An extensible interlocked arithmetic function (via lambdas)

Raymond Chen

Some time ago, I noted that you can build other interlocked operations out of Interlocked­Compare­Exchange. Here’s an example:

using System.Threading;
public static int InterlockedMax(ref int location, int value)
{
    int initialValue, newValue;
    do
    {
        initialValue = location;
        newValue = Math.Max(initialValue, value);
    }
    while (Interlocked.CompareExchange(ref location, newValue,
                                       initialValue) != initialValue);
    return initialValue;
}

(There’s a corresponding C++ version, which I leave as an exercise.)

This function atomically updates a “highest value seen so far” variable. It follows the usual pattern:

• Capture the starting value.
• Do a computation based on that value.
• Compare-exchange the new value in.
• If the compare-exchange failed, then start over.

(For bonus points, add an early-out if the operation should be abandoned.)

You can make this function extensible by use of lambdas, so that you can update the old value with any computation you like.

using System;
using System.Threading;
public static int InterlockedCombine(ref int location,
                                     Func<int, int> update)
{
    int initialValue, newValue;
    do
    {
        initialValue = location;
        newValue = update(initialValue);
    }
    while (Interlocked.CompareExchange(ref location, newValue,
                                       initialValue) != initialValue);
    return initialValue;
}
public static int InterlockedMax(ref int location, int value)
{
    return InterlockedCombine(ref location,
                              v => Math.Max(v, value));
}
public static int InterlockedMultiply(ref int location, int value)
{
    return InterlockedCombine(ref location,
                              v => v * value);
}
public static int InterlockedIncrementWithSaturation(
    ref int location, int maximum)
{
    return InterlockedCombine(ref location,
                              v => v < maximum ? v + 1 : maximum);
}
public static int InterlockedCompareExchangeIfNotEqual(
    ref int location, int newValue, int avoidValue)
{
    return InterlockedCombine(ref location,
                              v => v != avoidValue ? newValue : avoidValue);
}

Raymond Chen

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