Collection<T>.ClearItems Method
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Removes all elements from the Collection<T>.
Namespace: System.Collections.ObjectModel
Assembly: mscorlib (in mscorlib.dll)
Syntax
'Declaration
Protected Overridable Sub ClearItems
protected virtual void ClearItems()
Remarks
Count is set to zero, and references to other objects from elements of the collection are also released.
This method is an O(n) operation, where n is Count.
Notes to Inheritors
Derived classes can override this method to change the behavior of the Clear method.
Examples
The following code example shows how to derive a collection class from a constructed type of the Collection<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<T> to demonstrate the custom event.
Imports System.Collections.Generic
Imports System.Collections.ObjectModel
Imports System.Windows.Controls
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 Example
Private Shared _TextBlock As TextBlock
Public Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
Example._TextBlock = outputBlock
Dim dinosaurs As New Dinosaurs
AddHandler dinosaurs.Changed, AddressOf ChangedHandler
dinosaurs.Add("Psitticosaurus")
dinosaurs.Add("Caudipteryx")
dinosaurs.Add("Compsognathus")
dinosaurs.Add("Muttaburrasaurus")
Display(outputBlock, dinosaurs)
outputBlock.Text &= String.Format(vbLf & "IndexOf(""Muttaburrasaurus""): {0}" & vbCrLf, _
dinosaurs.IndexOf("Muttaburrasaurus"))
outputBlock.Text &= String.Format(vbLf & "Contains(""Caudipteryx""): {0}" & vbCrLf, _
dinosaurs.Contains("Caudipteryx"))
outputBlock.Text &= String.Format(vbLf & "Insert(2, ""Nanotyrannus"")") & vbCrLf
dinosaurs.Insert(2, "Nanotyrannus")
outputBlock.Text &= String.Format(vbLf & "dinosaurs(2): {0}" & vbCrLf, dinosaurs(2))
outputBlock.Text &= vbLf & "dinosaurs(2) = ""Microraptor""" & vbCrLf
dinosaurs(2) = "Microraptor"
outputBlock.Text &= vbLf & "Remove(""Microraptor"")" & vbCrLf
dinosaurs.Remove("Microraptor")
outputBlock.Text &= vbLf & "RemoveAt(0)" & vbCrLf
dinosaurs.RemoveAt(0)
Display(outputBlock, dinosaurs)
End Sub
Private Shared Sub Display(ByVal outputBlock As System.Windows.Controls.TextBlock, ByVal cs As Collection(Of String))
_TextBlock.Text &= vbCrLf
For Each item As String In cs
_TextBlock.Text &= item & vbCrLf
Next item
End Sub
Private Shared Sub ChangedHandler(ByVal source As Object, _
ByVal e As DinosaursChangedEventArgs)
If e.ChangeType = ChangeType.Replaced Then
_TextBlock.Text &= String.Format("{0} was replaced with {1}", _
e.ChangedItem, e.ReplacedWith) & vbCrLf
ElseIf e.ChangeType = ChangeType.Cleared Then
_TextBlock.Text &= "The dinosaur list was cleared." & vbCrLf
Else
_TextBlock.Text &= String.Format("{0} was {1}.", _
e.ChangedItem, e.ChangeType) & vbCrLf
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;
using System.Windows.Controls;
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 Example
{
private static TextBlock _textblock;
public static void Demo(TextBlock outputBlock)
{
Example._textblock = outputBlock;
Dinosaurs dinosaurs = new Dinosaurs();
dinosaurs.Changed += ChangedHandler;
dinosaurs.Add("Psitticosaurus");
dinosaurs.Add("Caudipteryx");
dinosaurs.Add("Compsognathus");
dinosaurs.Add("Muttaburrasaurus");
Display(dinosaurs);
outputBlock.Text += String.Format("\nIndexOf(\"Muttaburrasaurus\"): {0}\n",
dinosaurs.IndexOf("Muttaburrasaurus"));
outputBlock.Text += String.Format("\nContains(\"Caudipteryx\"): {0}\n",
dinosaurs.Contains("Caudipteryx"));
outputBlock.Text += String.Format("\nInsert(2, \"Nanotyrannus\")\n");
dinosaurs.Insert(2, "Nanotyrannus");
outputBlock.Text += String.Format("\ndinosaurs[2]: {0}\n", dinosaurs[2]);
outputBlock.Text += "\ndinosaurs[2] = \"Microraptor\"" + "\n";
dinosaurs[2] = "Microraptor";
outputBlock.Text += "\nRemove(\"Microraptor\")\n";
dinosaurs.Remove("Microraptor");
outputBlock.Text += "\nRemoveAt(0)\n";
dinosaurs.RemoveAt(0);
Display(dinosaurs);
}
private static void Display(Collection<string> cs)
{
_textblock.Text += "\n";
foreach (string item in cs)
{
_textblock.Text += item + "\n";
}
}
private static void ChangedHandler(object source,
DinosaursChangedEventArgs e)
{
if (e.ChangeType == ChangeType.Replaced)
{
_textblock.Text += String.Format("{0} was replaced with {1}\n", e.ChangedItem,
e.ReplacedWith);
}
else if (e.ChangeType == ChangeType.Cleared)
{
_textblock.Text += "The dinosaur list was cleared." + "\n";
}
else
{
_textblock.Text += String.Format("{0} was {1}.\n", 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
*/
Version Information
Silverlight
Supported in: 5, 4, 3
Silverlight for Windows Phone
Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0
XNA Framework
Supported in: Xbox 360, Windows Phone OS 7.0
Platforms
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.