{"id":993,"date":"2009-11-12T19:39:00","date_gmt":"2009-11-12T19:39:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/vbteam\/2009\/11\/12\/spot-the-bug-is-it-in-vb-or-xaml-jonathan-aneja\/"},"modified":"2024-07-05T12:42:22","modified_gmt":"2024-07-05T19:42:22","slug":"spot-the-bug-is-it-in-vb-or-xaml-jonathan-aneja","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/vbteam\/spot-the-bug-is-it-in-vb-or-xaml-jonathan-aneja\/","title":{"rendered":"Spot the Bug! – Is it in VB or XAML? (Jonathan Aneja)"},"content":{"rendered":"

What\u2019s wrong with the following WPF code?<\/font><\/p>\n

<\/font> <\/p>\n

Class<\/span> Window1 <\/span><\/p>\n

 <\/span><\/p>\n

  <\/span>Sub<\/span> OK_Click(sender As<\/span> Object<\/span>, e As<\/span> RoutedEventArgs) Handles <\/span>OK.Click<\/span><\/p>\n

      <\/span>MsgBox(“Button Clicked”<\/span>)<\/span><\/p>\n

  <\/span>End<\/span> Sub<\/span><\/span><\/p>\n

 <\/span><\/p>\n

End<\/span> Class<\/span><\/span><\/p>\n

 <\/span><\/p>\n

<<\/span>Window<\/span> x<\/span>:<\/span>Class<\/span>=”Window1″<\/span><\/p>\n

   <\/span> <\/span>xmlns<\/span>=”http:\/\/schemas.microsoft.com\/winfx\/2006\/xaml\/presentation”<\/span><\/span><\/p>\n

   <\/span> <\/span>xmlns<\/span>:<\/span>x<\/span>=”http:\/\/schemas.microsoft.com\/winfx\/2006\/xaml”<\/span><\/span><\/p>\n

   <\/span> <\/span>Title<\/span>=”Window1″<\/span> Height<\/span>=”300″<\/span> Width<\/span>=”300″><\/span><\/span><\/p>\n

    <\/span><\/span><<\/span>Grid<\/span>><\/span><\/p>\n

        <\/span><\/span><<\/span>Button<\/span> Name<\/span>=”OK”<\/span> Click<\/span>=”OK_Click”><\/span>OK<\/span><\/<\/span>Button<\/span>><\/span><\/p>\n

    <\/span><\/span><\/<\/span>Grid<\/span>><\/span><\/p>\n

<\/<\/span>Window<\/span>><\/span><\/p>\n

 <\/span><\/p>\n

.<\/font><\/p>\n

 <\/font><\/p>\n

.<\/font><\/p>\n

 <\/font><\/p>\n

.<\/font><\/p>\n

 <\/font><\/p>\n

.<\/font><\/p>\n

 <\/font><\/p>\n

.<\/font><\/p>\n

 <\/font><\/b><\/p>\n

Answer: <\/b>The message box will be displayed twice!  <\/span>To understand why, open up the project in Reflector, and in the System_Windows_Mark_IComponentConnector_Connect method you\u2019ll see these two lines:<\/font><\/font><\/p>\n

    <\/span>Me<\/span>.<\/font>OK<\/font><\/a> = DirectCast<\/span>(target, <\/font>Button<\/font><\/a>)<\/font><\/pre>\n
    <\/span>AddHandler Me<\/span>.<\/font>OK<\/font><\/a>.<\/font>Click<\/font><\/a>, New <\/span><\/font>RoutedEventHandler<\/font><\/a>(AddressOf Me<\/span>.<\/font>OK_Click<\/font><\/a>)<\/font><\/pre>\n
 <\/span><\/p>\n
Ok so we\u2019re adding one handler here, but click the link to navigate to \u201cOK\u201d.  <\/span>You\u2019ll see this is actually a property<\/i> of type Button (that\u2019s what the compiler does for WithEvents\/Handles \u2013 the full story of how WithEvents works is its own blog entry\u2026).  <\/span>In the Setter you\u2019ll see this code:<\/font><\/p>\n
 <\/font><\/p>\n
    <\/span>If<\/span> (Not<\/span> Me<\/span>.<\/font>_OK<\/font><\/a> Is<\/span> Nothing<\/span>) Then<\/span><\/font><\/pre>\n
        <\/span>AddHandler Me<\/span>.<\/font>_OK<\/font><\/a>.<\/font>Click<\/font><\/a>, handler<\/font><\/pre>\n
    <\/span>End If<\/span><\/font><\/pre>\n
 <\/font><\/p>\n
So now we can clearly see there\u2019s two AddHandler calls and thus the event gets handled multiple times by the same method (which is perfectly legal).  <\/span>One of them came from <\/font>Handles <\/span>OKButton.Click<\/span>, and the other came from <\/font>Click<\/span>=”OK_Click”<\/span>.  <\/span><\/font><\/font><\/p>\n
 <\/font><\/p>\n
The key thing to realize is that if you wire up the eventhandler in XAML, don\u2019t wire it up in code using Handles (and vice-versa).<\/i>  <\/span>Thankfully in this scenario it\u2019s obvious that there\u2019s a problem because there\u2019s a message box, but if the eventhandler was doing something more subtle (such as incrementing a counter or modifying some other state), this can lead to hard-to-diagnose bugs.<\/span><\/font><\/font><\/p>\n","protected":false},"excerpt":{"rendered":"
What\u2019s wrong with the following WPF code?   Class Window1     Sub OK_Click(sender As Object, e As RoutedEventArgs) Handles OK.Click       MsgBox(“Button Clicked”)   End Sub   End Class   <Window x:Class=”Window1″     xmlns=”http:\/\/schemas.microsoft.com\/winfx\/2006\/xaml\/presentation”     xmlns:x=”http:\/\/schemas.microsoft.com\/winfx\/2006\/xaml”     Title=”Window1″ Height=”300″ Width=”300″>     <Grid>         <Button Name=”OK” Click=”OK_Click”>OK<\/Button>     <\/Grid> <\/Window>   .   .   […]<\/p>\n","protected":false},"author":260,"featured_media":8818,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[195],"tags":[83,143,165,166,167],"class_list":["post-993","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-visual-basic","tag-jonathan-aneja","tag-spot-the-bug","tag-vb2005","tag-vb2008","tag-vb2010"],"acf":[],"blog_post_summary":"
What\u2019s wrong with the following WPF code?   Class Window1     Sub OK_Click(sender As Object, e As RoutedEventArgs) Handles OK.Click       MsgBox(“Button Clicked”)   End Sub   End Class   <Window x:Class=”Window1″     xmlns=”http:\/\/schemas.microsoft.com\/winfx\/2006\/xaml\/presentation”     xmlns:x=”http:\/\/schemas.microsoft.com\/winfx\/2006\/xaml”     Title=”Window1″ Height=”300″ Width=”300″>     <Grid>         <Button Name=”OK” Click=”OK_Click”>OK<\/Button>     <\/Grid> <\/Window>   .   .   […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/posts\/993","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/users\/260"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/comments?post=993"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/posts\/993\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/media\/8818"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/media?parent=993"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/categories?post=993"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/tags?post=993"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}