Why use finally block to prevent reentrancy in async code ?

dars · September 25th, 2015, 08:05 AM

Hi,

I am trying to understand the microsoft documentation on how to handle reentrancy issues. One of the methods it recommends is to disable the button so the user can't start another async operation before the original one is finished. The code microsoft provides is below:

Code:

private async void StartButton_Click(object sender, RoutedEventArgs e)
{
    // This line is commented out to make the results clearer in the output.
    //ResultsTextBox.Text = "";

    // ***Disable the Start button until the downloads are complete. 
    StartButton.IsEnabled = false; 

    try
    {
        await AccessTheWebAsync();
    }
    catch (Exception)
    {
        ResultsTextBox.Text += "\r\nDownloads failed.";
    }
    // ***Enable the Start button in case you want to run the program again. 
    finally
    {
        StartButton.IsEnabled = true;
    }
}

My question is why do we have to put StartButton.IsEnabled = true in a finally block ? Can't we put it right after the await statement ? I thought await prevented further code from executing in an async method unil the task was completed.

juliancorrea · October 6th, 2015, 04:56 PM

I understand that if you put "StartButton.IsEnabled = true" above the await statement, in try block, occuring an exception the StartButton never will be enabled again.
In Finally block is the best practice, because independent of any exception occur, the StartButton will enabled for a new action.

Regards

lucygrey · December 15th, 2015, 05:17 AM

Use the following steps to prevent the reentrancy in async code :-

1. Recognize Reentrancy

2. Handling Reentrancy

Disable the Start Button

Cancel and Restart the Operation

Run Multiple Operations and Queue the Output

3. Reviewing and Running the Example App

Thanks!!