Timer and Monitor

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Sateesh - 30 Jul 2004 08:25 GMT

Hi,

I have a method that is being called by a timer. I want to lock the entire
section of the code in the method to protect it from being called
simultaneously. So I am using Monitor.Enter(this) and Monitor.Exit(this)
where this is the form object where the method is present.

But what I observed was even after this lock still I see multiple calls
accessing the code. Am I doing something wrong? Is it the way to block a
section of a code in .Net?

Thanks in advance,
Sateesh.

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rajamanickam - 30 Jul 2004 14:25 GMT

Hi Sateesh..

Instead of Monitor class you can lock the block by a boolean variable....

eg

private bool IsAlreadyRunning =false;

private void timer_tick()
{
if(!IsAlreadyRunning)
{
IsAlreadyRunning=true;
your code here;
IsAlreadyRunning=false;
}
}

-rajaManickam

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Jon Skeet [C# MVP] - 31 Jul 2004 07:58 GMT

> Instead of Monitor class you can lock the block by a boolean variable....
>
[quoted text clipped - 11 lines]
> }
> }

No, that's not thread-safe.

1) Different threads may not see updated values, as there's no memory
barrier there

2) Two threads could both get inside the code before either of them
sets IsAlreadyRunning to true.

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Chris Dunaway - 30 Jul 2004 16:14 GMT

> Hi,
>
> I have a method that is being called by a timer. I want to lock the entire
> section of the code in the method to protect it from being called
> simultaneously. So I am using Monitor.Enter(this) and Monitor.Exit(this)

Why not stop the timer at the beginning of the code, when it is complete,
restart the timer.

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Jon Skeet [C# MVP] - 31 Jul 2004 07:59 GMT

> I have a method that is being called by a timer. I want to lock the entire
> section of the code in the method to protect it from being called
[quoted text clipped - 4 lines]
> accessing the code. Am I doing something wrong? Is it the way to block a
> section of a code in .Net?

Which kind of timer are you using? Are you calling Application.DoEvents
in your code?

Could you post a short but complete program which demonstrates the
problem?

See http://www.pobox.com/~skeet/csharp/complete.html for details of
what I mean by that.

See http://www.pobox.com/~skeet/csharp/multithreading.html for general
multi-threading advice.

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Timer and Monitor

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