c# - Plinq statement gets deadlocked inside static constructor

Question

I came across this situation where the following plinq statement inside static constructor gets deadlocked:

static void Main(string[] args)
{
    new Blah();
}

class Blah
{
     static Blah()
     {
         Enumerable.Range(1, 10000)
            .AsParallel()
            .Select(n => n * 3)
            .ToList();
     }
}

It happens only when a constructor is static. Can someone explain this to me please.

Is it TPL bug? Compiler? Me?

Answer 1

It is generally dangerous to call threading code from a static constructor. In order to ensure that the static constructor executes only once, the CLR executes the static constructor under a lock. If the thread running the static constructor waits on a helper thread, there is a risk that the helper thread is going to need the CLR-internal lock for some reason too, and the program will deadlock.

Here is a simpler code sample that demonstrates the problem:

using System.Threading;
class Blah
{
    static void Main() { /* Won’t run because the static constructor deadlocks. */ }

    static Blah()
    {
        Thread thread = new Thread(ThreadBody);
        thread.Start();
        thread.Join();
    }

    static void ThreadBody() { }
}

Section 10.5.3.3 "Races and deadlocks" of the ECMA CLI spec guarantees the following:

Type initialization alone shall not create a deadlock unless some code called from a type initializer (directly or indirectly) explicitly invokes blocking operations.

So, a type initializer (i.e., a static constructor) will not deadlock, provided that no operation in the static constructor blocks the thread. If the static constructor does block, it risks a deadlock.