IList

The IList interface is similar to IEnumerable, but it allows for both looping and a Count property. Types such as arrays and Lists implement IList.

By specifying a parameter of type IList, we can pass either an array or a List to a method. IList is generic, so we must specify a type parameter on it.

Example

Here we specify a subroutine called Display that receives an IList of type Integer. We call Display() twice in the Main sub.

Module Module1
    Sub Main()
        ' Version 1: create Integer array and use as IList.
        Dim array(2) As Integer
        array(0) = 1
        array(1) = 2
        array(2) = 3
        Display(array)

        ' Version 2: create Integer List and use as IList.
        Dim list As List(Of Integer) = New List(Of Integer)()
        list.Add(5)
        list.Add(7)
        list.Add(9)
        Display(list)
    End Sub

    Sub Display(list as IList(Of Integer))
        ' Receive an IList, and print its Count and elements.
        Console.WriteLine("Count: {0}", list.Count)
        For Each value In list
            Console.WriteLine(value)
        Next
    End Sub
End Module
Count: 3
1
2
3
Count: 3
5
7
9

IEnumerable

How is IList different from IEnumerable? With IList, we have a Count property, whereas on IEnumerable, we can only enumerate the collection with For Each.

Module Module1
    Sub Main()
        ' Use Integer array as IEnumerable.
        Dim array() As Integer = { 500, 600, 700 }
        X(array)
    End Sub

    Sub X(list as IEnumerable(Of Integer))
        ' Receive an IEnumerable, which does not have a Count property.
        For Each value In list
            Console.WriteLine(value)
        Next
    End Sub
End Module
500
600
700

Summary

With IList, we can build a function in VB.NET that receives either a List or an array—or any other type that implements IList. This can lead to more reusable code, and simpler programs.