how to lock on a method content

Question

I have a method that can be called from many threads, but I just want the 1st thread to do some logic inside the method. So, I'm planning to use a boolean variable. The first thread that comes in, will set the boolean variable to false (to prevent further threads to come inside), and execute the method logic.

Subsequent threads that come to this method, will check the boolean variable. Because it was set to false by the 1st thread, they will skip the method logic.

In code, something like this:

private void myMethod()
{
   if (firsTime)  //set to true in the constructor
   {
      firstTime = false; //To prevent other thread to come inside here.
      //method logic
   }
}

I want to use lock to perform this, but not sure where to put it.

If I lock inside the "if" to change firstTime to false, its possible 2 or more threads already come inside the if (don't want this).

If I lock outside the "if" to change firstTime to false, how can the 1st thread go inside the if to execute the method logic, if firstTime its already set to false??

My question is : how to do the lock to have the desired functionality? (1st thread that comes set the boolean and execute method logic).

I cannot lock over all the method logic, since it will be a very long time consuming operations.

Answer 1

You can use Interlocked.Exchange to solve this problem. It will set the value of the given variable to the specified value and return the value that used to be in the variable, and it will do it all atomically. Doing this will ensure that only one thread will ever run the code in the if:

private static int isFirst = 1;
public static void Foo()
{
    if (Interlocked.Exchange(ref isFirst, 0) == 1)
    {
        //DoStuff
    }
}

Note that Interlocked.Exchange has no overload that takes a bool, which is why you're forced to use an int (or some other type) instead, using 1 for true and 0 for false.

If you want a solution using lock, rather than Interlocked, you can do it through the use of an additional local bool value:

private static bool isFirst = true;
private static object key = new object();
public static void Foo()
{
    bool amFirst;
    lock (key)
    {
        amFirst = isFirst;
        isFirst = false;
    }

    if (amFirst)
    {
        //DoStuff
    }
}