Question

I know that is wrong to use lock(this) or any shared object.

I wonder if this usage is OK?

public class A
{
  private readonly object locker = new object();
  private List<int> myList;
  public A()
  {
    myList = new List<int>()
  }

  private void MethodeA()
  {
    lock(locker)
    {
      myList.Add(10);
    }
  }

  public void MethodeB()
  {
    CallToMethodInOtherClass(myList);
  }
}

public class OtherClass
{
  private readonly object locker = new object();
  public CallToMethodInOtherClass(List<int> list)
  {
   lock(locker)
   {
     int i = list.Count;
   }
  }
}

Is this thread safe? In OtherClass we lock with a private object so if the class A lock with its private lock can the list still change in the the lock block in OtherClass?

Answer 1

No, it’s not thread safe. Add and Count may be executed at the “same” time. You have two different lock objects.

Always lock your own lock object when passing the list:

  public void MethodeB()
  {
    lock(locker)
    {
      CallToMethodInOtherClass(myList);
    }
  }