Keeping boolean values in sync between two threads

Question

In one thread (thread2) I have changed a value e.g.

CheckSuccess = false;

Now the main thread (thread1 - GUI / Form) does not pickup the change, how would it be possible to "propogate" the changes around all threads?

I was under the impression that threads should manipulate the data, not work on seperate instances (unless told to do so)

Answer 1

It seems like a Race Condition. To avoid this you should synchronize an access to the shared variables.

• If CheckSuccess is a field - try out marking it by volatile keyword.

• If CheckSuccess is a property (which will be translated to a method call) you can use lock() statement:

  private static readonly object stateLock = new object();
  lock (stateLock)
  {
      // access a shared variable here
      CheckSuccess  = false;
  }
  

• If CheckSuccess is a property of UI control and you want changing it from a worker thread - you should use special techniques to push changes from a worker to the UI thread, it depends on which framework you are using WinForms of WPF.

  • WinForms - How to update GUI from another thread in C#?
  • WPF - Update WPF controls at run time

PS: Also if you have multiple threads reading a value and few threads writing (basically reads more often than writing) you may find useful ReaderWriterLock

Answer 2

I normally use Interlocked methods such as Exchange. This is much simpler than using a locking object and it's guaranteed to be atomic.

Answer 3

The problem is most likely compiler optimizations causing the caching of the boolean value in a CPU register. Declaring it as volatile should solve your problem.