Appropriate use of Monitor.Exit

Question

void MethodA()
{
   Monitor.Enter(this);
   if(someCondition)
   {
       Monitor.Exit(this);
       // This point
       MethodB();
    }
    else
    {
        // Set some values only
 Monitor.Exit(this);
    }
}

If I have the above method which can be called in multi threads:

• Assume thread 1 is at //This point
• Another thread enters Monitor.Enter while thread 1 is still at //This point
• Will this stop MethodB from being executed? If yes, is there a way of getting MethodB to execute.

I need to release MethodA before executing MethodB() because I can't wait for MethodB to complete before releasing MethodA. Also, I cannot start MethodB in a new thread.

Answer 1

Setting aside the issue of using 'this' as a lock (bad practice, as it exposes the lock publically), this form is cleaner and safer (releases the lock if exception is thrown):

void MethodA()
{
   bool condition;
   lock(this)
   {
     condition = someCondition;
     if(!condition)
     {
        // Set some values only
     }
   }
   if (condition)
       MethodB();
}

To answer your specific question, both this code and your original code will execute MethodB, even if something else acquires the lock.

Answer 2

Since, someCondition is not passed as a parameter, I could only assume the someCondition could be changing at any time (possibly an instance variable to the class). So, I would write the code this way:

void MethodA()
{
    if (someCondition)
    {
        bool conditionReached = false;

        Monitor.Enter(this);
        try
        {
            if (someCondition)
            {
                conditionReached = true;
            }
        }
        finally
        {
            Monitor.Exit(this);
        }

        if (conditionReached)
        {
            MethodB();
        }
    }
}

If not, than the previous answer with condition declared locally would suit you.