Can a C# Thread Ignore Value Changes from Other Threads?

Can a C# Thread Ignore Value Changes in Other Threads?

This question explores a potential anomaly in C# threading behavior, unrelated to race conditions or the need for locks. Specifically, the concern is whether a thread can cache a value and disregard changes made to that value on other threads, potentially leading to unexpected behavior.

The NET runtime, managing memory, is believed to mitigate this issue. However, some literature claims otherwise, asserting that certain code structures, like the following, may lead to infinite loops:

class BackgroundTaskDemo
{
    private bool stopping = false;

    static void Main()
    {
        BackgroundTaskDemo demo = new BackgroundTaskDemo();
        new Thread(demo.DoWork).Start();
        Thread.Sleep(5000);
        demo.stopping = true;
    }

    static void DoWork()
    {
         while (!stopping)
         {
               // Do something here
         }
    }
}

According to the articles referenced (http://www.yoda.arachsys.com/csharp/threads/volatility.shtml , http://softwareengineering.stackexchange.com/questions/104757/is-a-string-property-itself-threadsafe), the reading thread may cache the initial value of the stopping variable, rendering all subsequent updates invisible and causing the thread to run indefinitely.

However, expertos in memory modeling insist that such behavior is not guaranteed by the C# specification. While the provided code may usually function correctly, it relies on platform- and compiler-specific optimizations rather than concrete language semantics.

To further illustrate the issue, consider the following modified code:

using System.Threading;
using System;
static class BackgroundTaskDemo
{
    //make this volatile to fix it
    private static bool stopping = false;

    static void Main()
    {
        new Thread(DoWork).Start();
        Thread.Sleep(5000);
        stopping = true;

        Console.WriteLine("Main exit");
        Console.ReadLine();
    }

    static void DoWork()
    {
        int i = 0;
        while (!stopping)
        {
            i++;
        }

        Console.WriteLine("DoWork exit " + i);
    }
}

With this modification, the code predictably outputs "Main exit," but the DoWork method continues running indefinitely. This counter-example demonstrates that, despite the initial suspicions, the C# runtime does not always guarantee consistency in value updates across threads.

To resolve this ambiguity, the volatile keyword can be applied to the stopping variable. The volatile modifier enforces atomic access to the variable, preventing cached values from obscuring actual updates.

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