CODEWITHSUNDEEP
c#.netmultithreadingwinformsthread-safety
colosso
colosso

Reputation: 2525

Method called twice in same moment

I'm working on a windows forms application and fighting with a very harsh error. The application is supposed to run on a local machine and handle requests form a server applicaton. The client application looks like this:

public Reader mr_obj;

public Form1()
{
    mr_obj = new MyReader.Reader(7137);
    mr_obj.UserEvent += new ReaderEvent(UserEvent);
}

private void UserEvent(UserEvent e, long threadID)
{
   Thread.Sleep(1000);
   SafeSomethingToDB();
}

The Reader() object is connecting the client application to the server application. So after this, the server application is able to trigger the UserEvent() method in the client application. Ther problem is now, that the client application, which handles the UserEvents, crashes if the UserEvent() method gets triggered twice within one second. (Its actually not crashing just hanging untill you kill the task, a try catch wont return an error)

What I've tried so far is to delegate the Thread.Sleep() and SafeSomethingToDB() to another thread. This doesnt work because the server application does not wait until the tread is finished. So the server application does not find the data in the DB because its not waiting 1 second... The same problem happens when I did that with background workers.

Is there a possibility to handle these two triggers, which come from the same server application, in sort of a parallell way at the same time?

Any suggestions very apreciated

EDIT: I think locking the method does not cause the application to process both triggers in the same time. To make this visible I'v tried this:

private void UserEventHandler(UserEvent e, long threadID)
        {
            lock (_lockObject)
            {
                MessageBox.Show("Messagebox 1");
                MessageBox.Show("Messagebox 2");
            }
        }

When the first request triggers UserEvent() "MessageBox1" appeares. If you press OK, "MessageBox2" appeares. But if the UserEvent gets triggered a second time while "Messagebox2" is still opened, "MessageBox1" does not appear. Instead of that the application start hanging. Shouldn "MessageBox1" appear again triggered by the second trigger of UserEvent() when the two triggers really ar bbeing processed at the same time? So the two triggers are not beeing preformed parallel or am I mistaking here?

Upvotes: 1

Views: 1688

Answers (3)

Roy Dictus
Roy Dictus

Reputation: 33149

As you write in the comments, if SafeSomethingToDB() is called a second time before the first call has finished, then it crashes. So in other words: SafeSomethingToDB() is not re-entrant.

What you can do is use a Mutex (which stands for mutual exclusion), which defines a "critical section" in your code, meaning a code that can have only one thread executing it at any one time.

For instance:

private static Mutex mutex = new Mutex();

public void SafeSomethingToDB()
{
    mutex.WaitOne(); // wait until it is safe to enter the critical section

    // Critical section begins here
    DoWorkAndStuff();

    mutex.ReleaseMutex(); // indicate the end of the critical section
}

For more about System.Threading.Mutex, see http://msdn.microsoft.com/en-us/library/system.threading.mutex(v=vs.110).aspx.

Upvotes: 0

Swift
Swift

Reputation: 1881

I think a simple lock for synchronization will work for you, try this

public Reader mr_obj;
private static readonly object sync = new object();
public Form1()
{
    mr_obj = new MyReader.Reader(7137);
    mr_obj.UserEvent += new ReaderEvent(UserEvent);
}

private void UserEvent(UserEvent e, long threadID)
{
   lock(sync)
   {
      SafeSomethingToDB();
   }
}

Upvotes: 2

Sebastian Negraszus
Sebastian Negraszus

Reputation: 12215

Without knowing why you do the Sleep or what exactly SafeSomethingToDB does and what causes your problems, try to synchronize the calls:

private readonly object _lockObject = new object();

private void UserEvent(UserEvent e, long threadID)
{
   lock(_lockObject)
   {
       Thread.Sleep(1000);
       SafeSomethingToDB();
   }
}

Upvotes: 2

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