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

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.

Upvotes: 29

Answers (4)

JaredPar

Reputation: 754505

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

_{Option Infer On}

Dim combined = los1.Concat(los2)

This will return an IEnumerable(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.

Upvotes: 19

Dan Tao

Reputation: 128317

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.}

Upvotes: 8