How can I set a min value in .Net without using a lock?

Question

I have multiple threads accessing variables. I know how to write spinlocks and use the Threading.Interlocked methods to increment etc. variables.

However, I want to perform the equivalent of:

a = Math.Min(a, b)
or
a = a | 10

... but without using a critical section. Is this possible? I know the 2nd line is possible in assembler, but there is no Interlocked.Or method.

Answer 1

If you don't anticipate a great deal of contention then perhaps something like this? (If there's likely to be a lot of contention then a plain lock might well be more efficient.)

int original;    // assuming here that a is an int
do
{
    original = a;
} while (Interlocked.CompareExchange(ref a, original | 10, original) != original)

Answer 2

Here is the general pattern for simulating an interlocked operation.

public static T InterlockedOperation<T>(ref T location, T value)
{
  T initial, computed;
  do
  {
    initial = location;
    computed = op(initial, value); // initial | value
  } 
  while (Interlocked.CompareExchange(ref location, computed, initial) != initial);
  return computed;
}

The min operation is a completely different story. The issue here is that there are two memory locations in play. Furthermore, we are only interested in reading them. That means we really only need to worry about the memory barrier problem. Decorate your fields with volatile or do an explicit call to Thread.MemoryBarrier prior to computing the min.

Edit: I missed the fact that the result of the min operation is assigned to a. You can actually use the pattern I defined above, but instead of doing computed = initial | value do computed = initial < value ? initial : value. Everything else stays the same.