Can a C# Thread Cache a Boolean Value, Ignoring Updates from Other Threads?

Can a C# Thread Cache a Value and Ignore Changes on Other Threads?

The question posed concerns the behavior of a C# thread when reading a bool value modified on another thread. Some articles suggest that the reading thread may cache the old value, potentially leading to an infinite loop.

Understanding the Issue

While it's a good practice to lock variables for thread safety, the .NET runtime is designed to abstract away issues related to volatile memory. However, according to experts in the field, the behavior observed in the example provided is not guaranteed by the specification.

Potential Faulty Behavior

The example code presents a boolean value (stopping) shared between two threads. One thread sets the value to true after a 5-second delay, while another thread continuously reads the value in a loop. Theoretically, without proper synchronization (e.g., a lock), the reading thread could potentially cache the initial false value and never read the updated true value.

Counter-Example

Despite the claims in the articles, a repeatable counter-example has been demonstrated:

// using System.Threading;

private static bool stopping = false; // Make volatile to fix the issue

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

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

In this scenario, after setting stopping to true, the DoWork thread continues to run indefinitely, indicating that the old value is being cached.

Resolution

To ensure correct behavior, it's crucial to mark the shared variable as volatile, which adds a memory barrier and prevents optimizations that could lead to caching. Alternatively, using a lock to synchronize access to the variable would also resolve the issue.

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