Why does Interlocked.CompareExchange<T> only support reference types?

Question

Disclaimer: My posts are apparently always verbose. If you happen to know the answer to the title question, feel free to just answer it without reading my extended discussion below.

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The System.Threading.Interlocked class provides some very useful methods to assist in writing thread-safe code. One of the more complex methods is CompareExchange, which can be used for computing a running total that may be updated from multiple threads.

Since the use of CompareExchange is a bit tricky, I thought it a rather common-sense idea to provide some helper methods for it:

// code mangled so as not to require horizontal scrolling
// (on my monitor, anyway)
public static double Aggregate
(ref double value, Func<double, double> aggregator) {
    double initial, aggregated;

    do {
        initial = value;
        aggregated = aggregator(initial);
    } while (
        initial != Interlocked.CompareExchange(ref value, aggregated, initial)
    );

    return aggregated;
}

public static double Increase(ref double value, double amount) {
    return Aggregate(ref value, delegate(double d) { return d + amount; });
}

public static double Decrease(ref double value, double amount) {
    return Aggregate(ref value, delegate(double d) { return d - amount; });
}

Now, perhaps I am just guilty of being generic-happy (I will admit, this is often true); but it does feel silly to me to restrict the functionality provided by the above methods to double values only (or, more accurately, for me to have to write overloaded versions of the above methods for every type I want to support). Why can't I do this?

// the code mangling continues...
public static T Aggregate<T>
(ref T value, Func<T, T> aggregator) where T : IEquatable<T> {
    T initial, aggregated;

    do {
        initial = value;
        aggregated = aggregator(initial);
    } while (
        !initial.Equals(
            Interlocked.CompareExchange<T>(ref value, aggregated, initial)
        )
    );
}

I can't do this because Interlocked.CompareExchange<T> apparently has a where T : class constraint, and I don't understand why. I mean, maybe it's because there are already overloads for CompareExchange that accept Int32, Int64, Double, etc.; but that hardly seems a good rationale. In my case, for example, it would be quite handy to be able to use the Aggregate<T> method to perform a wide range of atomic calculations.

Answer 1

Interlocked.CompareExchange is meant to be implemented with native atomic instructions provided directly by the processor. It's pointless to have something like that use a lock internally (it's designed for lock-free scenarios).

Processors that provide atomic compare exchange instruction naturally support it as small, "register-sized" operations (e.g. the largest compare-exchange instruction on an Intel x64 processor is cmpxchg16b that works on 128 bit values).

An arbitrary value type can be potentially bigger than that and compare-exchanging it may not be possible with a single instruction. Compare-exchanging a reference type is easy. Regardless of its total size in memory, you'll be comparing and copying a small pointer of a known size. This is also true for primitive types like Int32 and Double—all of them are small.

Answer 2

Because that overload is specifically intended to compare and exchange a reference. It does not perform an equality check using the Equals() method. Since a value type would never have reference equality with the value you're comparing it against, my guess is that they constrained T to class in order to prevent misuse.

Answer 3

I would suspect that Interlocked.CompareExchange<T> just performs an atomic pointer-swap under the hood.

Trying to do that with a value type would likely not give the results you expect.

You could, of course, box value types up in an object before using them.