20:30

Tim Lloyd

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Why use a keyword for the variable? It's unnecessary and makes your answer harder to read. – Jeff Yates Jul 22 '10 at 20:31
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@Jeff Sure, it's just a hard thing to name for some sample code, and I've been writing a lot of code today. – Tim Lloyd Jul 22 '10 at 20:33
@chibacity: it's easy to write something other than event. `waitHandle`, `resetEvent`, `mre`, `myEvent`. The list is endless and it would make your example better. – Jeff Yates Jul 22 '10 at 20:35
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@Jeff You have no idea of my state of mental exhaustion at the moment - but I will comply! :) – Tim Lloyd Jul 22 '10 at 20:37
@chibacity: I can sympathise but if a job's worth doing, it's worth doing well. +1 – Jeff Yates Jul 22 '10 at 20:40
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@Jeff Cheers for the kick up the bum, you are quite right. :) – Tim Lloyd Jul 22 '10 at 20:52

Jeff Yates · Answer 3 · 2010-07-22T20:33:20.657

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Use one of the Wait... methods on WaitHandle that takes a timeout value, such as WaitOne, and pass a timeout of 0.

edited Jul 22 '10 at 20:33

answered Jul 22 '10 at 20:28

Jeff Yates

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score 1 · Answer 4 · answered Jul 22 '10 at 20:28

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You can use the WaitOne(int millisecondsTimeout, bool exitContext) method and pass in 0 for the timespan. It will return right away.

bool isSet = yourWaitHandle.WaitOne(0, true);

answered Jul 22 '10 at 20:28

Mike Dinescu

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Why should they use the one that takes an `exitContext` value? Considering that there are alternatives which do not require this field, you should explain its necessity. – Jeff Yates Jul 22 '10 at 20:34
Online help for VS2005 only shows WaitOne(), WaitOne(int,bool), and WaitOne(TimeSpan,bool). So, it's likely they didn't find WaitOne(int) – Lee Louviere Nov 15 '11 at 16:09

How to check if the WaitHandle was set?

4 Answers4