Concatenate multiple IEnumerable

Question

I'm trying to implement a method to concatenate multiple Lists e.g.

List<string> l1 = new List<string> { "1", "2" };
List<string> l2 = new List<string> { "1", "2" };
List<string> l3 = new List<string> { "1", "2" };
var result = Concatenate(l1, l2, l3);

but my method doesn't work:

public static IEnumerable<T> Concatenate<T>(params IEnumerable<T> List)
{
    var temp = List.First();
    for (int i = 1; i < List.Count(); i++)
    {
        temp = Enumerable.Concat(temp, List.ElementAt(i));
    }
    return temp;
}

Calling IEnumerable.Count() every cycle is a bit of a waste. Call it once and store it in a variable, or better, use a foreach loop: `var Temp = List.First(); foreach (IEnumerable sequence in List.Skip(1)) Temp = Enumerable.Concat(sequence);`. — Pieter Witvoet, Nov 21 '14 at 08:51

brz · Accepted Answer · 2015-08-18T03:23:48.703

90

Use SelectMany:

public static IEnumerable<T> Concatenate<T>(params IEnumerable<T>[] lists)
{
    return lists.SelectMany(x => x);
}

edited Aug 18 '15 at 03:23

answered Nov 21 '14 at 08:28

brz

5,926
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fubo · Answer 2 · 2018-05-14T06:02:59.747

8

Just for completeness another imo noteworthy approach:

public static IEnumerable<T> Concatenate<T>(params IEnumerable<T>[] List)
{
    foreach (IEnumerable<T> element in List)
    {
        foreach (T subelement in element)
        {
            yield return subelement;
        }
    }
}

edited May 14 '18 at 06:02

answered Nov 21 '14 at 09:19

fubo

44,811
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score 3 · Answer 3 · answered Nov 21 '14 at 08:31

3

If you want to make your function work you need an array of IEnumerable:

public static IEnumerable<T> Concartenate<T>(params IEnumerable<T>[] List)
{
    var Temp = List.First();
    for (int i = 1; i < List.Count(); i++)
    {
        Temp = Enumerable.Concat(Temp, List.ElementAt(i));
    }
    return Temp;
}

answered Nov 21 '14 at 08:31

Fabian S.

909
6
20

3

The parameter `List` is an array of IEnumerables and it could contain no items. This will cause `List.First()` to throw an exception. You should first check the length of this array. I will also use the `Length` property and the indexer `List[]` instead of the equivalent Linq extensions in the for loop. – Luca Cremonesi Feb 22 '16 at 16:51
3

Don't use a common class name as a variable name. =( – jpmc26 Jul 08 '17 at 00:35

score -2 · Answer 4 · edited May 23 '19 at 08:26

-2

All you have to do is to change:

public static IEnumerable<T> Concatenate<T>(params IEnumerable<T> lists)

to

public static IEnumerable<T> Concatenate<T>(params IEnumerable<T>[] lists)

Note the extra [].

edited May 23 '19 at 08:26

vsarunov

1,433
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answered Nov 21 '14 at 08:33

SmartDev

2,802
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Concatenate multiple IEnumerable

4 Answers4

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