(Sittin' on the) Dock of the Bay: In WPF, SelectionChanged does not mean that the selection changed

link管理

链接快照平台

输入网页链接，自动生成快照
标签化管理网页链接

相关文章推荐

奔跑的茄子 · C# ...· 1 周前 ·

犯傻的手链 · (Sittin' on the) Dock ...· 5 天前 ·

踏实的胡萝卜 · wpf ...· 5 天前 ·

千杯不醉的绿茶 · WPF combobox ...· 5 天前 ·

愤怒的显示器 · 热烈庆祝网龙公司在香港主板上市--网龙公司- ...· 2 月前 ·

踏实的风衣 · vue中的数据更新后的Dom操作 ...· 5 月前 ·

叛逆的斑马 · RTOS/TMS320C6657: ...· 6 月前 ·

傻傻的凳子 · Used Cars New and ...· 7 月前 ·

性感的香蕉 · 最新！宜宾主城区限行通告！_澎湃号·媒体_澎 ...· 8 月前 ·

Windows Presentation Foundation's Routed Events can lead to unexpected or at least nonintuitive behavior when using TabControls that contain ListViews and/or ComboBoxes. A routed event generally bubbles from the control that raised it, up the whole element tree until the root. On its way up it invokes handlers on multiple listeners. This makes a lot of sense in the ButtonBase.Click Event : if a button is clicked, then its containing element is also clicked.
By design, the Selector.SelectionChanged Event is such a routed event. TabItem, ListBox, and ComboBox all inherit from Selector, so if you put them in a hierarchy they will register on each other's events. A ComboBox that appears via a template in a ListBox will raise the SelectionChanged event of that ListBox - even if the user didn't select a new ListBoxItem. If you put that ListBox in a TabControl, then the SelectionChanged on that TabControl will also be fired - even if the user didn't select a new TabItem.
Enough talking: let's build a small demo. First we build a Window with a TabControl that has a ComboBox in its first TabItem:

< Window x : Class ="DockOfTheBay.SelectorSampleWindow"

xmlns ="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns : x ="http://schemas.microsoft.com/winfx/2006/xaml"

Title ="Selector sample" Height ="200" Width ="400" >

< TabControl

x : Name ="MainTabControl"

SelectionChanged ="MainTabControl_SelectionChanged" >

< TabItem Header ="Courses" >

< TabItem.Content >

< ListBox x : Name ="ListBox1" />

</ TabItem.Content >

</ TabItem >

< TabItem Header ="Classrooms" >

</ TabItem >

</ TabControl >

</ Grid >

</ Window >

/// <summary>

/// The selection in the main tab control was changed.

/// </summary>

/// <param name="sender"> Sender of the event: the Main Tab. </param>

/// <param name="e"> Event arguments. </param>

private void MainTabControl_SelectionChanged( object sender, SelectionChangedEventArgs e)

// First Tab

if ( this .MainTabControl.SelectedIndex == 0)

// (Re-)Populate ListBox

this .ListBox1.Items.Clear();

this .ListBox1.Items.Add( "Building Windows Applications with WPF, LINQ and WCF" );

this .ListBox1.Items.Add( "Building Cloud based Enterprise Applications for Windows Azure" );

this .ListBox1.Items.Add( "Designing Data Warehouses using Dimensional Modeling" );

this .ListBox1.Items.Add( "Upgrade to SharePoint 2010" );
Switching from one tab to another behaves nicely. Unfortunately, clicking in the ListBox now also triggers the SelectionChanged event from the TabControl itself, resulting in unexpected behavior:
Before diving into solutions, let's make it worse by implementing a very popular pattern. When the user navigates to a new TabItem, we populate a ListBox, and programatically select its first item . In theory there's nothing wrong with this, in practice it creates an infinite loop (well, it's not really infinite: it stops when you're out of stack space):
We can solve this by letting the child controls prevent the event from propagating -via the Handled property- like this:

< TabItem Header ="Courses" >

< TabItem.Content >

< ListBox x : Name ="ListBox1"

SelectionChanged ="ListBox1_SelectionChanged" />

</ TabItem.Content >

</ TabItem >

/// <summary>

/// The selection in the listbox was changed.

/// </summary>

/// <param name="sender"> Sender of the event: the ListBox. </param>

/// <param name="e"> Event arguments. </param>

private void ListBox1_SelectionChanged( object sender, SelectionChangedEventArgs e)

// Stop the event from bubbling.

e.Handled = true ;
Of course, not all child controls will have an event handler or even need one (e.g. a ComboBox bound to a property of a Business Entity). Moreover, the parent control can not depend on the implementation of his children. Fortunately the parent control can decide to ignore all bubbled events from child controls like this:

/// <summary>

/// The selection in the main tab control was changed.

/// </summary>

/// <param name="sender"> Sender of the event: the Main Tab. </param>

/// <param name="e"> Event arguments. </param>

private void MainTabControl_SelectionChanged( object sender, SelectionChangedEventArgs e)

// Ignore Routed Events from children

if (e.OriginalSource == this .MainTabControl)

// First Tab

if ( this .MainTabControl.SelectedIndex == 0)

// (Re-)Populate ListBox

this .ListBox1.Items.Clear();

this .ListBox1.Items.Add( "Building Windows Applications with WPF, LINQ and WCF" );

this .ListBox1.Items.Add( "Building Cloud based Enterprise Applications for Windows Azure" );

this .ListBox1.Items.Add( "Designing Data Warehouses using Dimensional Modeling" );

this .ListBox1.Items.Add( "Upgrade to SharePoint 2010" );

// Select first item (no more 'Kaboom')

this .ListBox1.SelectedIndex = 0;
In practice this means that you should program this check on OriginalSource not only in every TabControl (because its TabItems can contain ListBoxes and ComboBoxes), but also in every ListBox (because its template can contain ComboBoxes).

Learning's Notes October 28, 2009 at 4:13 PM

Thanks, your article is very useful.
Reply Delete
Replies
Anonymous March 17, 2011 at 4:04 PM

Thank you, well described and very useful.
Reply Delete
Replies
xadriel September 20, 2012 at 12:12 AM

Thanks! I used "if(sender == e.OriginalSource)" instead, in order to recycle the method.
Reply Delete
Replies
Zeeshan Ahmed August 6, 2013 at 1:06 PM

thanks, how to do that in xaml for mvvm?
Reply Delete
Replies
Julie Arnold January 30, 2025 at 1:48 PM

This is a very helpful explanation and demonstration.
Reply Delete
Replies

Add comment

Diederik Krols is Principal Consultant for U2U Consult, a Microsoft partner. Diederik is an application architect, developer and development manager. He focuses on Microsoft .NET technology, the Microsoft SQL Server suite (relational engine as well as business intelligence platform), and SharePoint.

View my complete profile