If you’ve ever had reason to try to extend the standard Microsoft web TreeView control, you will have no doubt noticed that MS was quite unkind to you and sealed (or declared NotInheritable for you VB.NET types) the System.Web.UI.WebControls.TreeNodeCollection class.

The problem arises when you want to overload the default behavior that is implemented by the TreeNodeCollection class. For example, when a node was added to my TreeView class (via the TreeView.Nodes.add method), I needed to be able to analyze it for the ultimate purpose of my subclass.

However, this was not possible because the TreeNodeCollection class is sealed, so I wasn’t able to inherit from it and overload the add method behavior as I should have been able to do.

There are always possibilities.

Anyone who knows me knows I don’t give up easily (if ever), and I eventually plowed through many false starts but hit upon a solution.

I decided to wrap the underlying TreeNodeCollection class with my own, and overload the Nodes property on the TreeView class, and the ChildNodes property on the TreeNode class.

The Wrapper.

Here’s what the wrapper looks like:
[vb language=".net"]

Public Class MyTreeNodeCollection

Private mtvwChildNodes As System.Web.UI.WebControls.TreeNodeCollection
Private mMyTreeViewOwner As IMyNodeContainer

Public Sub New(ByVal Owner As IMyNodeContainer, _
ByVal TreeViewChildren As System.Web.UI.WebControls.TreeNodeCollection)
mtvwChildNodes = TreeViewChildren
mMyTreeViewOwner = Owner
End Sub

Public Sub Add(ByVal child As MyTreeNode)
mtvwChildNodes.Add(CType(child, System.Web.UI.WebControls.TreeNode))
mMyTreeViewOwner.RegisterNodeForLookup(child)
End Sub
End Class
[/vb]

As you can see, the constructor takes the real, underlying TreeNodeCollection to pass all node to prior to being “registered” (analyzed). It also takes something that implements the IMyNodeContainer interface.

The IMyNodeContainer interface.

I had to implement this because I wanted to be able to use this wrapper class for both TreeView and TreeNode objects, but the signature of their properties is different - TreeView.Nodes and TreeNode.ChildNodes, respectively. So I opted for the interface to keep things clean.

Here’s what the interface looks like:

[vb language=".net"]
Public Interface IMyNodeContainer

Sub RegisterNodeForLookup(ByVal node As MyTreeNode)
ReadOnly Property Nodes() As MyTreeNodeCollection

End Interface
[/vb]

Wrapping things up.

This is where the magic happens.

To put the wrapper (MyTreeNodeCollection) in place, I overload the collection getting properties in the base class - TreeView.Nodes and TreeNode.ChildNodes, respectively, like so:

[vb language=".net"]
Public Overloads ReadOnly Property Nodes() As MyTreeNodeCollection Implements IMyNodeContainer.Nodes
Get
Return mNodeCollection
End Get
End Property
[/vb]

So, for those of you playing along at home, when the developer calls .Nodes on an instance of MyTreeView class, an instance of MyTreeNodeCollection is returned and the resulting add method in performed on the underlying TreeNodeCollection, thusly:

[vb language=".net"]
Public Sub Add(ByVal child As MyTreeNode)
mtvwChildNodes.Add(CType(child, System.Web.UI.WebControls.TreeNode))
mMyTreeViewOwner.RegisterNodeForLookup(child)
End Sub
[/vb]

You can see why I had to use the IMyNodeContainer interface here. The RegisterNodeForLookup performs the functionality I was originally trying to subclass the TreeNodeCollection for.

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I’ve been writing a subclass of the MS Webcontrol.TreeView control for one of our Web Applications at work. I figured this would be a fairly easy task, since I only needed to extended it with a few properties. It turns out that I was caught on a nit-picky annoyance in the TreeView control itself.

The TreeView control renders verticals lines with gaps.

Here’s a screen cap of the problem.

As you can see, the TreeView control renders the vertical line with a gap, or break (looks like a dashed line!). It didn’t matter how I loaded the data - dynamic/runtime/design time - I get the same gap no matter what!

I was able to see that it was not a problem in IE 6 or less, but what good is that? Well, it turns out that it was a bit of a clue because starting with version 5, IE didn’t render things correctly per the CSS boxing specification. This was fixed in IE 7, but to provide backwards compatibility, Microsoft carried this busted form of rendering forward in IE 6 as QuirksMode. The line gap problem only occurs in strict mode (default for IE 7 and 8, as well as firefox). I could make the line gap go away by forcing the browser into QuirksMode (by adding a comment, ex:

< ! - - QUIRK! - - >

to the very top of the HTML file), but I was writing a web control and would not always have the luxury of controlling my container.

I needed to find a long term solution to this problem.

Next, I looked at the HTML source of the rendered page, and saw this:

[html]
<table cellpadding="0" cellspacing="0" style="border-width:0;">
<tr>
<td>
<div style="width:20px;height:1px">
<img src="/TreeviewControlTest/WebResource.axd?d=OYmDnVppVECKIpxOWC8o8Y7DO6QwB2J3EY4s4RR8zAU1&amp;t=633765128008804061" alt="">
</div>
</td>
<td>
<img src="/TreeviewControlTest/WebResource.axd?d=OYmDnVppVECKIpxOWC8o8UGy0bLoCc8gOB1oQm6Pzj81&amp;t=633765128008804061" alt="">
</td>
<td class="TreeView1_WebTree_1" style="white-space:nowrap;">
<a class="TreeView1_WebTree_0" href="javascript:__doPostBack(’TreeView1$WebTree’,'Root//Tree’)" onclick="TreeView_SelectNode(TreeView1_WebTree_Data, this,’TreeView1_WebTreet6′);" id="TreeView1_WebTreet6" name="TreeView1_WebTreet6">Tree</a>
</td>
</tr>
</table>
[/html]

Each node is rendered as a table, with the vertical line and expand/collapse icons being in their own table cell and wrapped in a div. The problem was the style applied to the outer div - style=”width:20px;height:1px“.

That 1px height was causing the vertical line image to be compressed, but where did it come from?

Reflecting on System.Web.UI.WebControls.TreeNode

I spent almost an hour delving into the various (and copious!) style properties for the tree and its nodes, looking for where this height setting was generating from. I couldn’t find it! I eventually opened the System.Web.UI.WebControls.dll in Reflector to see what the render code for the node was doing:

Well, once I saw that the code was hard-wired to render this CSS style, I was done. Or was I?

!important

Well, it wasn’t going to be as easy as setting the style in the code behind, but I could override the style in my own class.

The trick is to define, and apply the following CSS class:

[css]
<style type="text/css">
.MyTreeView TD Div
{
height: 20px!important;
}
</style>
[/css]

The !important CSS directive overrides the style applied in the System.Web.UI.WebControls.TreeNode render method.

Again, just as with the QuirksMode comment above, I was able to add the CSS style to the page and voila - problem solved. But this still wasn’t good enough. I needed this to work out of the box for any consumers of my control.

The ultimate answer was to override the RenderBeginTag of the TreeView control, and render this style before the control itself:

[vb language=".net"]

Public Overrides Sub RenderBeginTag(ByVal writer As System.Web.UI.HtmlTextWriter)
‘/////////////////////////////////////////////////////////////////////////////////////////
‘/// This is a total hack to get around some Microsoft BS which hardwires
‘/// a style attribute on the node div to set the height = 1px!
‘///
‘/// This renders a css override to force the div to the proper height
‘/////////
Me.CssClass = String.Concat(Me.CssClass, " MyTreeView")
writer.WriteBeginTag(HtmlTextWriterTag.Style.ToString)
writer.WriteAttribute(HtmlTextWriterAttribute.Type.ToString, "text/css")
writer.WriteLine(HtmlTextWriter.TagRightChar)
writer.Write(".MyTreeView TD Div ")
writer.WriteLine("{ height: 20px!important; }")
writer.WriteEndTag(HtmlTextWriterTag.Style.ToString)
writer.WriteLine()
‘//////////////////////////////////////////////////

‘/// Render the Standard Begin Tag
MyBase.RenderBeginTag(writer)
End Sub
[/vb]

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The other day I was writing an in-house tool to assist in some upgrades we were performing on client installations. This tool was supposed to perform its operations on a batch of items, and display the results upon completion.

Since processing this batch of items was a lengthy endeavor, I wanted the failure to process one of the items to simply be recorded and allow the processing of the others to continue. Part of the processing of each item was a call to multiple web services, so I would need a way to handle the collection of errors along the way and make them available for their eventual display.

I had what I thought was a clever idea: a private Arraylist of exceptions that occurred during processing.

[vbnet]
Public Class BatchExceptions
Implements ICollection(Of System.Exception)

Private mExceptionlist As ArrayList
.
.
.
End Class
[/vbnet]

That way, I could simply iterate over the list and perform the standard exception handling, like so:

[vbnet]
Dim exc As Exception
For Each exc In BatchExceptions
HandleError(exc)
Next
[/vbnet]

Of course, in order to make use of the “For Each” construct, I had to implement the GetEnumerator of the ICollection interface.

[vbnet]
Public Function GetEnumerator() As System.Collections.Generic.IEnumerator(Of System.Exception) _
Implements System.Collections.Generic.IEnumerable(Of System.Exception).GetEnumerator
Return (mExceptionlist.GetEnumerator)
End Function
[/vbnet]

Cool. Only one problem:

That was OK though, because I was using generics after all. The compiler was being helpful and reminding me that I had to specify ‘IEnumerator(Of Exception)’:

[vbnet]
Public Function GetEnumerator() As System.Collections.Generic.IEnumerator(Of System.Exception) _
Implements System.Collections.Generic.IEnumerable(Of System.Exception).GetEnumerator
Return (DirectCast(mExceptionlist.GetEnumerator, IEnumerator(Of Exception)))
End Function
[/vbnet]

Everything compiled fine, but at run-time I got the following RTE:

Unable to cast object of type ‘ArrayListEnumeratorSimple’ to type ‘System.Collections.Generic.IEnumerator`1[System.Exception]‘.

This was frustrating. It seemed like the compiler failed to warn me of this incompatibility, and simply kicked the can on down the road to the run-time to deal with.

Solution:

Well, I did a little poking around and finally ended up replacing my Arraylist with a list like so:

[vbnet]
private mExceptionlist as System.Collections.Generic.list
[/vbnet]

Well, that did the trick but I can’t say it was as intuitive as it would seem. It makes sense, in hindsight, but why did I have to get some cryptic RTE? Why couldn’t the compiler have picked up on my use of an ArrayList and say, “Hey dummy - use a generic list!”? Still, I have a new trick to toss in my bag for the time I want to implement an enumerator on an Arraylist!

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The other day I thought it might be nice to “do the right thing” and give my code a run against Microsoft’s FxCop.

I ran it right out of the box - I didn’t bother making my own rules or changing the defaults. I was bored. Anyway, here’s one of the results that actually made be chuckle once I read it carefully:

Warning, Certainty 90, for DoNotInitializeUnnecessarily
{

Target : #.ctor() (IntrospectionTargetMember)
Location : <735>> (String)
Resolution : “‘MyDocument.New()’ initializes field ‘MyDocument.disposedValue’
of type ‘Boolean’ to false. Remove this initialization
because it will be done automatically by the runtime.”
Help : (String)
Category : Microsoft.Performance (String)
CheckId : CA1805 (String)
RuleFile : Performance Rules (String)
Info : “Do not make initializations that have already been
done by the runtime.”
Created : 3/6/2009 6:58:21 PM (DateTime)
LastSeen : 3/6/2009 8:36:25 PM (DateTime)
Status : Active (MessageStatus)
Fix Category : NonBreaking (FixCategories)

}

The reason for the chuckle was that the code which triggered this violation of the rule was written by the Microsoft IDE! My role in this infraction was really quite simple: I typed “Implements IDisposable” and hit Enter. The IDE was “nice” enough to plugin the rest for me:

[vb language=".net"]

‘ To detect redundant calls
Private disposedValue As Boolean = False

‘ IDisposable
Protected Overridable Sub Dispose(ByVal disposing As Boolean)
If Not Me.disposedValue Then
If disposing Then
Array.Clear(mDocumentContent, 0, mDocumentContent.Length)
End If
End If
Me.disposedValue = True
End Sub
[/vb]

It’s bad enough that the IDE writes code for me without prompting, but maybe the FxCop team should talk to the IDE team to avoid such embarrassing nuisances (I had dozens of similar warnings to weed through) in the future.

According to the Code Analysis Team, this is the new default for FxCop 1.36.

Here’s how to avoid it:

Using an FxCop project:

1. Open your FxCop project in FxCop
2. Choose Project -> Options -> Spelling & Analysis
3. Check Suppress analysis results against generated code
4. Click OK

OR, if you prefer the command-line:

1. Pass the /ignoregeneratedcode switch, for example:

FxCopCmd.exe /file:MyAssembly.dll /out:AnalysisResults.xml /ignoregeneratedcode

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