Illustrating usage of the volatile keyword in C#

I've achieved a working example!

The main idea received from wiki, but with some changes for C#. The wiki article demonstrates this for static field of C++, it is looks like C# always carefully compile requests to static fields... and i make example with non static one:

If you run this example in Release mode and without debugger (i.e. using Ctrl+F5) then the line while (test.foo != 255) will be optimized to 'while(true)' and this program never returns. But after adding volatile keyword, you always get 'OK'.

class Test
{
    /*volatile*/ int foo;

    static void Main()
    {
        var test = new Test();

        new Thread(delegate() { Thread.Sleep(500); test.foo = 255; }).Start();

        while (test.foo != 255) ;
        Console.WriteLine("OK");
    }
}

Yes, it's hardware dependent (you are unlikely to see the problem without multiple processors), but it's also implementation dependent. The memory model specifications in the CLR spec permit things which the Microsoft implementation of the CLR do not necessarily do.

It's not really a matter of a fault happening when the 'volatile' keyword isn't specified, more that an error could happen when it hasn't been specified. Generally you are going to know when this is the case better than the compiler!

The easiest way of thinking about it would be that the compiler could, if it wanted to, inline certain values. By marking the value as volatile, you are telling yourself and the compiler that the value may actually change (even if the compiler doesn't think so). This means the compiler should not in-line values, keep cache or read the value early (in an attempt to optimize).

This behaviour isn't really the same keyword as in C++.

MSDN has a short description here. Here is a perhaps a more in depth post on the subjects of Volatility, Atomicity and Interlocking