Question

In the following code:

a = Interlocked.Exchange(ref b, c);

I know b is set to c atomically. But is a also set to b in the same atomic operation? Or is this outside of the atomic operation.

What I need is to ensure both a and b are set in the same atomic operation.

c => b, b => a

This is in C#.Net.

Answer 1

I assume you’re considering code like this:

using System;
using System.Threading;

class Test
{
    static int x = 1;
    static int y = 2;

    static void Main()
    {
        x = Interlocked.Exchange(ref y, 5);
    }
}

In that case, no, the operation isn’t atomic. In IL, there are two separate actions:

• Calling the method
• Copying the value from the notional stack to the field

It would be entirely possible for another thread to “see” y become 5 before the return value of Interlocked.Exchange was stored in x.

Personally, if I were looking at something where you need multiple field values to be changed atomically, I’d be considering locks instead of atomic lock-free operations.