Can multiple threads write safely to different array indices without synchronization?

[MartyIX]

Hi,

suppose there is this code sample:

object[] objects = new object[2];

_ = Parallel.For(fromInclusive: 0, toExclusive: 2, (i) => { // This is specially crafted so that threads do write to the same array locations.
    objects[i] = new object();
});

Console.WriteLine($"{objects[0]} {objects[1]}"); // Access the array (in any way really).

Now I would expect that one needs to change it to

object lockObj = new();
object[] objects = new object[2];

_ = Parallel.For(fromInclusive: 0, toExclusive: 2, (i) => {
    lock (lockObj) 
    {
        objects[i] = new object();
    }
});

lock (lockObj) 
{
    Console.WriteLine($"{objects[0]} {objects[1]}");
}

to be sure that:

• No "wrong" compiler optimizations took place.
• Observability of changes is correct (no stale read)

Is the first sample correct or not?

Thank you

PS: I know that the memory model for .NET application is described¹ but I don't understand it fully (yet).

Footnotes

1. https://github.com/dotnet/runtime/blob/main/docs/design/specs/Memory-model.md ↩

[EgorBo]

I'm not sure I understand what is the expected behavior here, but Parallel.For is blocking so all array elements will be initialized as is, lock here makes no sense.

[EgorBo]

Is it? Parallel.For(fromInclusive: 0, toExclusive: 2, (i) => { Console.WriteLine(Environment.CurrentManagedThreadId); }); prints different thread ids.

Does it contradict to what I said?

[MartyIX]

My question is if objects need to be synchronized given that multiple threads access it (but in a way that the same array index is not accessed in parallel).

Think of a field where two people work on a single field but each works exclusively on its assigned half of the field.

[MartyIX]

The expected behavior is that Console.WriteLine prints the objects that were assigned in the Parralel.For. Not null, for example.

[huoyaoyuan]

Is it? Parallel.For(fromInclusive: 0, toExclusive: 2, (i) => { Console.WriteLine(Environment.CurrentManagedThreadId); }); prints different thread ids.

Does it contradict to what I said?

I was thinking about the two write operations, not their relationship with the read operation.

[MartyIX]

all array elements will be initialized as is

@EgorBo So after the Parallel.For, there is a memorry barrier (or how it is called) so that visibility of all array elements is as one expects after the For call..? Sorry, if the question is dumb. I just want to make sure I understand it.

[Clockwork-Muse]

Parallel.For(..) is going to have to call Wait(..)/Join(..) or similar in order to be able to complete the call, which will use barriers.

This is one of the "traditional" (As in, across multiple languages) ways that multi-threaded work has been done safely - have multiple threads work on separate array elements, then do something with the combined results afterwards.