Modify a var at the same time

Question

Hi there,
I create a timer and worker. What will be the result if they try to modify a var at the same time?
e.g.
var = 10
timer { var--; }
worker { var++; }

Thank you.

Answer 1

I think that the behaviour is undefined because the "++" and "--" operators are not atomic (https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/variables#96-atomicity-of-variable-references). You should use Interlocked.Increment and Interlocked.Decrement. Then the variable will contain the result of the most recent thread (that waited for completion of operation performed by other threads).

Answer 2

Hi,@Leon NG . Welcome Microsoft Q&A.
For the problem of modifying var at the same time, here is an example of Timer and Worker. And you can see the resulting graph at the end.

Xaml:

<StackPanel>  
        <TextBox Text="{Binding Num,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Width="100" Height="50" Background="AliceBlue"/>  
        <ListBox x:Name="lb1" Width="100" Height="100"/>  
        <Button x:Name="btn" Content="timer" Click="btn_Click" Height="50" Width="100" />  
        <Button x:Name="btn1" Content="worker" Click="btn1_Click" Height="50" Width="100" />  
    </StackPanel>  

Codebehind:

253680-timerand-worker.txt
The result:
Set the number of worker job(loop) to 5 for testing, if more than 5 times, only the timer is working.

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