What's the most efficient way to combine two List(Of String)?

Question

Let's say I've got:

Dim los1 as New List(Of String)
los1.Add("Some value")

Dim los2 as New List(Of String)
los2.Add("More values")

What would be the most efficient way to combine the two into a single List(Of String)?

Edit: While I love the solutions everyone has provided, I probably should have also mentioned I'm stuck using the .NET 2.0 framework.

I guess I should have been more specific. I'm mainly looking for the fastest solution possible, but memory usage should also be a consideration. :) — Jason Towne, Mar 16 '10 at 20:14

score 41 · Accepted Answer · answered Mar 16 '10 at 19:56

41

I think

los1.AddRange(los2)

answered Mar 16 '10 at 19:56

Javier

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This works so long as you don't mind that los1 is altered. Also, it is slower than Concat since it will need to allocate memory for the new items in los1. – Ryan Brunner Mar 16 '10 at 19:57
1

Yes, I supposed that los1 can be altered. What you say about being slower than Concat is very interesting. Thanks – Javier Mar 16 '10 at 20:13
This is the method that worked for me, primarily because I'm stuck using the .NET 2.0 framework. – Jason Towne Mar 16 '10 at 20:27

score 19 · Answer 2 · edited Aug 15 '13 at 14:33

19

If you just want to see a sequence of the two lists then use the Enumerable.Concat method.

Dim combined = los1.Concat(los2)

This will return an IEnemurable(Of String) which contains all of the elements in los1 and los2. It won't allocate a huge new collection but instead will iterate over the 2 separate collections.

edited Aug 15 '13 at 14:33

basher

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answered Mar 16 '10 at 19:53

JaredPar

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especially if need only to foreach them. memory will not be wasted. – Andrey Mar 16 '10 at 19:54
4

If you use this method, for the love of God turn Option Infer on. (Otherwise `combined` is an `Object` and VB will let you attempt *anything* with it.) Alternatively, you could just type `Dim combined As IEnumerable(Of String)`... By the way, you've got a lingering semi-colon there ;) – Dan Tao Mar 17 '10 at 11:51
This works but only of course only with `List (Of String)`. For myself I needed something that worked with other Objects. – JFreeman Apr 17 '18 at 08:26

Dan Tao · Answer 3 · 2010-03-17T11:48:36.287

JaredPar's answer will give you an object that will enumerate over both lists. If you actually want a List(Of String) object containing these values, it's as simple as:

Dim combined As New List(Of String)(los1.Concat(los2));

EDIT: You know, just because you're using .NET 2.0 doesn't mean you can't roll your own versions of some of the LINQ extension methods you personally find useful. The Concat method in particular would be quite trivial to implement in C#*:

public static class EnumerableUtils {
    public static IEnumerable<T> Concat<T>(IEnumerable<T> first, IEnumerable<T> second) {
        foreach (T item in first)
            yield return item;

        foreach (T item in second)
            yield return item;
    }
}

Then, see here:

Dim los1 as New List(Of String)
los1.Add("Some value")

Dim los2 as New List(Of String)
los2.Add("More values")

Dim combined As New List(Of String)(EnumerableUtils.Concat(los2, los2))

* _{To be fair, this is a lot more straightforward in C# thanks to the yield keyword. It could be done in VB.NET, but it'd be trickier to provide deferred execution in the same manner that the LINQ extensions do.}

You know, I never thought of recreating the LINQ extensions for my .NET 2.0 projects (although I probably should have). +1 to you and thank you for opening my eyes to something new! — Jason Towne, Mar 17 '10 at 15:24
@Jason: Yeah, the main hiccup that I could see you running into would be the lack of extension methods, making it tougher to get clean-looking code through method chaining. However, this can easily be accomplished by writing a class that wraps an `IEnumerable(Of T)` and provides equivalents of all the LINQ extensions (that you care about) as methods. I have in fact written a library that does precisely this, and was thinking about putting it up on an OSS hosting site. Let me know if you're interested. — Dan Tao, Mar 17 '10 at 15:41

score 1 · Answer 4 · answered Mar 16 '10 at 19:56

1

Union() potentially, if you want a distinct list of entries (I beleive it only does a distinct), is another alternative.

answered Mar 16 '10 at 19:56

Brian Mains

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You are correct, `Union` does return a list of the distinct values: as explained in the MSDN topic http://msdn.microsoft.com/en-us/library/bb341731.aspx – MarkJ Mar 17 '10 at 09:24

What's the most efficient way to combine two List(Of String)?

4 Answers4