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Let's say I have two sequences returning integers 1 to 5. The first returns 1, 2 and 3 very fast, but 4 and 5 take 200ms each. public static IEnumerable<int> FastFirst() { for (int i = 1; i < 6; i++) { if (i > 3) Thread.Sleep(200); yield return i; } } The second returns 1, 2 and 3 with a 200ms delay, but 4 and 5 are returned fast. public static IEnumerable<int> SlowFirst() { for (int i = 1; i < 6; i++) { if (i < 4) Thread.Sleep(200); yield return i; } } Unioning both these sequences give me just numbers 1 to 5. FastFirst().Union(SlowFirst()); I cannot guarantee which of the two methods has delays at what point, so the order of the execution cannot guarantee a solution for me. Therefore, I would like to parallelise the union, in order to minimise the (artifical) delay in my example. A real-world scenario: I have a cache that returns some entities, and a datasource that returns all entities. I'd like to be able to return an iterator from a method that internally parallelises the request to both the cache and the datasource so that the cached results yield as fast as possible. Note 1: I realise this is still wasting CPU cycles; I'm not asking how can I prevent the sequences from iterating over their slow elements, just how I can union them as fast as possible. Update 1: I've tailored achitaka-san's great response to accept multiple producers, and to use ContinueWhenAll to set the BlockingCollection's CompleteAdding just the once. I just put it here since it would get lost in the lack of comments formatting. Any further feedback would be great! public static IEnumerable<TResult> SelectAsync<TResult>( params IEnumerable<TResult>[] producer) { var resultsQueue = new BlockingCollectio

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