C# volatile array items?

Question

I need an array with volatile items, and can't find a way to do that.

private volatile T[] _arr;

This means that the _arr reference is volatile, however it does not guarantee anything about the items inside the _arr object itself.

Is there any way to mark the _arr's Items as volatile?

Thanks.

EDIT:

The following code built according to binarycoder's answer. Is this code thread-safe to use?

public class VolatileArray<T>
{
    private T[] _arr;

    public VolatileArray(int length)
    {
        _arr = new T[length];
    }

    public VolatileArray(T[] arr)
    {
        _arr = arr;
    }

    public T this[int index]
    {
        get
        {
            T value = _arr[index];
            Thread.MemoryBarrier();
            return value;
        }

        set
        {
            Thread.MemoryBarrier();
            _arr[index] = value;
        }
    }

    public int Length
    {
        get { return _arr.Length; }
    }
}

It's not thread safe when T is `long` or `double`, as these are 64 bit values, and you have to read and write them using `Volatile.Read()`and `Volatile.Write()` or some such, otherwise reads and writes are not guaranteed to be atomic. — Evgeniy Berezovsky, Oct 23 '18 at 02:28

score 11 · Accepted Answer · edited May 23 '17 at 11:33

11

Since it is possible to pass array elements by reference, you can use Thread.VolatileRead and Thread.VolatileWrite.

It is useful to understand that the volatile keyword works behind the scenes by using Thread.MemoryBarrier. You could write:

// Read
x = _arr[i];
Thread.MemoryBarrier();

// Write
Thread.MemoryBarrier();
_arr[i] = x;

Note that volatile and MemoryBarrier are advanced techniques that are both easy to get wrong. For example, see How do I Understand Read Memory Barriers and Volatile. Usually you are better off with higher level constructs such as lock, Monitor, ReaderWriterLockSlim, and others.

edited May 23 '17 at 11:33

Community

1
1

answered Dec 05 '09 at 17:05

Jason Kresowaty

16,105
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1

IIRC, then strictly, the `volatile` keyword entails only a read or write barrier as appropriate, while `Thread.MemoryBarrier()` has a full barrier. The above therefore is slightly heavier than the equivalent use of `volatile` even before we consider that it involves a method call. It is still the best we can do (and as you say, the current implementation of `VolatileRead()` and `VolatileWrite()`). – Jon Hanna Nov 08 '11 at 20:29

score 6 · Answer 2 · answered Jul 15 '16 at 15:31

6

Use Volatile.Read(ref array[index]) and Volatile.Write(ref array[index], value).

Class Volatile is available since .NET 4.5. It allows to read/write from/to fields, array elements, ref parameters, pointers.

answered Jul 15 '16 at 15:31

OmariO

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He's concerned about the volatility of the array reference, not the items in it. – Evgeniy Berezovsky Oct 23 '18 at 01:18
3

@EugeneBeresovsky He clearly stated that he needed volatile access to array items. – OmariO Oct 23 '18 at 01:54

Nick Strupat · Answer 3 · 2013-10-16T19:46:18.323

3

I made a little struct that helps keep things clean and OO

struct VolatileBoolean {
    public volatile Boolean Value;
}

VolatileBoolean[] arrayOfVolatileBooleans;
public void SomeMethod() {
    if (arrayOfVolatileBooleans[4].Value)
        DoSomething();
}

edited Oct 16 '13 at 19:46

answered Mar 22 '13 at 15:29

Nick Strupat

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score 2 · Answer 4 · answered Dec 05 '09 at 16:54

2

~~I don't think that you can~~

You can't, volatile is defined as a field-modifier (ECMA 334).

And I don't think it will accomplish what you want either.
Consider:

 private T[] _arr;

 volatile T v;
 ....  v = _arr[x];
 ....  _arr[x] = v;

answered Dec 05 '09 at 16:54

H H

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C# volatile array items?

4 Answers4

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