`IEnumerable`

An IEnumerable collection contains elements. We use the For-Each loop in VB.NET to loop over these elements. Lists and arrays implement this interface.

We can use Extensions, such as ToList, on an IEnumerable type. Query expressions, part of LINQ, can be done with IEnumerables as well.

Example

We can use the IEnumerable type to reference types like lists and arrays. This can help programs become smaller and easier to maintain.

Step 1 We create a List and a String array. These types both implement IEnumerable.

Step 2 We pass the list and array to a custom Display Sub—it receives an IEnumerable(Of String) argument.

Step 3 In the Display Sub, we use the For-Each loop to enumerate the elements within the IEnumerable collection argument.

Important We can act upon a List and an array with the same code, by using the IEnumerable interface implemented by both types.

Module Module1

    Sub Main()
        ' Step 1: create a List and String array.
        Dim list1 As List(Of String) = New List(Of String)
        list1.Add("one")
        list1.Add("two")
        list1.Add("three")

        Dim array1(2) As String
        array1(0) = "ONE"
        array1(1) = "TWO"
        array1(2) = "THREE"

        ' Step 2: pass both to the Display sub.
        Display(list1)
        Display(array1)
    End Sub

    Sub Display(ByVal argument As IEnumerable(Of String))
        ' Step 3: loop over IEnumerable.
        For Each value As String In argument
            Console.WriteLine("Value: {0}", value)
        Next
        Console.WriteLine()
    End Sub

End ModuleValue: one
Value: two
Value: three

Value: ONE
Value: TWO
Value: THREE

Example 2

IEnumerable has other uses. Many Extension methods that act upon IEnumerable are implemented. These are available in all VB.NET programs based on recent .NET Frameworks.

Here We employ the ToList extension. This converts any IEnumerable type into a List. The element type remains the same.

Detail In this example, we implicitly cast the array into an IEnumerable reference. This step is not required.

Note We use this statement to show how the array is an IEnumerable instance, as well as an array.

Module Module1

    Sub Main()
        ' An array.
        ' ... This implements IEnumerable.
        Dim array1() As String = {"cat", "dog", "mouse"}

        ' Cast the array to IEnumerable reference.
        Dim reference1 As IEnumerable(Of String) = array1

        ' Use an extension method on IEnumerable.
        Dim list1 As List(Of String) = reference1.ToList()

        ' Display result.
        Console.WriteLine(list1.Count)
    End Sub

End Module
3

Discussion

The IEnumerable interface is a critical abstraction in .NET. Many Functions are built upon this abstraction. Even the For-Each loop, a core construct, directly uses IEnumerable.

Tip In most programs, we will not need to implement IEnumerable ourselves. This is possible, but is not covered here.

Detail A thorough understanding of how to use IEnumerable on built-in types, including Lists and arrays, often has more benefit.

We explored the IEnumerable type in VB.NET. This interface is needed for the For-Each loop. It can be used to act upon arrays, Lists and many other collections with a unified code path.