Elegant way to combine multiple collections of elements?

Question

Say I have an arbitrary number of collections, each containing objects of the same type (for example, List foo and List bar).

Answer 1

Use Enumerable.Concact:

var query = foo.Concat(bar);

Answer 2

Use Enumerable.Concat like so:

var combined = foo.Concat(bar).Concat(baz)....;

Answer 3

A couple techniques using Collection Initializers --

assuming these lists:

var list1 = new List<int> { 1, 2, 3 };
var list2 = new List<int> { 4, 5, 6 };

SelectMany with an array initializer (not really that elegant to me, but doesn't rely on any helper function):

var combined = new []{ list1, list2 }.SelectMany(x => x);

Define a list extension for Add which allows IEnumerable<T> in the List<T> initializer:

public static class CollectionExtensions
{
    public static void Add<T>(this ICollection<T> collection, IEnumerable<T> items)
    {
        foreach (var item in items) collection.Add(item);
    }
}

Then you can create a new list containing the elements of the others like this (it also allows single items to be mixed in).

var combined = new List<int> { list1, list2, 7, 8 };

Answer 4

You could always use Aggregate combined with Concat...

        var listOfLists = new List<List<int>>
        {
            new List<int> {1, 2, 3, 4},
            new List<int> {5, 6, 7, 8},
            new List<int> {9, 10}
        };

        IEnumerable<int> combined = new List<int>();
        combined = listOfLists.Aggregate(combined, (current, list) => current.Concat(list)).ToList();

Answer 5

The only way I see is to use Concat()

 var foo = new List<int> { 1, 2, 3 };
 var bar = new List<int> { 4, 5, 6 };
 var tor = new List<int> { 7, 8, 9 };

 var result = foo.Concat(bar).Concat(tor);

But you should decide what is better:

var result = Combine(foo, bar, tor);

or

var result = foo.Concat(bar).Concat(tor);

One point why Concat() will be a better choice is that it will be more obvious for another developer. More readable and simple.