Convert list of elements with list to single list using lambda

Question

I have list of households and each households includes a list of residents. I want to have a function that returns a list of all residents of all households.

What I have so far:

public List<Resident> Residents() {
    var list = new List<Resident>();
    Households.ForEach(x => list.AddRange(x.Residents));
    return list;
}

Is it possible to shorten this with a lambda? Something like this:

public List<Resident> Residents => { ??? }

I know if Households were a list of lists I could use SelectMany, however it is not.

EDIT1:

To make it clear, Household is a class, not a container.

EDIT2:

I tried the following

public List<Resident> Residents => Households.SelectMany(h => h.Residents);

However this gives me the error:

error CS1061: 'List< Household>' does not contain a definition for 'SelectMany' and no extension method 'SelectMany' accepting a first argument of type 'List< Household>' could be found (are you missing a using directive or an assembly reference?)

EDIT3:

Sorry, I thought I was clear. Household is a class which has a list of residents, like this:

class Household {
    public List<Resident> Residents { get; }
}

I don't understand your remark about SelectMany. Why can't you do Households.SelectMany(x => x.Residents)? — Kevin Gosse
– Kevin Gosse, Commented Mar 16, 2016 at 10:34
What type is Hoseholds if not a list? Can't you do .ToList() on it before? Or have you forgot to add a using statement and that's why you don't see the SelectMany? — Niklas
– Niklas, Commented Mar 16, 2016 at 10:35
As @KooKiz noted, from your code it looks like you can simply do SelectMany. — Cetin Basoz
– Cetin Basoz, Commented Mar 16, 2016 at 10:35
Well err...no, it won't contain a definition for 'SeletMany', but it should have 1 for 'SelectMany'. — sr28
– sr28, Commented Mar 16, 2016 at 10:48

Enigmativity · Accepted Answer · 2016-03-16 10:47:35Z

4

You can do this in C# 6.0:

public List<Resident> Residents => Households.SelectMany(x => x.Residents).ToList();

You need to make sure that you have a using System.Linq; at the top of your code to get the IEnumerable<T> extension methods - which includes SelectMany.

answered Mar 16, 2016 at 10:47

Enigmativity

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1 Comment

gartenriese Over a year ago

This is the only answer that mentioned using System.Linq in their answer, so I'll accept this one.

Jamiec · Accepted Answer · 2016-03-16 10:37:34Z

3

Your code can be shortened to

var residents = Households.SelectMany(x => x.Residents).ToList();

residents will then be a IEnumerable<Resident> ( or whatever type Residents is, I've assumed its a type called Resident).

You can tag .ToList() on the end if you actually need a List<Resident>.

answered Mar 16, 2016 at 10:37

Jamiec

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Comments

Menelaos Vergis · Accepted Answer · 2016-03-16 12:14:48Z

2

make sure that you are using System.Linq

using System.Linq;

Households.SelectMany(x => x.Residents) should work no matter if Residents is an array or a List<>

edited Mar 16, 2016 at 12:14

answered Mar 16, 2016 at 10:38

Menelaos Vergis

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Comments

Mr. B · Accepted Answer · 2016-03-16 10:43:00Z

1

The following is the shortest I think:

public List<Resident> Residents() 
{
   return Households.SelectMany(x => x.Residents).ToList();
}

edited Mar 16, 2016 at 10:43

answered Mar 16, 2016 at 10:41

Mr. B

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2 Comments

Jamiec Over a year ago

You dont need the AddRange, and actually that wont compile as list does not exist in your code.

Mr. B Over a year ago

Yep just realized that I didn't pull that out

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