This section contains two code examples. The first example demonstrates several properties and methods of the Collection<(Of <(T>)>) class. The second example shows how to derive a collection class from a constructed type of Collection<(Of <(T>)>), and how to override the protected methods of Collection<(Of <(T>)>) to provide custom behavior.
Example 1
The following code example demonstrates many of the properties and methods of Collection<(Of <(T>)>). The code example creates a collection of strings, uses the Add method to add several strings, displays the Count, and lists the strings. The example uses the IndexOf method to find the index of a string and the Contains method to determine whether a string is in the collection. The example inserts a string using the Insert method and retrieves and sets strings using the default Item property (the indexer in C#). The example removes strings by string identity using the Remove method and by index using the RemoveAt method. Finally, the Clear method is used to clear all strings from the collection.
Imports System
Imports System.Collections.Generic
Imports System.Collections.ObjectModel
Public Class Demo
Public Shared Sub Main()
Dim dinosaurs As New Collection(Of String)
dinosaurs.Add("Psitticosaurus")
dinosaurs.Add("Caudipteryx")
dinosaurs.Add("Compsognathus")
dinosaurs.Add("Muttaburrasaurus")
Console.WriteLine("{0} dinosaurs:", dinosaurs.Count)
Display(dinosaurs)
Console.WriteLine(vbLf & "IndexOf(""Muttaburrasaurus""): {0}", _
dinosaurs.IndexOf("Muttaburrasaurus"))
Console.WriteLine(vbLf & "Contains(""Caudipteryx""): {0}", _
dinosaurs.Contains("Caudipteryx"))
Console.WriteLine(vbLf & "Insert(2, ""Nanotyrannus"")")
dinosaurs.Insert(2, "Nanotyrannus")
Display(dinosaurs)
Console.WriteLine(vbLf & "dinosaurs(2): {0}", dinosaurs(2))
Console.WriteLine(vbLf & "dinosaurs(2) = ""Microraptor""")
dinosaurs(2) = "Microraptor"
Display(dinosaurs)
Console.WriteLine(vbLf & "Remove(""Microraptor"")")
dinosaurs.Remove("Microraptor")
Display(dinosaurs)
Console.WriteLine(vbLf & "RemoveAt(0)")
dinosaurs.RemoveAt(0)
Display(dinosaurs)
Console.WriteLine(vbLf & "dinosaurs.Clear()")
dinosaurs.Clear()
Console.WriteLine("Count: {0}", dinosaurs.Count)
End Sub
Private Shared Sub Display(ByVal cs As Collection(Of String))
Console.WriteLine()
For Each item As String In cs
Console.WriteLine(item)
Next item
End Sub
End Class
' This code example produces the following output:
'
'4 dinosaurs:
'
'Psitticosaurus
'Caudipteryx
'Compsognathus
'Muttaburrasaurus
'
'IndexOf("Muttaburrasaurus"): 3
'
'Contains("Caudipteryx"): True
'
'Insert(2, "Nanotyrannus")
'
'Psitticosaurus
'Caudipteryx
'Nanotyrannus
'Compsognathus
'Muttaburrasaurus
'
'dinosaurs(2): Nanotyrannus
'
'dinosaurs(2) = "Microraptor"
'
'Psitticosaurus
'Caudipteryx
'Microraptor
'Compsognathus
'Muttaburrasaurus
'
'Remove("Microraptor")
'
'Psitticosaurus
'Caudipteryx
'Compsognathus
'Muttaburrasaurus
'
'RemoveAt(0)
'
'Caudipteryx
'Compsognathus
'Muttaburrasaurus
'
'dinosaurs.Clear()
'Count: 0
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
public class Demo
{
public static void Main()
{
Collection<string> dinosaurs = new Collection<string>();
dinosaurs.Add("Psitticosaurus");
dinosaurs.Add("Caudipteryx");
dinosaurs.Add("Compsognathus");
dinosaurs.Add("Muttaburrasaurus");
Console.WriteLine("{0} dinosaurs:", dinosaurs.Count);
Display(dinosaurs);
Console.WriteLine("\nIndexOf(\"Muttaburrasaurus\"): {0}",
dinosaurs.IndexOf("Muttaburrasaurus"));
Console.WriteLine("\nContains(\"Caudipteryx\"): {0}",
dinosaurs.Contains("Caudipteryx"));
Console.WriteLine("\nInsert(2, \"Nanotyrannus\")");
dinosaurs.Insert(2, "Nanotyrannus");
Display(dinosaurs);
Console.WriteLine("\ndinosaurs[2]: {0}", dinosaurs[2]);
Console.WriteLine("\ndinosaurs[2] = \"Microraptor\"");
dinosaurs[2] = "Microraptor";
Display(dinosaurs);
Console.WriteLine("\nRemove(\"Microraptor\")");
dinosaurs.Remove("Microraptor");
Display(dinosaurs);
Console.WriteLine("\nRemoveAt(0)");
dinosaurs.RemoveAt(0);
Display(dinosaurs);
Console.WriteLine("\ndinosaurs.Clear()");
dinosaurs.Clear();
Console.WriteLine("Count: {0}", dinosaurs.Count);
}
private static void Display(Collection<string> cs)
{
Console.WriteLine();
foreach( string item in cs )
{
Console.WriteLine(item);
}
}
}
/* This code example produces the following output:
4 dinosaurs:
Psitticosaurus
Caudipteryx
Compsognathus
Muttaburrasaurus
IndexOf("Muttaburrasaurus"): 3
Contains("Caudipteryx"): True
Insert(2, "Nanotyrannus")
Psitticosaurus
Caudipteryx
Nanotyrannus
Compsognathus
Muttaburrasaurus
dinosaurs[2]: Nanotyrannus
dinosaurs[2] = "Microraptor"
Psitticosaurus
Caudipteryx
Microraptor
Compsognathus
Muttaburrasaurus
Remove("Microraptor")
Psitticosaurus
Caudipteryx
Compsognathus
Muttaburrasaurus
RemoveAt(0)
Caudipteryx
Compsognathus
Muttaburrasaurus
dinosaurs.Clear()
Count: 0
*/
Example 2
The following code example shows how to derive a collection class from a constructed type of the Collection<(Of <(T>)>) generic class, and how to override the protected InsertItem, RemoveItem, ClearItems, and SetItem methods to provide custom behavior for the Add, Insert, Remove, and Clear methods, and for setting the Item property.
The custom behavior provided by this example is a Changed notification event that is raised at the end of each of the protected methods. The Dinosaurs class inherits Collection<string> (Collection(Of String) in Visual Basic) and defines the Changed event, which uses a DinosaursChangedEventArgs class for the event information, and an enumeration to identify the kind of change.
The code example calls several properties and methods of Collection<(Of <(T>)>) to demonstrate the custom event.
Imports System
Imports System.Collections.Generic
Imports System.Collections.ObjectModel
Public Class Dinosaurs
Inherits Collection(Of String)
Public Event Changed As EventHandler(Of DinosaursChangedEventArgs)
Protected Overrides Sub InsertItem( _
ByVal index As Integer, ByVal newItem As String)
MyBase.InsertItem(index, newItem)
RaiseEvent Changed(Me, New DinosaursChangedEventArgs( _
ChangeType.Added, newItem, Nothing))
End Sub
Protected Overrides Sub SetItem(ByVal index As Integer, _
ByVal newItem As String)
Dim replaced As String = Items(index)
MyBase.SetItem(index, newItem)
RaiseEvent Changed(Me, New DinosaursChangedEventArgs( _
ChangeType.Replaced, replaced, newItem))
End Sub
Protected Overrides Sub RemoveItem(ByVal index As Integer)
Dim removedItem As String = Items(index)
MyBase.RemoveItem(index)
RaiseEvent Changed(Me, New DinosaursChangedEventArgs( _
ChangeType.Removed, removedItem, Nothing))
End Sub
Protected Overrides Sub ClearItems()
MyBase.ClearItems()
RaiseEvent Changed(Me, New DinosaursChangedEventArgs( _
ChangeType.Cleared, Nothing, Nothing))
End Sub
End Class
' Event argument for the Changed event.
'
Public Class DinosaursChangedEventArgs
Inherits EventArgs
Public ReadOnly ChangedItem As String
Public ReadOnly ChangeType As ChangeType
Public ReadOnly ReplacedWith As String
Public Sub New(ByVal change As ChangeType, ByVal item As String, _
ByVal replacement As String)
ChangeType = change
ChangedItem = item
ReplacedWith = replacement
End Sub
End Class
Public Enum ChangeType
Added
Removed
Replaced
Cleared
End Enum
Public Class Demo
Public Shared Sub Main()
Dim dinosaurs As New Dinosaurs
AddHandler dinosaurs.Changed, AddressOf ChangedHandler
dinosaurs.Add("Psitticosaurus")
dinosaurs.Add("Caudipteryx")
dinosaurs.Add("Compsognathus")
dinosaurs.Add("Muttaburrasaurus")
Display(dinosaurs)
Console.WriteLine(vbLf & "IndexOf(""Muttaburrasaurus""): {0}", _
dinosaurs.IndexOf("Muttaburrasaurus"))
Console.WriteLine(vbLf & "Contains(""Caudipteryx""): {0}", _
dinosaurs.Contains("Caudipteryx"))
Console.WriteLine(vbLf & "Insert(2, ""Nanotyrannus"")")
dinosaurs.Insert(2, "Nanotyrannus")
Console.WriteLine(vbLf & "dinosaurs(2): {0}", dinosaurs(2))
Console.WriteLine(vbLf & "dinosaurs(2) = ""Microraptor""")
dinosaurs(2) = "Microraptor"
Console.WriteLine(vbLf & "Remove(""Microraptor"")")
dinosaurs.Remove("Microraptor")
Console.WriteLine(vbLf & "RemoveAt(0)")
dinosaurs.RemoveAt(0)
Display(dinosaurs)
End Sub
Private Shared Sub Display(ByVal cs As Collection(Of String))
Console.WriteLine()
For Each item As String In cs
Console.WriteLine(item)
Next item
End Sub
Private Shared Sub ChangedHandler(ByVal source As Object, _
ByVal e As DinosaursChangedEventArgs)
If e.ChangeType = ChangeType.Replaced Then
Console.WriteLine("{0} was replaced with {1}", _
e.ChangedItem, e.ReplacedWith)
ElseIf e.ChangeType = ChangeType.Cleared Then
Console.WriteLine("The dinosaur list was cleared.")
Else
Console.WriteLine("{0} was {1}.", _
e.ChangedItem, e.ChangeType)
End If
End Sub
End Class
' This code example produces the following output:
'
'Psitticosaurus was Added.
'Caudipteryx was Added.
'Compsognathus was Added.
'Muttaburrasaurus was Added.
'
'Psitticosaurus
'Caudipteryx
'Compsognathus
'Muttaburrasaurus
'
'IndexOf("Muttaburrasaurus"): 3
'
'Contains("Caudipteryx"): True
'
'Insert(2, "Nanotyrannus")
'Nanotyrannus was Added.
'
'dinosaurs(2): Nanotyrannus
'
'dinosaurs(2) = "Microraptor"
'Nanotyrannus was replaced with Microraptor
'
'Remove("Microraptor")
'Microraptor was Removed.
'
'RemoveAt(0)
'Psitticosaurus was Removed.
'
'Caudipteryx
'Compsognathus
'Muttaburrasaurus
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
public class Dinosaurs : Collection<string>
{
public event EventHandler<DinosaursChangedEventArgs> Changed;
protected override void InsertItem(int index, string newItem)
{
base.InsertItem(index, newItem);
EventHandler<DinosaursChangedEventArgs> temp = Changed;
if (temp != null)
{
temp(this, new DinosaursChangedEventArgs(
ChangeType.Added, newItem, null));
}
}
protected override void SetItem(int index, string newItem)
{
string replaced = Items[index];
base.SetItem(index, newItem);
EventHandler<DinosaursChangedEventArgs> temp = Changed;
if (temp != null)
{
temp(this, new DinosaursChangedEventArgs(
ChangeType.Replaced, replaced, newItem));
}
}
protected override void RemoveItem(int index)
{
string removedItem = Items[index];
base.RemoveItem(index);
EventHandler<DinosaursChangedEventArgs> temp = Changed;
if (temp != null)
{
temp(this, new DinosaursChangedEventArgs(
ChangeType.Removed, removedItem, null));
}
}
protected override void ClearItems()
{
base.ClearItems();
EventHandler<DinosaursChangedEventArgs> temp = Changed;
if (temp != null)
{
temp(this, new DinosaursChangedEventArgs(
ChangeType.Cleared, null, null));
}
}
}
// Event argument for the Changed event.
//
public class DinosaursChangedEventArgs : EventArgs
{
public readonly string ChangedItem;
public readonly ChangeType ChangeType;
public readonly string ReplacedWith;
public DinosaursChangedEventArgs(ChangeType change, string item,
string replacement)
{
ChangeType = change;
ChangedItem = item;
ReplacedWith = replacement;
}
}
public enum ChangeType
{
Added,
Removed,
Replaced,
Cleared
};
public class Demo
{
public static void Main()
{
Dinosaurs dinosaurs = new Dinosaurs();
dinosaurs.Changed += ChangedHandler;
dinosaurs.Add("Psitticosaurus");
dinosaurs.Add("Caudipteryx");
dinosaurs.Add("Compsognathus");
dinosaurs.Add("Muttaburrasaurus");
Display(dinosaurs);
Console.WriteLine("\nIndexOf(\"Muttaburrasaurus\"): {0}",
dinosaurs.IndexOf("Muttaburrasaurus"));
Console.WriteLine("\nContains(\"Caudipteryx\"): {0}",
dinosaurs.Contains("Caudipteryx"));
Console.WriteLine("\nInsert(2, \"Nanotyrannus\")");
dinosaurs.Insert(2, "Nanotyrannus");
Console.WriteLine("\ndinosaurs[2]: {0}", dinosaurs[2]);
Console.WriteLine("\ndinosaurs[2] = \"Microraptor\"");
dinosaurs[2] = "Microraptor";
Console.WriteLine("\nRemove(\"Microraptor\")");
dinosaurs.Remove("Microraptor");
Console.WriteLine("\nRemoveAt(0)");
dinosaurs.RemoveAt(0);
Display(dinosaurs);
}
private static void Display(Collection<string> cs)
{
Console.WriteLine();
foreach( string item in cs )
{
Console.WriteLine(item);
}
}
private static void ChangedHandler(object source,
DinosaursChangedEventArgs e)
{
if (e.ChangeType==ChangeType.Replaced)
{
Console.WriteLine("{0} was replaced with {1}", e.ChangedItem,
e.ReplacedWith);
}
else if(e.ChangeType==ChangeType.Cleared)
{
Console.WriteLine("The dinosaur list was cleared.");
}
else
{
Console.WriteLine("{0} was {1}.", e.ChangedItem, e.ChangeType);
}
}
}
/* This code example produces the following output:
Psitticosaurus was Added.
Caudipteryx was Added.
Compsognathus was Added.
Muttaburrasaurus was Added.
Psitticosaurus
Caudipteryx
Compsognathus
Muttaburrasaurus
IndexOf("Muttaburrasaurus"): 3
Contains("Caudipteryx"): True
Insert(2, "Nanotyrannus")
Nanotyrannus was Added.
dinosaurs[2]: Nanotyrannus
dinosaurs[2] = "Microraptor"
Nanotyrannus was replaced with Microraptor
Remove("Microraptor")
Microraptor was Removed.
RemoveAt(0)
Psitticosaurus was Removed.
Caudipteryx
Compsognathus
Muttaburrasaurus
*/