Unique, Random Selections from a List

[jmcilhinney]

C #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(Of T) rather than an ArrayList.

Dim list As New ArrayList
 
        For i As Integer = 1 To 10
            'Add the numbers to the collection.
            list.Add(i)
        Next i
 
        Dim rand As New Random
        Dim index As Integer
        Dim item As Object
 
        '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())
        End While