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    copied!<p>The general idea is simple: you merge P and Q, use BufferWithCount(2) to get pairs of values and then process pairs according to your logic:</p> <pre> <code> P.Merge(Q).BufferWithCount(2).Select(values =&gt; { var first = values[0]; var second = values[1]; if (first is P && second is P || first is Q && second is Q) { return 0; } if (first is P) { return selector(first, second); } else // suppose Q, P is a valid sequence as well. { return selector(second, first); } }); </code> </pre> <p>Now the hard part is to merge P and Q if they are of different type and then distinguish between them in Select. If they are of the same type you could use something simple like approach proposed by Enigmativity, i.e.</p> <pre> <code> var pqs = (from p in ps select new { k = "p", v = p }) .Merge(from q in qs select new { k = "q", v = q }); </code> </pre> <p>Now the hard part is if they are of different types, to merge them we'll need some common wrapper type, something like, e.g. <a href="http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Either.html" rel="nofollow">Data.Either</a> from Haskell:</p> <pre> <code> public abstract class Either&lt;TLeft, TRight&gt; { private Either() { } public static Either&lt;TLeft, TRight&gt; Create(TLeft value) { return new Left(value); } public static Either&lt;TLeft, TRight&gt; Create(TRight value) { return new Right(value); } public abstract TResult Match&lt;TResult&gt;( Func&lt;TLeft, TResult&gt; onLeft, Func&lt;TRight, TResult&gt; onRight); public sealed class Left : Either&lt;TLeft, TRight&gt; { public Left(TLeft value) { this.Value = value; } public TLeft Value { get; private set; } public override TResult Match&lt;TResult&gt;( Func&lt;TLeft, TResult&gt; onLeft, Func&lt;TRight, TResult&gt; onRight) { return onLeft(this.Value); } } public sealed class Right : Either&lt;TLeft, TRight&gt; { public Right(TRight value) { this.Value = value; } public TRight Value { get; private set; } public override TResult Match&lt;TResult&gt;( Func&lt;TLeft, TResult&gt; onLeft, Func&lt;TRight, TResult&gt; onRight) { return onRight(this.Value); } } } </code> </pre> <p>Funny enough, there is already similar Either class in System.Reactive.dll, unfortunately it's internal, so we need our own implementation. Now we can put both P and Q into Either and proceed with out solution (I've generalized it a bit, so you can return any result instead of int only):</p> <pre> <code> public static IObservable&lt;TResult&gt; SmartZip&lt;TLeft, TRight, TResult&gt;( IObservable&lt;TLeft&gt; leftSource, IObservable&lt;TRight&gt; rightSource, Func&lt;TLeft, TRight, TResult&gt; selector) { return Observable .Merge( leftSource.Select(Either&lt;TLeft, TRight&gt;.Create), rightSource.Select(Either&lt;TLeft, TRight&gt;.Create)) .BufferWithCount(2) .Select(values =&gt; { // this case was not covered in your question, // but I've added it for the sake of completeness. if (values.Count &lt; 2) { return default(TResult); } var first = values[0]; var second = values[1]; // pattern-matching in C# is really ugly. return first.Match( left =&gt; second.Match( _ =&gt; default(TResult), right =&gt; selector(left, right)), right =&gt; second.Match( left =&gt; selector(left, right), _ =&gt; default(TResult))); }); } </code> </pre> <p>And here is a small demo for all this scary ugly stuff.</p> <pre> <code> private static void Main(string[] args) { var psource = Observable .Generate(1, i =&gt; i &lt; 100, i =&gt; i, i =&gt; i + 1) .Zip(Observable.Interval(TimeSpan.FromMilliseconds(10.0)), (i, _) =&gt; i); var qsource = Observable .Generate(1, i =&gt; i &lt; 100, i =&gt; (double)i * i, i =&gt; i + 1) .Zip(Observable.Interval(TimeSpan.FromMilliseconds(30.0)), (i, _) =&gt; i); var result = SmartZip( psource, qsource, (p, q) =&gt; q / p).ToEnumerable(); foreach (var item in result) { Console.WriteLine(item); } } </code> </pre>
 

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