Can you enumerate a collection in C# out of order?

Question

Is there a way to use a foreach loop to iterate through a collection backwards or in a completely random order?

Answer 1

Using System.Linq you could do...

// List<...> list;
foreach (var i in list.Reverse())
{
}

For a random order you'd have to sort it randomly using list.OrderBy (another Linq extension) and then iterate that ordered list.

var rnd = new Random();
var randomlyOrdered = list.OrderBy(i => rnd.Next());
foreach (var i in randomlyOrdered)
{
}

Answer 2

As other answers mention, the Reverse() extension method will let you enumerate a sequence in reverse order.

Here's a random enumeration extension method:

public static IEnumerable<T> OrderRandomly<T>(this IEnumerable<T> sequence)
{
    Random random = new Random();
    List<T> copy = sequence.ToList();

    while (copy.Count > 0)
    {
        int index = random.Next(copy.Count);
        yield return copy[index];
        copy.RemoveAt(index);
    }
}

Your usage would be:

foreach (int n in Enumerable.Range(1, 10).OrderRandomly())
    Console.WriteLine(n);