Is it possible to create Array Formulas/CSE formulas when manually creating an excel file?
I believe that this functionality is supported. I will build a quick sample to demonstrate how you can do it. I expect to complete the sample within the week. Please let me know if you have any questions.
While Mike is making the sample, I can tell you that yes, array formulas are supported. You create one by using one of the static ArrayFormula.Parse or TryParse methods. This will return you an ArrayFormula instance if the formula was parsed correctly. Then call the ApplyTo method on that instance. Use the ApplyTo overload which takes a WorksheetRegion as its parameter and pass the region where you want the formula applied.
FYI, you can also use the WorksheetRegion.ApplyArrayFormula(string) instance method, which is a bit simpler, but doesn't give as many options regarding parsing.