CODEWITHSUNDEEP
.netvb.netlinqienumerable
Rory Becker
Rory Becker

Reputation: 15701

How do I Aggregate multiple IEnumerables of T

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.

Upvotes: 18

Views: 4403

Answers (5)

Mark Hurd
Mark Hurd

Reputation: 10931

Just to provide true VB.NET answers:

' Identical to Per Erik Stendahl's and Oliver Hanappi's C# answers
Dim children1 = MasterList.SelectMany(Function(master) master.SubItems)

' Using VB.NET query syntax
Dim children2 = From master In MasterList, child in master.SubItems Select child

' Using Aggregate, as the question title referred to
Dim children3 = Aggregate master In MasterList Into SelectMany(master.SubItems)

These all compile down to the same IL, except children2 requires the equivalent of Function(master, child) child.

Upvotes: 3

yfeldblum
yfeldblum

Reputation: 65435

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).

Upvotes: 8

Oliver Hanappi
Oliver Hanappi

Reputation: 12336

You can achieve this by Linq with SelectMany

C# Code

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


Best Regards

Upvotes: 8

Per Erik Stendahl
Per Erik Stendahl

Reputation: 883

Are you looking for SelectMany()?

MasterList.SelectMany(master => master.SubItems)

Sorry for C#, don't know VB.

Upvotes: 27

Daniel A. White
Daniel A. White

Reputation: 190897

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.

Upvotes: 1

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