Array

In an array, one element is stored after another. And with For Each, we can loop over these elements. The size of a VB.NET array cannot be changed once created.

Other collections are built with internal arrays. There are many ways to initialize arrays. We can use Array class methods to change them.

`String` array

A string array is created in various ways. In the VB.NET language we can create the array with all its data in an initialization statement.

Version 1 The first array is created with an initialization statement. We do not need to specify the size of the array on the left side.

Version 2 The next array uses the longer syntax. You must specify the capacity of the array in the Dim statement.

Info We loop over the arrays with For Each. And we pass the arrays to a Sub M, which prints their Lengths.

Module Module1

    Sub Main()
        ' Version 1: create an array with the simple initialization syntax.
        Dim array() As String = {"dog", "cat", "fish"}

        ' Loop over the array.
        For Each value As String In array
            Console.WriteLine(value)
        Next

        ' Pass array as argument.
        M(array)

        ' Version 2: create an array in several statements.
        ' ... Use the maximum index in the declaration.
        ' ... Begin indexing at zero.
        Dim array2(2) As String
        array2(0) = "bird"
        array2(1) = "dog"
        array2(2) = "gopher"

        ' Loop.
        For Each value As String In array2
            Console.WriteLine(value)
        Next

        ' Pass as argument.
        M(array2)
    End Sub

    Sub M(ByRef array() As String)
        ' Write length.
        Console.WriteLine(array.Length)
    End Sub

End Moduledog
cat
fish
3
bird
dog
gopher
3

First and last

We get the first element with the index 0. For the last element, we take the array Length and subtract one. This works on all non-empty, non-Nothing arrays.

Warning Arrays that are Nothing will cause a NullReferenceException to occur if you access elements on them.

Also An empty array (one with zero elements) has no first or last element, so this code also will fail.

Module Module1
    Sub Main()
        ' Get char array of 3 letters.
        Dim values() As Char = "abc".ToCharArray()

        ' Get first and last chars.
        Dim first As Char = values(0)
        Dim last As Char = values(values.Length - 1)

        ' Display results.
        Console.WriteLine(first)
        Console.WriteLine(last)
    End Sub
End Modulea
c

`For`-loop

We can use the For-loop construct. This allows us to access the index of each element. This is useful for additional computations or logic.

Also Using the For-loop syntax is better for when you need to modify elements within the array.

Module Module1
    Sub Main()
        ' New array of integers.
        Dim array() As Integer = {100, 300, 500}

        ' Use for-loop.
        For i As Integer = 0 To array.Length - 1
            Console.WriteLine(array(i))
        Next
    End Sub
End Module100
300
500

`For Each`, integers

This example creates an integer array. It specifies the maximum index in the first statement—we specify the maximum index, not the actual array element count.

Then We use For Each to enumerate its elements in order. This is clearer code than For.

Module Module1
    Sub Main()
        ' Create an array.
        Dim array(2) As Integer
        array(0) = 100
        array(1) = 10
        array(2) = 1

        For Each element As Integer In array
            Console.WriteLine(element)
        Next
    End Sub
End Module100
10
1

Integer array, initializer

Often we can initialize an array with a single expression. Here we use the curly-bracket initializer syntax to create an array of 3 elements.

Module Module1
    Sub Main()
        ' Create array of 3 integers.
        Dim array() As Integer = {10, 30, 50}

        For Each element As Integer In array
            Console.WriteLine(element)
        Next
    End Sub
End Module10
30
50

Array argument

We can pass an array to a subroutine or function. Please note that the entire array is not copied. Just a reference to the array is copied.

Detail When we pass the array ByVal, we can still modify the array's elements and have them changed elsewhere in the program.

Module Module1
    Sub Main()
        Dim array() As Integer = {5, 10, 20}
        ' Pass array as argument.
        Console.WriteLine(Example(array))
    End Sub

    ''' <summary>
    ''' Receive array parameter.
    ''' </summary>
    Function Example(ByVal array() As Integer) As Integer
        Return array(0) + 10
    End Function
End Module
15

Return

We can return an array. This program shows the correct syntax. The Example function creates a 2-element array and then returns it.

Then The Main subroutine displays all the returned results by calling String.Join on them.

Module Module1
    Sub Main()
        Console.WriteLine(String.Join(",", Example()))
    End Sub

    ''' <summary>
    ''' Return array.
    ''' </summary>
    Function Example() As String()
        Dim array(1) As String
        array(0) = "Perl"
        array(1) = "Python"
        Return array
    End Function
End Module
Perl,Python

`If`-test

Often we must check that an index is valid—that it exists within the array. We can use an If-statement. Here we check that the index 2 is less than the length (which is 3).

So The index 2 can be safely accessed. But the index 50, tried next, causes an IndexOutOfRangeException.

Module Module1
    Sub Main()
        Dim values() As Integer = {5, 10, 15}

        ' Check that index is a valid position in array.
        If 2 < values.Length Then
            Console.WriteLine(values(2))
        End If

        ' This causes an exception.
        Dim value = values(50)
    End Sub
End Module15

Unhandled Exception: System.IndexOutOfRangeException:
Index was outside the bounds of the array.

`Length`, `LongLength`

An array stores its element count. The Length property returns this count as an Integer. LongLength, useful for large arrays, returns a Long value.

Note The length of every array is stored directly inside the array object. It is not computed when we access it. This makes it fast.

Module Module1
    Sub Main()
        ' Create an array of 3 strings.
        Dim array() As String = {"Dot", "Net", "Perls"}
        Console.WriteLine(array.Length)
        Console.WriteLine(array.LongLength)

        ' Change the array to have two strings.
        array = {"OK", "Computer"}
        Console.WriteLine(array.Length)
        Console.WriteLine(array.LongLength)
    End Sub
End Module3
3
2
2

Empty array

How does one create an empty array in VB.NET? We specify the maximum index -1 when we declare the array—this means zero elements.

Detail To test for an empty array, use an if-statement and access the Length property, comparing it against 0.

Module Module1
    Sub Main()
        ' Create an empty array.
        Dim array(-1) As Integer

        ' Test for empty array.
        If array.Length = 0 Then
            Console.WriteLine("ARRAY IS EMPTY")
        End If
    End Sub
End Module
ARRAY IS EMPTY

Object arrays

In VB.NET, Strings inherit from the Object class. So we can pass an array of Strings to a method that requires an array of Objects.

Module Module1
    Sub Main()
        ' Use string array as an object array.
        PrintLength(New String() {"Mac", "PC", "Android"})
    End Sub

    Sub PrintLength(ByVal array As Object())
        Console.WriteLine("OBJECTS: {0}", array.Length)
    End Sub
End Module
OBJECTS: 3

`IEnumerable`

Arrays in VB.NET implement the IEnumerable interface (as does the List type). So we can pass an array to a method that accepts an IEnumerable.

Info We can then loop over the elements in the IEnumerable argument with For Each.

Module Module1
    Sub Main()
        ' Pass an array to a method that requires an IEnumerable.
        LoopOverItems(New Integer() {100, 500, 0})
    End Sub

    Sub LoopOverItems(ByVal items As IEnumerable)
        ' Use For Each loop over IEnumerable argument.
        For Each item In items
            Console.WriteLine("ITEM: {0}", item)
        Next
    End Sub
End ModuleITEM: 100
ITEM: 500
ITEM: 0

Benchmark, array creation

Should we just use Lists instead of arrays in all places? The List generic in VB.NET has an additional performance cost over an equivalent array.

Version 1 This code creates many zero-element Integer arrays and then tests their Lengths.

Version 2 Here we create many zero-element (empty) Integer Lists, and then we access their Count properties.

Result Creating an array is faster than creating an equivalent List. Consider replacing Lists with arrays in hot code.

Module Module1

    Sub Main()
        Dim m As Integer = 10000000
        ' Version 1: create an empty Integer array.
        Dim s1 As Stopwatch = Stopwatch.StartNew
        For i As Integer = 0 To m - 1
            Dim array(-1) As Integer
            If array.Length <> 0 Then
                Return
            End If
        Next
        s1.Stop()

        ' Version 2: create an empty List of Integers.
        Dim s2 As Stopwatch = Stopwatch.StartNew
        For i As Integer = 0 To m - 1
            Dim list As List(Of Integer) = New List(Of Integer)()
            If list.Count <> 0 Then
                Return
            End If
        Next
        s2.Stop()

        Dim u As Integer = 1000000
        Console.WriteLine(((s1.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
        Console.WriteLine(((s2.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
    End Sub

End Module3.68 ns     array(-1)
10.46 ns    New List

Review

Arrays are an important type. They are used inside other types, such as List and Dictionary, to implement those types storage. They are often faster.