Merge hot source1 with a cold source2

Question

source1 emits A,B,C,D etc and never completes

source2 emit 1,2 and completes

I want to merge to A1, B2, C1, D2 etc

update

my initial attemp was to Zip and Repeat as suggested from Theodor however this creates a lock cause source2 generation is expensive.

Last comment from Enigmativity addresses that problem

source1.Zip(source2.ToEnumerable().ToArray().Repeat())

Answer 1

Since you want to repeat source2 indefinitely and you say that it is cold (in the sense that it produces the same set of values each time, and generally in the same sort of cadence) and it is expensive, we want to turn the IObservable<T> into a T[] to ensure it's computed once and only once.

var array = source2.ToEnumerable().ToArray();
var output = source1.Zip(array.Repeat(), (x, y) => (x, y));