C# (.NET, Mono) Use array for transfer data between threads

Question

Today I accidentally stumbled upon a MemoryBarrier and realized that I do not fully understand the work of the CPU with RAM. The search did not give me an unambiguous answer to the question, so I decided to ask a new question.

There are two arrays:

int[] dataStates; //default items value is 0
MyStruct[] data; //MyStruct is a struct with several fields

There are two threads (ThreadA, ThreadB)

ThreadA performs the following code:

data[index] = ... //new value
//change to "1" after setting data
Interlocked.Exchange(ref dataStates[index], 1);

ThreadB code:

//wait for "1" in array item and replace to "2"
while(Interlocked.CompareExchange(ref dataStates[index], 2, 1) != 1)
{
     Thread.Sleep(0);
}

//read actual data
var value = data[index];

Is it possible that the Thread will read the data from the data[index] and they will be obsolete? By the word obsolete, I mean that the data received from the array will not match the data that was set before the Interlocked.Exchange call.

In general, I try data transfer between threads in the most productive way. Do use this approach (without locks) is it appropriate or are there more acceptable approaches?

Not familiar with Interlocked class. Normally do this with a Singleton. But your Thread.Sleep(0) is a problem since it does not force a context change. See: https://stackoverflow.com/a/3257751/2245849. — sthames42, Mar 05 '18 at 17:11

score 0 · Answer 1 · answered Mar 05 '18 at 17:14

I don't know if this is the solution but as far as I understood your sample you could do the following:

var queue = new ConcurrentQueue<DateTime>();
var tcs = new CancellationTokenSource();
var token = tcs.Token;
Task.Run(async () => {
    for (var i = 0; i < 2; i++) 
    {
        queue.Enqueue(DateTime.Now);
        await Task.Delay(2000);
    }
    tcs.Cancel();
}, token);
Task.Run(() => {
    while (!token.IsCancellationRequested) 
    {
        if (queue.Any())
        {
            DateTime result;
            queue.TryDequeue(out result);
            Console.WriteLine($"Received {result}...");
        }
    }
}, token).ContinueWith(t =>
{
    Console.WriteLine("Stop");
});
Console.ReadLine();
tcs.Cancel();

You will need some namespace-imports:

using System.Threading;
using System.Threading.Tasks
using System.Collections.Concurrent;

This is a complete different apporach avodiding manual sync between threads. I used DateTime instead of MyStruct.

score 0 · Answer 2 · answered Mar 06 '18 at 08:01

From what I read in the reference to the methods Exchange and CompareExchange they do not imply a memory barrier. Hence, writing the value to data[index] may be swapped with the interlocked setting of dataStates[index], which means that the second thread may actually read invalid data.

I agree with sprinter252 that there is probably a better way to implement this. Isn't it a normal producer-consumer problem? This is solvable with semaphores or it can be rewritten to use a task queue as in sprinter's answer.

C# (.NET, Mono) Use array for transfer data between threads

2 Answers2