Collection(Of T).ClearItems Method
Removes all elements from the Collection(Of T).
Namespace: System.Collections.ObjectModel
Assembly: mscorlib (in mscorlib.dll)
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.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
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