Unique, Random Selections from a List

[jmcilhinney]

VB version here.

This is pretty simple stuff but it's come up more than once so I thought I'd post it. If you want to randomly select objects from a list where each object can only be chosen once you can use a collection and a Random object to select indexes into that collection. Below is an example using an ArrayList and the numbers 1 to 10. The objects could be anything though, not just numbers. In .NET 2.0 you'd more likely use a Generic.List<T> rather than an ArrayList.

ArrayList list = new ArrayList();

for (int i = 1; (i <= 10); i++)
{
    // Add the numbers to the collection.
    list.Add(i);
}

Random rand = new Random();
int index;
object item;

// Display the items in random order.
while ((list.Count > 0))
{
    // Choose a random index.
    index = rand.Next(0, list.Count);

    // Get the item at that index.
    item = list[index];

    // Remove the item so that it cannot be chosen again.
    list.RemoveAt(index);

    // Display the item.
    MessageBox.Show(item.ToString());
}