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.

classA
  creates instance of classB
  has methodA that can update & uses classB's myVar
  has methodB that can update & uses classB's myVar

classB
  has a variable myVar

methodA and methodB can both run in separate threads (called in new threads from main). How do I ensure this is thread safe?

Answer 1

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 2

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 3

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.