What is the volatile keyword purpose in c#?

Question

I want to see the real time use of Volatile keyword in c#. but am unable to project the best example. the below sample code works without Volatile keyword how can it possible?

class Program
{
    private static int a = 0, b = 0;

    static void Main(string[] args)
    {
        Thread t1 = new Thread(Method1);
        Thread t2 = new Thread(Method2);

        t1.Start();
        t2.Start();

        Console.ReadLine();
    }

    static void Method1()
    {
        a = 5;
        b = 1;
    }

    static void Method2()
    {
        if (b == 1)
            Console.WriteLine(a);
    }
}

In the above code i am getting a value as 5. how it works without using volatile keyword?

Answer 1

The volatile keyword tells the compiler that a variable can change at any time, so it shouldn't optimise away reading and writing of the variable.

Consider code like this:

int sum;
for (var i = 0; i < 1000; i++) {
  sum += x * i;
}

As the variable x doesn't change inside the loop, the compiler might read the variable once outside the loop and just use the same value throughout the loop.

If you make the variable x volatile, the compiler will read the value of the variable each time that it is used, so if you change the value in a different thread, the new value will be used immediately.