(.NET 3.5) Randomise a List of Items

[jmcilhinney]

C# 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:

Imports System.Runtime.CompilerServices

Public Module Enumerable

    <Extension()> _
    Public Function Randomise(Of T)(ByVal items As IEnumerable(Of T)) As IEnumerable(Of T)
        Dim result As T() = Nothing

        If items IsNot Nothing Then
            Dim rng As New Random

            result = (From item In items Order By rng.NextDouble()).ToArray()
        End If

        Return result
    End Function

End Module

Note that it obeys the rules for extension methods, i.e. it's a member of a module, is decorated with the Extension attribute and takes the target type as the first argument. You might use it something like this:

Dim numbers As Integer() = {1, 2, 3, 4, 5, 6, 7, 8, 9}
 
For Each number In numbers.Randomise()
    MessageBox.Show(number.ToString())
Next

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:

Imports System.Runtime.CompilerServices
 
Public Module Enumerable
 
    <Extension>
    Public Function Randomise(Of T)(ByVal items As IEnumerable(Of T)) As IEnumerable(Of T)
        Dim result As T() = Nothing
 
        If items IsNot Nothing Then
            Dim rng As New Random
 
            result = items.ToArray()
 
            'Generate one random value per item.
            Dim keys = Array.ConvertAll(result, Function(item) rng.NextDouble())
 
            'Sort the items by their corresponding random keys, thus randomising them.
            Array.Sort(keys, result)
        End If
 
        Return result
    End Function
 
End Module