How do I Aggregate multiple IEnumerables of T

Question

Given....

Public MasterList as IEnumerable(Of MasterItem)
Public Class MasterItem(Of T) 
    Public SubItems as IEnumerable(Of T)
End Class

I would like a single IEnumerable(Of T) which will iterate through all SubItems of all MasterItems in MasterList

I would like to think that there is a Linq facility to do this, or an extension method I am overlooking. I need a mechanism that works in VB9 (2008) and hence does not use Yield.

Answer 1

I know in C# there is the yield operator for loops. Just iterate and yield return each sub item recursively. Apparently, there is no yield for VB, sorry.

Answer 2

Are you looking for SelectMany()?

MasterList.SelectMany(master => master.SubItems)

Sorry for C#, don't know VB.

Answer 3

Hi!

You can achieve this by Linq with SelectMany

C# Code

masterLists.SelectMany(l => l.SubItems);

Best Regards

Answer 4

Enumerable.SelectMany is the key to the IEnumerable monad, just as its Haskell equivalent, concatMap, is the key to Haskell's list monad.

As it turns out, your question goes right to the heart of a deep aspect of computer science.

You will want to be careful with your phrasing, because Aggregate means something very different from SelectMany - even the opposite. Aggregate combines an IEnumerable of values into a single value (of possibly another type), while SelectMany uncombines an IEnumerable of values into even more values (of possibly another type).