How to lock a variable used in multiple threads

Question

I have asked a question badly over here Lock on a variable in multiple threads so for clarity I am going to ask it here and hope I can ask it correctly.

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Answer 1

I've written a blog post about having multiple threads add values to a list and using lock() to prevent the writes from colliding along with why this needs to be done.

Answer 2

Use the lock keyword to guard code that can be executed simultaneously by more than one thread.

public class ClassA
{
  private ClassB b = new ClassB();

  public void MethodA()
  {
    lock (b)
    {
      // Do anything you want with b here.
    }
  }

  public void MethodB()
  {
    lock (b)
    {
      // Do anything you want with b here.
    }
  }
}

It is important to note that lock does not guard or lock the object instance used in the statement. Instead, the object is used as a way to identify a section of code that should be prevented from executing if another section using the same object instance is already executing. In other words, you could use any object instance you like in the lock statements and it would still be safe to access members of ClassB.

Answer 3

ClassB shouldn't be exposing a variable (by which you probably mean data member). Expose a property or set of methods instead, and use ReaderWriterLockSlim to handle multiple threads.

Answer 4

The easiest solution: Don't share the instance of ClassB among your threads.

In other words, instantiate a new ClassB with your thread declaration and send it as a parameter.

Answer 5

Unfortunately, the question is somewhat ambiguous about what is considered success in terms of thread safety. Thread safety just means that the operation would work correctly if multiple threads are executing.

What seems to be missing is whether or not classA.methodA or classA.methodB needs to finish its operation with classB.myVar before either another thread calling classA.methodA(...) or classA.methodB(...). It would determine what type of locking pattern you would need.

For example, if you need a guarantee on reading a value, it would look like the following:

public class classA
{
    private classB b = new classB();

    public void methodA()
    {
        lock (b)
        {
            // Operation without calling methodA() or methodB() 
            // Read b.myVar
            // Update b.myVar
        }
    }

    public void methodB()
    {
        lock (b)
        {
            // Operation without calling methodA() or methodB()
            // Read b.myVar
            // Update b.myVar
        }
    }
}

In another example, if b.myVar is some type of collection that needs to be synchronized like a cache, it would look like the following:

public class classA
{
    private classB b = new classB();

    public void methodA()
    {
        // Read b.myVar for missing collection item

        lock (b)
        {
            // Check for missing collection item again. If not missing, leave lock
            // Operation without calling methodA() or methodB() 
            // Read b.myVar
            // Update b.myVar with new array item
        }
    }

    public void methodB()
    {
        // Read b.myVar for missing collection item
        lock (b)
        {
            // Check for missing collection item again. If not missing, leave lock
            // Operation without calling methodA() or methodB() 
            // Read b.myVar
            // Update b.myVar with new array item
        }
    }
}

Answer 6

Check out the lock statement documentation.