How to get all the unique n-long combinations of a set of duplicatable elements?

Question

I have found many solutions giving a collection elements combined in all possible orders but they all use every element just once in every result while I need them to be tr

Answer 1

Eric Lippert has presented a general-purpose method of generating a cartesian product from any number of sequences which he blogs about here.

He wrote an extension method that looks like this:

public static class Combinations
{
    public static IEnumerable> CartesianProduct(this IEnumerable> sequences)
    {
        IEnumerable> emptyProduct = new[] { Enumerable.Empty() };
        return sequences.Aggregate(
            emptyProduct,
            (accumulator, sequence) =>
            from accseq in accumulator
            from item in sequence
            select accseq.Concat(new[] { item }));
    }
}

Given that extension method, the solution to the original problem can be done like this:

var items = new[] { "a", "b", "c" };

int numInEachSelection = 2;

var combs = Enumerable.Repeat(items, numInEachSelection).CartesianProduct();

foreach (var comb in combs)
    Console.WriteLine(string.Join(", ", comb));

Note that combs is an IEnumerable> - it is a sequence of enumerables each of which is a sequence representing one combination.

If you don't need a general-purpose method like that and you are happy to have each combination in a separate object with properties called Item1 and Item2 for the two combined items, the easiest way is this:

var items = new[] { "a", "b", "c" };

var combs = from Item1 in items from Item2 in items select new {Item1, Item2};

foreach (var comb in combs)
    Console.WriteLine(comb);