Initialize `List`

A VB.NET List starts out empty. But we can initialize it in a single line with an initializer. This makes programs easier to read and shorter.

And with the List constructor, we can use a capacity to improve performance. For optimal VB.NET programs, using a capacity and the Add() method is a good choice.

First example

Sometimes we want a String List. We can use several syntax forms for String Lists—these same syntax forms can be used for other Lists.

Version 1 We create a List by passing an array of values to the List constructor. We print the first value.

Version 2 We use a more verbose syntax for the List constructor call. We print the first value with Console.WriteLine.

Version 3 We directly call the Add() method and append strings to the String list.

Module Module1
    Sub Main()
        ' Version 1.
        Dim list As New List(Of String)({"carrot", "fox", "explorer"})
        Console.WriteLine(list(0))

        ' Version 2.
        Dim list2 As New List(Of String)(New String() {"carrot", "fox", "explorer"})
        Console.WriteLine(list2(0))

        ' Version 3.
        Dim list3 As New List(Of String)()
        list3.Add("carrot")
        list3.Add("fox")
        list3.Add("explorer")
        Console.WriteLine(list3(0))
    End Sub
End Modulecarrot
carrot
carrot

Integer `List` examples

Here we create 3 integer lists, with 3 different initialization approaches. The shortest syntax comes first.

Part 1 Here we use an expression. We can omit the "New Integer" part of the argument to the List constructor.

Part 2 We can create an array containing the elements we want the List to have, and pass that to the List constructor.

Part 3 We can add elements to a list with imperative statements (the Add method).

Module Module1
    Sub Main()
        ' Part 1: initialize with curly brackets.
        Dim list As New List(Of Integer)({20, 30, 500})
        For Each element In list
            Console.Write(element)
            Console.Write(";")
        Next
        Console.WriteLine()

        ' Part 2: initialize with a temporary array.
        Dim list2 As New List(Of Integer)(New Integer() {20, 30, 500})
        For Each element In list2
            Console.Write(element)
            Console.Write(";")
        Next
        Console.WriteLine()

        ' Part 3: initialize with Add calls.
        Dim list3 As New List(Of Integer)()
        list3.Add(20)
        list3.Add(30)
        list3.Add(500)
        For Each element In list3
            Console.Write(element)
            Console.Write(";")
        Next
        Console.WriteLine()
    End Sub
End Module20;30;500;
20;30;500;
20;30;500;

Benchmark, initialization

In this benchmark we see how a capacity can speed up list initialization. We create many Lists in a tight loop.

Version 1 This version of the code creates a List by passing an array to the List constructor. Two allocations will occur per call.

Version 2 Here we set a capacity of 5 in the List constructor. Then we directly invoke the Add() method with the values.

Result Version 2 is 4 times faster than version 1. A capacity will speed up List programs.

Module Module1

    Sub Main()
        Dim m As Integer = 10000000
        A()
        B()

        Dim s1 As Stopwatch = Stopwatch.StartNew
        ' Version 1: initialize list with an array argument.
        For i As Integer = 0 To m - 1
            A()
        Next
        s1.Stop()

        Dim s2 As Stopwatch = Stopwatch.StartNew
        ' Version 2: initialize list with Add() calls.
        For i As Integer = 0 To m - 1
            B()
        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

    Sub A()
        ' Add with initialization statement.
        Dim a As List(Of Integer) = New List(Of Integer)({400, 500, 600, 700, 800})
        If Not a(0) = 400 Then
            Console.WriteLine("X")
        End If
    End Sub

    Sub B()
        ' Add with Add() calls, specify capacity.
        Dim a As List(Of Integer) = New List(Of Integer)(5)
        a.Add(400)
        a.Add(500)
        a.Add(600)
        a.Add(700)
        a.Add(800)
        If Not a(0) = 400 Then
            Console.WriteLine("X")
        End If
    End Sub
End Module83.48 ns    Initialize with one statement
20.41 ns    Initialize with capacity, Add() calls

A summary

In VB.NET programs we often use Lists. These contain Integers, Strings and many other types of elements. With a capacity, we can initialize lists in the fastest way.

Initialize List

First example

Integer List examples

Benchmark, initialization

A summary

Initialize `List`

Integer `List` examples