Threading in Form_Load - Application now hangs suddenly

Question

Sorry for the lengthy post, I just want to illustrate my situation as best as possible. Read the items in bold and check the code if you want the quick gist of the issue.

I use ClickOnce to deploy a C# application, and have opted to have my application check for updates manually using the ApplicationDeployment Class rather than letting it do the update checking for me.

The program is a specialized network scanner that searches for network devices made by the company I work for. Once the main window is loaded, a prompt is displayed asking if the user would like to scan the network. If they say Yes, a scan begins which can take a minute or two to complete depending on their network settings; otherwise it just waits for the user to do some action.

One of the last things I do in Form_Load is create a new thread that checks for updates. This had all been working fine for several months through about 12 releases and has suddenly stopped working. I didn't change the update code at all, nor change the sequence of what happens when the app starts.

In staring at the code, I think I see why it is not working correctly and wanted to confirm if what I think is correct. If it is, it begs the question as to why it DID work before - but I'm not too concerned with that either.

Consider the following code:

frmMain.cs

private void Form1_Load(object sender, EventArgs e)
{
    // set up ui, load settings etc

    Thread t = new Thread(new ParameterizedThreadStart(StartUpdateThread));
    t.Start(this);
}

private void StartUpdateThread(object param)
{
    IWin32Window owner = param as IWin32Window;

    frmAppUpdater.CheckForUpdate(owner);
}

frmAppUpdater.cs

public static void CheckForUpdate(IWin32Window owner)
{
    if (ApplicationDeployment.IsNetworkDeployed) {
        Console.WriteLine("Going to check for application updates.");
        parentWindow = owner;

        ApplicationDeployment ad = ApplicationDeployment.CurrentDeployment;
        ad.CheckForUpdateCompleted += new CheckForUpdateCompletedEventHandler(ad_CheckForUpdateCompleted);
        ad.CheckForUpdateProgressChanged += new DeploymentProgressChangedEventHandler(ad_CheckForUpdateProgressChanged);

        ad.CheckForUpdateAsync();
        // CAN/WILL THE THREAD CREATED IN FORM1_LOAD BE TERMINATED HERE???
    }
}

When the CheckForUpdateAsync() callback completes, if no update is available the method call simply returns; if an update IS available, I use a loop to block until 2 things occur: The user has dismissed the "Would you like to scan prompt" AND no scan is currently running.

The loop looks like this, which takes place in ad_CheckForUpdateCompleted:

while (AppGlobals.ScanInProgress || AppGlobals.ScanPromptVisible) {
    System.Threading.Thread.Sleep(5000);
}

I sleep for 5 seconds because I figured this was happening in a separate thread and it has seemed to work well for a while.

My main question about the above code is:
When ad.CheckForUpdateAsync(); is called from CheckForUpdate does the thread I created in Form1_Load terminate (or might it terminate)? I suspect it may because the subsequent Async call causes the method to return, and then start another thread?

The only reason I am confused is because this method WAS working for so long without hanging the application and now all of the sudden it hangs and my best effort at debugging revealed that it was that Sleep call blocking the app.

I'd be happy to post the full code for frmAppUpdater.cs if it would be helpful.

Answer 1

When ad.CheckForUpdateAsync(); is called from CheckForUpdate does the thread I created in Form1_Load terminate (or might it terminate)?

If the CheckForUpdateAsync() call is asynchronous then yes, the thread will terminate, no it won't otherwise.

If you suspect the Sleep to have caused the application hang then these two variables AppGlobals.ScanInProgress and AppGlobals.ScanPromptVisible are probably always set to true! You should start looking at the code that is setting them to true and see what is going on there.

In order to avoid an application hang, you could introduce a variable to avoid sleeping indefinitely:

int nTrials = 0;
while ((AppGlobals.ScanInProgress || AppGlobals.ScanPromptVisible) && (nTrials < 5)) {
    System.Threading.Thread.Sleep(5000);
    nTrials++;
}
// Check the results and act accordingly

I personally do not like using Sleep for thread synchronization. .NET offers a bunch of classes that are perfect for thread synchronization, WaitHandle being one of them.

Answer 2

See this post at Asynchronous Delegates Vs Thread/ThreadPool?

your form load method seems to be doing synchronous work. you mention that you are using clickonce deployment. Has the binary location changed after the previous release or has permissions on this resource changed. Looks like the work (checkupdates) in the Thread is never finishing and is never handed back to the form.

as an immediate fix, I would change the Thread approach to Delegate - if you use delegate, then this becomes less of a customer issue (the form will respond to end user) but the underlying problem remains.

as the next step, i would go through http://msdn.microsoft.com/en-us/library/ms229001.aspx and do the troubleshoot