Friday, September 25, 2009

Problem rendering AJAX PopupExtender over ActiveX in IE 7, IE 8 and FireFox.

If you happen to have some sort of ActiveX control on your page in an IFrame, such as PDF viewer, then some of the AJAX controls which use PopupExtender will be obscured by the ActiveX or any other browser plug-in which rendering happens out of the main page context.

For example menus, and CalendarExtender suffer this problem, since they inherit PopupBehavior.

What happens is Ajax PopupExtender behavior explicitly checks for browser version, in particular IE 6. In IE 6 there was a known problem with z-index so you had to use IFrames to render some popup controls, so PopupBehavior checks whether browser is IE 6 and creates an IFrame around control, otherwise no IFrame created, hence the problem in newer IEs and FireFox with browser plug-ins.

To fix this problem, first download source code for AJAX Control Toolkit from CodePlex. And find PopupExtender folder in VS project. Then find this function:

addBackgroundIFrame : function()

Remember to duplicate your effort for debug and release versions of code, since there are two separate files.

Now lets take a look at the original code:

addBackgroundIFrame : function() {
/// <summary>
/// Add an empty IFRAME behind the popup (for IE6 only) so that SELECT, etc., won't
/// show through the popup.
/// </summary>

// Get the child frame
var element = this.get_element();
if ((Sys.Browser.agent === Sys.Browser.InternetExplorer) && (Sys.Browser.version < 7)) {
var childFrame = element._hideWindowedElementsIFrame;

// Create the child frame if it wasn't found
if (!childFrame) {
childFrame = document.createElement("iframe");
childFrame.src = "javascript:'<html></html>';"; = "absolute"; = "none";
childFrame.scrolling = "no";
childFrame.frameBorder = "0";
childFrame.tabIndex = "-1"; = "progid:DXImageTransform.Microsoft.Alpha(style=0,opacity=0)";
element.parentNode.insertBefore(childFrame, element);
element._hideWindowedElementsIFrame = childFrame;
this._moveHandler = Function.createDelegate(this, this._onMove);
Sys.UI.DomEvent.addHandler(element, "move", this._moveHandler);

// Position the frame exactly behind the element
$common.setBounds(childFrame, $common.getBounds(element)); =; =; =;

if (element.currentStyle && element.currentStyle.zIndex) { = element.currentStyle.zIndex;
} else if ( { =;

Notice the section above, which has IF statement.

Remove that IF statement completely. Recompile your AjaxControlToolKit project and reference this new dll in your ASP.NET project. Drum roll… it works! ActiveX does not render over your popup control anymore.

Keep in mind, if you have many IFrames on your page it would slower rendering, so try to keep number of IFrames to a minimum.

Another ISolvable problem :).

Wednesday, September 16, 2009

Restoring WPF window of another process

Recently I was working on a WPF single instance application, which hides it’s main window and shows system tray icon. My task was to restore main window whenever user tries to open another instance of the application. The good thing I have control over source code, which means I can do anything to the target app. However there are 3 bad things related to WPF

WPF shortfalls
  • no out of the box support for single instance applications (in WinForms it is just a flag in project properties tab)
  • it is hard to get a handle of main window of another process, if that window is hidden. Process.MainWindowHandle would return 0.
  • when restoring window of a different process WPF does not listen for window events and thus WPF thread does not start rendering, as a result you get black window with XP blue frame around it.
1st problem

First problem is easily ISolvable either using Process.GetProcessByName(yourAppName) when returns true and process id is different from current process, then most likely there is another instance of the app is running, unless name of your app for some reason, is the same as some other app running on the box. In that case you can use mutex to solve the problem. In fact using Mutex is a more robust approach. You can find implementations in here.

2nd Problem

Second problem turned out to be lengthy and requires use of SharedMemoryFile and a bunch of other APIs. You can see how it is solved in the same article. This article however does not address WPF issue, since it was written prior WPF release.

3d Problem

While easily solvable, it took me some time to figure it out.
When you have a code like this:

'' if mutex was not created that means other instance is running, 
'' so we need to restore window of other application.
If Not IsMutexCreated Then
Dim mainWindowHanle As IntPtr = System.IntPtr.Zero

SyncLock GetType(FilesView)
mainWindowHanle = MemoryMappedFile.ReadHandle("Local\sharedMemoryFilesView")
End SyncLock

If mainWindowHanle <> IntPtr.Zero Then
Dim result As Boolean
result = ShowWindowAsync(mainWindowHanle, SW_SHOWDEFAULT)
result = SetForgroundWindow(mainWindowHanle)
result = UpdateWindow(mainWindowHanle)
End If
Catch ex As Exception

End Try

Catch ex As Exception

End Try
End If

the ShowWindow or ShowWindowAsync function will indeed restore the window, only with a little problem. It is going to be black. As WPF rendering runs on a separate thread and apparently not listening for main window events :(. Notice that I am not using GC.KeepAlive and GC.Collect like in the original article, but I declared mutex as a class member of Application class, which is a main class in WPF applications. In my case reference to mutex object is kept until Application class is disposed, which is when application shuts down. So GC (Garbage Collector) will not reclaim memory occupied by object mutex because it would have active reference.


So WPF thread is silent when some other process calls window restore or window show. Well, here is a good thing I mentioned in the beginning. I have complete control of the source code. And it means I can include event listener in the application main message loop and from there restore WPF window.

Since I know I am calling ShowWindow and SetFocus I can concentrate on the events which are fired in those two cases. It is most likely would be GotFocus, Activated or IsVisibleChanged events for WPF window.
In there I call Show method, and … it works!

Private Sub FilesView_Activated(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Activated
End Sub

Private Sub FileView_GotFocus(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles MyBase.GotFocus
' or simply
' Me.Show() 'if you are already inside main window class
End Sub

' this should also work
Private Sub FilesView_IsVisibleChanged(ByVal sender As System.Object, ByVal e As System.Windows.DependencyPropertyChangedEventArgs) Handles MyBase.IsVisibleChanged
End Sub

That’s it. A lot of sweat for a simple problem, but hey it is ISolvable :).

Monday, September 14, 2009

VB.NET short circuit IF statement and Nullable(of T)

Now this was an interesting bug to find. May be it is not a bug, but I can’t explain this behavior otherwise.

When working with databases and trying to cover impedance mismatch cases it is common to use the following logic:

Dim primaryUserId As Integer?
Using reader As IDataReader = dataAccess.ExecuteReader("usp_CostCenter_Get", params)
With reader
While .Read()
returnResult = True
costCenter = If(.IsDBNull(0), "", .GetString(0))
description = If(.IsDBNull(1), "", .GetString(1))
primaryUserId = If(.IsDBNull(2), Nothing, .GetInt32(2)) '' <<<<-- incorrect behavior,
'' if condition is true primaryUserId will get 0 instead of Nothing.
'' 0 - is default value for type Integer, but not for type Integer? or Nullable(of Integer)
'' Nothing - should be the correct value in this case.
End While
End With
End Using

In case when condition is true you would expect true part of IF statement to execute, while something else happens and primaryUserId receives default value for type Integer not for type Nullable(of Integer) or Integer? .

This is how to correct such behavior:

Using reader As IDataReader = dataAccess.ExecuteReader("usp_CostCenter_Get", params)
With reader
While .Read()
returnResult = True
costCenter = If(.IsDBNull(0), "", .GetString(0))
description = If(.IsDBNull(1), "", .GetString(1))

'' expanding IF statement
If .IsDBNull(2) Then
primaryUserId = Nothing
primaryUserId = .GetInt32(2)
End If

End While
End With
End Using

Please let me know if you had experienced this before and agree or disagree with me.

Thank you!

Visual Studio 2008 XAML designer crashes

There are several reasons why Visual Studio can crash.

and some other which I don’t remember now. But I am not going to talk about the above issues, rather about some other case when Visual Studio could crash.

Here is what happens.
Visual Studio Designer could create instances of some of the controls if for example you placed a child control on a form the base class for that child would be instantiated, similar if you have referenced external assemblies which have controls that are on a form, those controls could be instantiated, it depends on a control logic.

If you are creating a control you have to check for Design time compilation versus run-time. See my previous blog on how to handle it in Win Forms and there is plenty of topics on the web how to handle a similar issue for WPF.

Now if control resides in an assembly which is located on a shared or networking folder, then logic of that control would be executed in a different security context, and AccessDenied exception could be thrown. In my case Visual Studio was not handling this exception properly and was crashing. DWatson was executing and collecting crash data but that did help.

What should happen is VS IDE should handle such exception and cancel rendering with a proper message and type of the exception.

I solved it by copying all referenced assemblies into local project folder, then recompiling project and restarting Visual Studio IDE.

I usually copy assemblies for real projects. I ran into this problem by trying to create real quick prototype and was lazy to transfer dlls locally.

Try and let me know.
This was another ISolvable problem. I am sorry for being out of touch, but I will try to get back and post since I have a lot of things to share.