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    <p>Here's a pure Linq solution, inspired by Eric Lippert's <a href="http://blogs.msdn.com/b/ericlippert/archive/2010/06/28/computing-a-cartesian-product-with-linq.aspx" rel="nofollow">blog post</a> about computing a cartesian product. I modified the <code>CartesianProduct</code> method slightly so that it returns combinations:</p> <pre class="lang-cs prettyprint-override"><code>public static IEnumerable&lt;IEnumerable&lt;T&gt;&gt; Combinations&lt;T&gt;(this IEnumerable&lt;IEnumerable&lt;T&gt;&gt; sequences) { IEnumerable&lt;IEnumerable&lt;T&gt;&gt; emptyProduct = new[] { Enumerable.Empty&lt;T&gt;() }; return sequences.Aggregate( emptyProduct, (accumulator, sequence) =&gt; from accseq in accumulator // Exclude items that were already picked from item in sequence.Except(accseq) // Enforce ascending order to avoid same sequence in different order where !accseq.Any() || Comparer&lt;T&gt;.Default.Compare(item, accseq.Last()) &gt; 0 select accseq.Concat(new[] {item})).ToArray(); } </code></pre> <p>Based on this extension method, you can produce the desired result as follows:</p> <pre class="lang-cs prettyprint-override"><code>IEnumerable&lt;string&gt; items = new[] {"Coffee", "Tea", "Milk"}; IEnumerable&lt;IEnumerable&lt;string&gt;&gt; result = Enumerable.Range(1, items.Count()) .Aggregate( Enumerable.Empty&lt;IEnumerable&lt;string&gt;&gt;(), (acc, i) =&gt; acc.Concat(Enumerable.Repeat(items, i).Combinations())); </code></pre> <p>(it concatenates all combinations of 1, 2... N items)</p> <p>Note that it's probably not a very efficient solution, but I think it's an interesting use of Linq...</p> <hr> <p>EDIT: here's a new version of the <code>Combinations</code> method that maintains the original order:</p> <pre class="lang-cs prettyprint-override"><code>public static IEnumerable&lt;IEnumerable&lt;T&gt;&gt; Combinations&lt;T&gt;(this IEnumerable&lt;IEnumerable&lt;T&gt;&gt; sequences) { var indexedSequences = sequences.Select(seq =&gt; seq.Select((item, idx) =&gt; new IndexedItem&lt;T&gt;(item, idx))); IEnumerable&lt;IEnumerable&lt;IndexedItem&lt;T&gt;&gt;&gt; emptyProduct = new[] { Enumerable.Empty&lt;IndexedItem&lt;T&gt;&gt;() }; var indexedResult = indexedSequences.Aggregate( emptyProduct, (accumulator, sequence) =&gt; from accseq in accumulator // Exclude items that were already picked from item in sequence.Except(accseq) // Enforce ascending order of indexes to avoid same sequence in different order where !accseq.Any() || item.Index &gt; accseq.Last().Index select accseq.Concat(new[] {item})).ToArray(); return indexedResult.Select(seq =&gt; seq.Select(i =&gt; i.Item)); } class IndexedItem&lt;T&gt; { public IndexedItem(T item, int index) { this.Item = item; this.Index = index; } public T Item { get; private set; } public int Index { get; set; } } </code></pre> <p>Probably even more inefficient than the previous version, but it gets the job done...</p>
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    1. POGenerate All Unique Combinations of Elements of a IEnumerable(Of T)
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    1. USThomas Levesque
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    1. COThanks for your input. Correct number of results (and I like that it didn't even include the empty set), but the ordering doesn't satisfy the problem statement, but it is VERY close (closest answer yet). The ordering rules noticeably break down once a 4th item is added to 'items.' The results are ordered properly from top to bottom, but are not ordered properly within the set. I could do some post-processing ordering to get them back in the correct order.
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    2. CO@ckittel, see my updated answer. I associated an index with each item so that I can maintain the original order
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    3. COA+. Thank you so much! This solution is ideal for my needs. I'll probably write up an answer myself later today to show the VB.NET version for others' benefit, but this solution is definitely the accepted answer. Thanks for being so diligent in your solution.
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