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by Jiří {x2} Činčura

Visualizing tearing on decimal

30 May 2017 .NET, .NET Core, C#, Multithreading/Parallelism/Asynchronous/Concurrency

Today, as I’m teaching my parallel/threading/async course, I was showing how the tearing on reads happens. I used the same code I published couple of months ago. And then idea came (yes, it happens sometimes 😉). I wanted to show that the decimal, being 128-bit structure, splits on underlying values. Here’s the code.

The code is mostly the same as the original one. I just used decimal.MaxValue and 0 as values to have some nice bits and simple method (GetDecimalBits in the code) to get me these bits as string.

static class Program
{
    const decimal Value1 = decimal.MaxValue;
    const decimal Value2 = 0;
    static readonly TimeSpan Delay = TimeSpan.FromMilliseconds(100);

    static decimal value;

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

        Console.WriteLine(GetDecimalBits(Value1));
        Console.WriteLine(GetDecimalBits(Value2));

        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} {GetDecimalBits(t)}");
                Thread.Sleep(Delay);
            }
        }
    }

    static string GetDecimalBits(decimal value)
    {
        var bits = decimal.GetBits(value);
        return $"{bits[3]:X8}{bits[2]:X8}{bits[1]:X8}{bits[0]:X8}";
    }

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

Running this on .NET Core (but works fine on .NET as well) shows nicely the splitting happening (again full optimizations need to be turned on).

.NET Core 4.6.25211.01
00000000FFFFFFFFFFFFFFFFFFFFFFFF
00000000000000000000000000000000
00:00:00.0000036: 79228162495817593519834398720 00000000FFFFFFFF0000000000000000
00:00:00.1003531: 79228162495817593519834398720 00000000FFFFFFFF0000000000000000
00:00:00.2005971: 79228162495817593524129366015 00000000FFFFFFFF00000000FFFFFFFF
00:00:00.3008787: 79228162495817593524129366015 00000000FFFFFFFF00000000FFFFFFFF
00:00:00.4011402: 79228162495817593519834398720 00000000FFFFFFFF0000000000000000
00:00:00.5014158: 79228162495817593524129366015 00000000FFFFFFFF00000000FFFFFFFF
00:00:00.6022055: 79228162495817593519834398720 00000000FFFFFFFF0000000000000000
00:00:00.7030075: 79228162495817593524129366015 00000000FFFFFFFF00000000FFFFFFFF
00:00:00.8037584: 79228162495817593524129366015 00000000FFFFFFFF00000000FFFFFFFF
00:00:00.9041407: 79228162495817593524129366015 00000000FFFFFFFF00000000FFFFFFFF
00:00:01.0049634: 79228162495817593519834398720 00000000FFFFFFFF0000000000000000
00:00:01.1058126: 79228162495817593519834398720 00000000FFFFFFFF0000000000000000
00:00:01.2062336: 79228162495817593524129366015 00000000FFFFFFFF00000000FFFFFFFF
00:00:01.3063833: 79228162495817593519834398720 00000000FFFFFFFF0000000000000000
00:00:01.4071562: 79228162495817593519834398720 00000000FFFFFFFF0000000000000000

What a nice result. The underlying values are clearly visible (the boundaries between them).