VB.NET Initialize ListInitialize Lists of Strings and Integers, using capacity to make List initialization faster.
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 A Here we use an expression. We can omit the "New Integer" part of the argument to the List constructor.
Part B We can create an array containing the elements we want the List to have, and pass that to the List constructor.
Part C We can add elements to a list with imperative statements (the Add method).
Module Module1
Sub Main()
' Part A: 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 B: 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 C: 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.
