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ListView.VirtualMode Property

Gets or sets a value indicating whether you have provided your own data-management operations for the ListView control.

Namespace:  System.Windows.Forms
Assembly:  System.Windows.Forms (in System.Windows.Forms.dll)

'Declaration
Public Property VirtualMode As Boolean
'Usage
Dim instance As ListView 
Dim value As Boolean 

value = instance.VirtualMode

instance.VirtualMode = value

Property Value

Type: System.Boolean
true if ListView uses data-management operations that you provide; otherwise, false. The default is false.

ExceptionCondition
InvalidOperationException

VirtualMode is set to true and one of the following conditions exist:

Setting the VirtualMode property to true puts the ListView into virtual mode. In Virtual mode, the normal Items collection is unused. Instead, ListViewItem objects are created dynamically as the ListView requires them.

Virtual mode can be useful under many circumstances. If a ListView object must be populated from a very large collection already in memory, creating a ListViewItem object for each entry can be wasteful. In virtual mode, only the items required are created. In other cases, the values of the ListViewItem objects may need to be recalculated frequently, and doing this for the whole collection would produce unacceptable performance. In virtual mode, only the required items are calculated.

In order to use virtual mode, you must handle the RetrieveVirtualItem event, which is raised every time the ListView requires an item. This event handler should create the ListViewItem object that belongs at the specified index. In addition, the VirtualListSize property must be set to the size of the virtual list.

Handling the SearchForVirtualItem event enables searching in virtual mode. If this event is not handled, the FindItemWithText and FindNearestItem methods will return Nothing.

You can handle the CacheVirtualItems event in order to maintain a cache of ListViewItem objects. If the calculation or lookup to create a ListViewItem object is expensive, maintaining a cache can improve performance.

If the View property is set to Tile, the value will automatically be changed to LargeIcon when VirtualMode is set to true.

In virtual mode, the Items collection is disabled. Attempting to access it results in an InvalidOperationException. The same is true of the CheckedItems collection and the SelectedItems collection. If you want to retrieve the selected or checked items, use the SelectedIndices and CheckedIndices collections instead.

This example illustrates a simple ListView whose contents are the first ten thousand squares. It handles searching and uses a cache for increased performance.

Public Class Form1
    Inherits Form
    Private myCache() As ListViewItem 'array to cache items for the virtual list
    Private firstItem As Integer 'stores the index of the first item in the cache
    Private WithEvents listView1 As ListView

    Public Shared Sub Main()
        Application.Run(New Form1)
    End Sub 

    Public Sub New()
        'Create a simple ListView.
        listView1 = New ListView()
        listView1.View = View.SmallIcon
        listView1.VirtualMode = True
        listView1.VirtualListSize = 10000

        'Add ListView to the form. 
        Me.Controls.Add(listView1)

        'Search for a particular virtual item. 
        'Notice that we never manually populate the collection! 
        'If you leave out the SearchForVirtualItem handler, this will return null. 
        Dim lvi As ListViewItem = listView1.FindItemWithText("111111")

        'Select the item found and scroll it into view. 
        If Not (lvi Is Nothing) Then
            listView1.SelectedIndices.Add(lvi.Index)
            listView1.EnsureVisible(lvi.Index)
        End If 

    End Sub 

    'The basic VirtualMode function.  Dynamically returns a ListViewItem 
    'with the required properties; in this case, the square of the index. 
    Private Sub listView1_RetrieveVirtualItem(ByVal sender As Object, ByVal e As RetrieveVirtualItemEventArgs) Handles listView1.RetrieveVirtualItem
        'Caching is not required but improves performance on large sets. 
        'To leave out caching, don't connect the CacheVirtualItems event  
        'and make sure myCache is null. 
        'check to see if the requested item is currently in the cache 
        If Not (myCache Is Nothing) AndAlso e.ItemIndex >= firstItem AndAlso e.ItemIndex < firstItem + myCache.Length Then 
            'A cache hit, so get the ListViewItem from the cache instead of making a new one.
            e.Item = myCache((e.ItemIndex - firstItem))
        Else 
            'A cache miss, so create a new ListViewItem and pass it back. 
            Dim x As Integer = e.ItemIndex * e.ItemIndex
            e.Item = New ListViewItem(x.ToString())
        End If 


    End Sub 

    'Manages the cache.  ListView calls this when it might need a  
    'cache refresh. 
    Private Sub listView1_CacheVirtualItems(ByVal sender As Object, ByVal e As CacheVirtualItemsEventArgs) Handles listView1.CacheVirtualItems
        'We've gotten a request to refresh the cache. 
        'First check if it's really neccesary. 
        If Not (myCache Is Nothing) AndAlso e.StartIndex >= firstItem AndAlso e.EndIndex <= firstItem + myCache.Length Then 
            'If the newly requested cache is a subset of the old cache,  
            'no need to rebuild everything, so do nothing. 
            Return 
        End If 

        'Now we need to rebuild the cache.
        firstItem = e.StartIndex
        Dim length As Integer = e.EndIndex - e.StartIndex + 1 'indexes are inclusive
        myCache = New ListViewItem(length) {}

        'Fill the cache with the appropriate ListViewItems. 
        Dim x As Integer = 0
        Dim i As Integer 
        For i = 0 To length
            x = (i + firstItem) * (i + firstItem)
            myCache(i) = New ListViewItem(x.ToString())
        Next i

    End Sub 

    'This event handler enables search functionality, and is called 
    'for every search request when in Virtual mode. 
    Private Sub listView1_SearchForVirtualItem(ByVal sender As Object, ByVal e As SearchForVirtualItemEventArgs) Handles listView1.SearchForVirtualItem
        'We've gotten a search request. 
        'In this example, finding the item is easy since it's 
        'just the square of its index.  We'll take the square root 
        'and round. 
        Dim x As Double = 0
        If [Double].TryParse(e.Text, x) Then 'check if this is a valid search
            x = Math.Sqrt(x)
            x = Math.Round(x)
            e.Index = Fix(x)
        End If 
        'Note that this only handles simple searches over the entire 
        'list, ignoring any other settings.  Handling Direction, StartIndex, 
        'and the other properties of SearchForVirtualItemEventArgs is up 
        'to this handler. 
    End Sub 

End Class

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0

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