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    <p>Something like the following should do the job, I think.</p> <pre><code>public static IEnumerable&lt;IEnumerable&lt;int&gt;&gt; AllSequences(int start, int end, int size) { return size &lt;= 0 ? new[] { new int[0] } : from i in Enumerable.Range(start, end - size - start + 2) from seq in AllSequences(i + 1, end, size - 1) select Enumerable.Concat(new[] { i }, seq); } </code></pre> <p>The key to the solution is the <a href="http://msdn.microsoft.com/en-us/library/bb383978.aspx" rel="noreferrer">compound <code>from</code> clause</a>, which is quite handy for dealing with nested enumerables.</p> <p>Notice that I've changed the method signature slightly to <code>IEnumerable&lt;IEnumerable&lt;int&gt;&gt;</code>, since this is more convenient when using (pure) LINQ. You can always convert it easily to a <code>IEnumerable&lt;ICollection&lt;int&gt;&gt;</code> at the end if you like, however.</p> <p>Let me know if the code needs any explanation, but I'm hoping the LINQ syntax makes it reasonably clear.</p> <p><strong>Edit 1:</strong> Fixed bug and improved conciseness.</p> <p><strong>Edit 2:</strong> Because I'm bored and have nothing better to do (no, not really), I thought I'd write an extension method that compute the combinations of a given list of elements, making use of the <code>AllSequences</code> method.</p> <pre><code>public static IEnumerable&lt;IEnumerable&lt;T&gt;&gt; Combinations&lt;T&gt;(this IList&lt;T&gt; source, int num) { return AllSequences(0, source.Count - 1, num).Select( seq =&gt; seq.Select(i =&gt; source[i])); } </code></pre> <p>Perhaps not the most efficient way of computing combinations, but certainly pretty compact code!</p>
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    1. POGenerate number sequences with LINQ
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    1. COYou stole my thunder! I was just working this out =D +1
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    2. COI just ran it and it causes a stackoverflow :-(, maybe with a where size > 0, just before the second from?
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    3. COIt needs to be `from seq in AllSequences(i+1, end, size-1)` Other than that, nice! I just wrote one too, but yours is a lot more concise. +1
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