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    <p>This returns one set of a powerset at a time. It is based on python code <a href="http://groups.google.com/group/comp.lang.python/browse_thread/thread/d9211cd6c65e1d3a/" rel="noreferrer">here</a>. It works for powersets of over 32 elements. If you need fewer than 32, you can change long to int. It is pretty fast -- faster than my previous algorithm and faster than (my modified to use yield return version of) P Daddy's code.</p> <pre><code>static class PowerSet4&lt;T&gt; { static public IEnumerable&lt;IList&lt;T&gt;&gt; powerset(T[] currentGroupList) { int count = currentGroupList.Length; Dictionary&lt;long, T&gt; powerToIndex = new Dictionary&lt;long, T&gt;(); long mask = 1L; for (int i = 0; i &lt; count; i++) { powerToIndex[mask] = currentGroupList[i]; mask &lt;&lt;= 1; } Dictionary&lt;long, T&gt; result = new Dictionary&lt;long, T&gt;(); yield return result.Values.ToArray(); long max = 1L &lt;&lt; count; for (long i = 1L; i &lt; max; i++) { long key = i &amp; -i; if (result.ContainsKey(key)) result.Remove(key); else result[key] = powerToIndex[key]; yield return result.Values.ToArray(); } } } </code></pre> <p>You can download all the fastest versions I have tested <a href="http://www.iwebthereforeiam.com/files/TestYieldReturn.zip" rel="noreferrer">here</a>.</p> <p>I really think that using yield return is the change that makes calculating large powersets possible. Allocating large amounts of memory upfront increases runtime dramatically and causes algorithms to fail for lack of memory very early on. Original Poster should figure out how many sets of a powerset he needs at once. Holding all of them is not really an option with >24 elements.</p>
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    1. COI think you are correct that it is the yeild that makes the difference. I am very impressed by your tenacity to provide the fastest possible implementation, as well as several very valid points made. Honourable mention to P Daddy as well, and to all who have provided input.
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    2. COP Daddy had a cool idea that I have not called out yet -- the allocation of fixed-size arrays that are the right length. In contrast, resizing Lists is expensive. My other code makes Lists of maximum length so as not to reallocate.
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    3. COIf I were doing this I'd write a custom IList<T> class that wraps around a array whose size is fixed at currentGroupList.Length. You could easily get better performance than the dictionary-based implementation here.
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