Appending/concatenating two IEnumerable sequences

Question

I have two sets of datarows. They are each IEnumerable. I want to append/concatenate these two lists into one list. I\'m sure this is doable. I don\'t want to do a for loop

Answer 1

Delayed invocation of the second and subsequent enumerables

I usually use Linq IEnumerable.Concat() but today I needed to be 100% sure that the second enumeration was not enumerated until the first one has been processed until the end. (e.g. two db queries that I didn't want to run simultaneously). So the following function made the trick to delay the enumerations.

    IEnumerable DelayedConcat(params Func>[] enumerableList)
    {
        foreach(var enumerable in enumerableList)
        {
            foreach (var item in enumerable())
            {
                yield return item;
            }
        }
    }

Usage:

    return DelayedConcat(
                () => GetEnumerable1(),
                () => GetEnumerable2(),
 // and so on.. () => GetEnumerable3(),
                );

In this example GetEnumerable2 function invocation will be delayed until GetEnumerable1 has been enumerated till the end.