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by Jiří {x2} Činčura

Decimal tearing example on .NET/.NET Core

19 Mar 2017 2 mins .NET, .NET Core, C#, Multithreading/Parallelism/Asynchronous/Concurrency

I realized I have some pieces of code to show some specific behavior, mostly around threading and synchronization, all over my notes. Some of these pieces may be 10+ years old. I use these pieces during my “threading/parallel/async” course, but why not to share it publicly. Maybe I’ll stumble on it after some years, maybe .NET will be history, and it will be interesting to re-read and re-think the code. The code isn’t unique or something where I’m the first to realize it. It’s really just an example code.

Here’s the most recent one. I wanted to show torn reads happening. So I took a decimal value, because it is 128-bit value it will not be atomic in any way, and started looping using plain reads and writes.

static class Program
{
    const decimal Value1 = 1;
    const decimal Value2 = 9999999999999999999;
    static readonly TimeSpan Delay = TimeSpan.FromMilliseconds(100);

    static decimal value;

    static void Main(string[] args)
    {
        Console.WriteLine(FrameworkVersion);

        var writer = new Thread(_ => Writer());
        writer.Start();
        var reader = new Thread(_ => Reader());
        reader.Start();

        Console.ReadLine();
    }

    static void Writer()
    {
        while (true)
        {
            value = Value1;
            value = Value2;
        }
    }

    static void Reader()
    {
        var sw = Stopwatch.StartNew();
        while (true)
        {
            var t = value;
            if (t != Value1 && t != Value2)
            {
                Console.WriteLine($"{sw.Elapsed}: {t}");
                Thread.Sleep(Delay);
            }
        }
    }

    static string FrameworkVersion =>
#if NETCOREAPP1_1
        System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription;
#else
        Environment.Version.ToString();
#endif
}

Running this code with full optimizations (aka Release build) it starts “failing” pretty quickly (just a few seconds on my machine). And on my machine the torn values are 2313682943 or 9999999997686317057 (yours might vary).

At the time of writing it works successfully on good old .NET Framework 4.0.30319.42000 and also .NET Core 4.6.25009.03 using netcoreapp1.1.

Follow-up post.

Profile Picture Jiří Činčura is an independent developer focusing on data and business layers, language constructs, parallelism and databases. Specifically Entity Framework, asynchronous and parallel programming, cloud and Azure. He's Microsoft Most Valuable Professional and you can read his articles, guides, tips and tricks at www.tabsoverspaces.com.