(.NET 3.5) Randomise a List of Items

[jmcilhinney]

VB version here.

The following is an extension method for getting the contents of any IEnumerable(Of T) object, e.g. an array or List(Of T), in random order:

using System;
using System.Collections.Generic;
using System.Linq;

public static class Enumerable
{
    public static IEnumerable<T> Randomise<T>(this IEnumerable<T> items)
    {
        T[] result = null;

        if (items != null)
        {
            var rng = new Random();

            result = (from item in items
                      orderby rng.NextDouble()
                      select item).ToArray();
        }

        return result;
    }
}

Note that it obeys the rules for extension methods, i.e. it's a member of a static class and the first argument is declared with the 'this' keyword. You might use it something like this:

int[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9};

foreach (var number in numbers.Randomise())
{
    MessageBox.Show(number.ToString());
}

Try running that repeatedly and you'll get a different order each time.

Note that you could write a regular method that does the same thing in earlier versions.

EDIT: It's been brought to my attention that there is a significant issue with this code that, while unlikely to be encountered, could cause the operation to fail. Here's a reimplementation that isn't susceptible to this issue:

using System;
using System.Collections.Generic;
using System.Linq;
 
public static class Enumerable
{
    public static IEnumerable<T> Randomise<T>(this IEnumerable<T> items)
    {
        T[] result = null;
 
        if (items != null)
        {
            var rng = new Random();
 
            result = items.ToArray();
 
            var keys = Array.ConvertAll(result, item => rng.NextDouble());
 
            Array.Sort(keys, result);
        }
 
        return result;
    }
}