List. The VB.NET List is a resizable array. It stores a variable number of elements in an expandable collection. And it changes its size as needed, on its own.
Type. The List type offers many useful methods. We add elements, count elements, and can locate elements with IndexOf or Find. With special syntax we initialize lists.
First example. Here we create 2 Lists—each list has 4 elements: 4 Integers of the values 2, 3, 5 and 7. We call Add() to append to the end of the list. Count returns the length.
Version 1 We call Add 4 times to add the 4 Integers. We access Count, and get the last element (which is 7) by subtracting 1 from Count.
Version 2 This code does the same thing as version 1, but it uses a shorter syntax form—the List initializer—to create the list.
Module Module1
Sub Main()
' Version 1: create list of 4 Integers with Add.
Dim list As New List(Of Integer)
list.Add(2)
list.Add(3)
list.Add(5)
list.Add(7)
Console.WriteLine("LIST 1 COUNT: {0}", list.Count)
Console.WriteLine("LIST 1 LAST: {0}", list(list.Count - 1))
' Version 2: create list with initializer.
Dim list2 As New List(Of Integer)({2, 3, 5, 7})
Console.WriteLine("LIST 2 COUNT: {0}", list2.Count)
Console.WriteLine("LIST 2 LAST: {0}", list2(list2.Count - 1))
End Sub
End ModuleLIST 1 COUNT: 4
LIST 1 LAST: 7
LIST 2 COUNT: 4
LIST 2 LAST: 7
For Each, For. Looping is an important operation on Lists. We can use For-Each and For. When an index is not required, the For-Each loop is a better choice as it is simpler.
Part 1 We create a new List of Integers. Then, we add 3 integers to the List contents.
Part 2 We loop over the elements in the List with a For-Each loop construct. On each value, "number" refers to the current element.
Part 3 We use a For-loop. In the upper bound, we subtract 1 from list.Count. Item() gets each element by its index.
Module Module1
Sub Main()
' Part 1: create List and add 3 Integers.
Dim list As New List(Of Integer)
list.Add(2)
list.Add(3)
list.Add(7)
' Part 2: loop through List elements.
For Each number As Integer In list
Console.WriteLine("FOR EACH: {0}", number)
Next
' Part 3: loop through list with a for-to loop.
For i As Integer = 0 To list.Count - 1
Console.WriteLine("FOR: {0}", list.Item(i))
Next i
End Sub
End ModuleFOR EACH: 2
FOR EACH: 3
FOR EACH: 7
FOR: 2
FOR: 3
FOR: 7
First, last. Often we have a List and want to access the first and last elements. The first element is at index 0. The last is at Count minus one.
Module Program
Sub Main()
Dim items As New List(Of String)({"X", "T", "Z"})
' Get first.
Dim first As String = items(0)
Console.WriteLine("FIRST: {0}", first)
' Get last.
Dim last As String = items(items.Count - 1)
Console.WriteLine(" LAST: {0}", last)
End Sub
End ModuleFIRST: X
LAST: Z
Count, Clear. The Count property returns the total number of elements in the List. The Clear function removes all the elements so that the next invocation of Count will return zero.
Here We use the Count property and the Clear function on the List type instance.
Module Module1
Sub Main()
' Create a list of booleans.
Dim list As New List(Of Boolean)
list.Add(True)
list.Add(False)
list.Add(True)
' Write the count.
Console.WriteLine("COUNT: {0}", list.Count)
' Clear the list elements.
list.Clear()
' Write the count again.
Console.WriteLine("COUNT: {0}", list.Count)
End Sub
End ModuleCOUNT: 3
COUNT: 0
If-statement. We often need to scan (or loop) through the elements in the List and use logical tests on them. Perhaps we need to test each element against a specific value.
Tip To do this, we can use a For-Each loop and then an enclosed If-statement in the program.
Module Module1
Sub Main()
' Create a list of 3 integers.
Dim list As New List(Of Integer)(New Integer() {2, 3, 5})
' Loop through each number in the list.' ... Then check it against the integer 3.
For Each num As Integer In list
If (num = 3) Then
Console.WriteLine("Contains 3")
End If
Next
End Sub
End ModuleContains 3
Join. The String.Join function combines an array of strings into a single string, with a specific delimiter character dividing the parts. A List, or any IEnumerable type, can be used with Join.
Module Module1
Sub Main()
' Create a List of strings, and then use the String.Join method on it.
Dim colors As New List(Of String)
colors.Add("orange")
colors.Add("yellow")
colors.Add("mauve")
Console.WriteLine(String.Join(",", colors))
End Sub
End Moduleorange,yellow,mauve
Dictionary keys. The List and Dictionary collections can be used together. We can acquire a List of all of the keys from a Dictionary if needed.
Step 1 We create a Dictionary and add 2 keys to it—each key is an Integer, each value is a Boolean.
Step 2 We get the Keys by accessing the Keys property, and then create a new List with the List copy constructor.
Step 3 We use a For-Each loop to enumerate all the Integer keys, writing them to the screen.
Module Module1
Sub Main()
' Step 1: create a dictionary, and add 2 pairs.
Dim dictionary As New Dictionary(Of Integer, Boolean)
dictionary.Add(3, True)
dictionary.Add(5, False)
' Step 2: get the list of keys.
Dim list As New List(Of Integer)(dictionary.Keys)
' Step 3: loop and display the keys.
For Each number As Integer In list
Console.WriteLine("KEY: {0}", number)
Next
End Sub
End ModuleKEY: 3
KEY: 5
Insert. The List is optimized for end-insertion with the Add function. But we can perform a slower insertion in the middle of the List if needed.
Argument 1 The first argument must be the desired index for the element. The index 1 will make the element the second element.
Argument 2 The second argument is the element to insert. Here we insert a dog breed name into our List.
Module Module1
Sub Main()
' Create a list of strings.
Dim list As New List(Of String)
list.Add("spaniel")
list.Add("beagle")
' Insert a string into the list.
list.Insert(1, "dalmatian")
' Loop through the entire list.
For Each str As String In list
Console.WriteLine(str)
Next
End Sub
End Modulespaniel
dalmatian
beagle
AddRange, InsertRange. Suppose we want to append or insert an array of Integers into a List of Integers. We can use the AddRange or InsertRange Subs.
Tip The type of the array we insert (like Integer or String) must match the target List's element type.
Tip 2 We can insert one List into another with AddRange and InsertRange—the syntax is the same (just pass a list instead of an array).
Module Module1
Sub Main()
' Initial list.
Dim list As List(Of Integer) = New List(Of Integer)({1, 2})
' Add ints on the end.
list.AddRange(New Integer() {3, 4})
' Insert ints at the start.
list.InsertRange(0, New Integer() {-2, -1})
' Display final list.
For Each value In list
Console.WriteLine("FINAL LIST: {0}", value)
Next
End Sub
End ModuleFINAL LIST: -2
FINAL LIST: -1
FINAL LIST: 1
FINAL LIST: 2
FINAL LIST: 3
FINAL LIST: 4
GetRange. This function returns a range of elements within a List. We specify (as the first argument) the starting index of the range, and then the number of elements.
Step 1 We create a List of several strings. We specify string literals containing the names of shapes.
Step 2 We call GetRange in a For-Each loop—the loop only calls GetRange once, and it returns shape names starting at the specified index.
Module Module1
Sub Main()
' Step 1: create a List.
Dim list As New List(Of String)({"circle", "rectangle", "ellipse", "square", "triangle"})
' Step 2: call GetRange, and loop through the strings returned by GetRange.
For Each value As String In list.GetRange(1, 2)
Console.WriteLine("SHAPE: {0}", value)
Next
End Sub
End ModuleSHAPE: rectangle
SHAPE: ellipse
Remove. This function removes the first matching element by value. When we pass the value 20 to Remove, the first matching element is removed from the List.
Note If we call Remove() with a value that is not found in the List, nothing happens.
Module Module1
Sub Main()
Dim numbers = New List(Of Integer)({10, 20, 30})
' Remove this element by value.
numbers.Remove(20)
For Each number In numbers
Console.WriteLine("NOT REMOVED: {0}", number)
Next
' This will not change anything.
numbers.Remove(3000)
End Sub
End ModuleNOT REMOVED: 10
NOT REMOVED: 30
IndexOf. This function locates a value within a List. It searches from the first element and continues through to the last element.
Part 1 Here we call IndexOf and the value is located in the list. The value 20 occurs at index 1, so 1 is returned.
Part 2 We try to find 100, but it does not exist. IndexOf returns -1 if no matching value exists in the List.
Module Module1
Sub Main()
Dim sizes As List(Of Integer) = New List(Of Integer)
sizes.Add(10)
sizes.Add(20)
sizes.Add(30)
' Part 1: the value 20 occurs at the index 1.
Dim index20 As Integer = sizes.IndexOf(20)
Console.WriteLine(index20)
' Part 2: the value 100 does not occur, so IndexOf returns -1.
Dim index100 As Integer = sizes.IndexOf(100)
If index100 = -1 Then
Console.WriteLine("Not found")
End If
End Sub
End Module1
Not found
IEnumerable. The List (like an array) can be used as an IEnumerable type—we can pass List to methods where an IEnumerable is required. We can then loop over (with For Each) the collection.
Here We create a List and an array in Main. We pass those of those to the PrintElements Sub—it handles all IEnumerable types.
Tip Often we can use IEnumerable-receiving functions to share code that must act upon Lists and arrays.
Module Module1
Sub Main()
' Create List and pass it as an IEnumerable.
Dim list As List(Of Integer) = New List(Of Integer)({10, 20, 30})
PrintElements(list)
' Use array as an IEnumerable.
Dim array As Integer() = {100, 200, 300}
PrintElements(array)
End Sub
Sub PrintElements(ByVal elements As IEnumerable(Of Integer))
' Handle elements in IEnumerable.
For Each element In elements
Console.WriteLine("Element: {0}", element)
Next
End Sub
End ModuleElement: 10
Element: 20
Element: 30
Element: 100
Element: 200
Element: 300
Summary. The List type is a generic, constructed type. It has some performance advantages over other structures. In many programs, the List type is useful.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Oct 4, 2024 (new example).
