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ASP.NET has been missing a good, free tab control for a long time.  With the ACT Tab Container, we were finally given one.  It typically runs in client-side only mode, but can interact with server-code if we set its AutoPostback property to true.

Compared to what we had before, it is a huge improvement.  The peculiar thing about it, however, is that it isn't actually an Ajax control.  It doesn't use asynchronous postbacks or web service calls to talk to the server -- instead you just have these two mode: run it using client script only, or run it using server-side events and code-behind only.

So a few months ago I rectified this for a project, and only found out afterwards that Matt Berseth had already outlined the technique on his blog.  You basically run the tab container in client-side mode, and add update panels to the tab panels that you want to be ajaxy.  You then hook up the client-side ActivePageChanged event in such a way that it spoofs the Update Panel contained in the tab, causing an asynchronous (or partial) postback.

Matt also gave this technique a cool name.  He called it 'lazy loading the tab panel'.  Like lazy loading in OOP, using this technique the update panels inside each tab panel only do something when its tab is selected.  Information is loaded only when its needed, and not before.

I must admit that I hold some resentment against Matt for coming up with this first, and for coming up with the cool moniker for it.  On the other hand, the solution I came up with encapsulates all of the javascript needed for this into a nice simple extender control that you can drop on your page, which his does not, and I'm rather proud of this.

The VS 2008 project for this extender is linked at the top of this post.  To use it, you need to compile the project and add the compiled assembly to your project, or else just add the project to your solution and add a project reference.

1. Drop the TabContainerExtender control into your page.

2. Set the Extender's TargetControlID property to your TabContainer's ID.

3. In the RegisterUpdatePanels element of the Extender, map your tabs to your update panels.  This mapping tells the extender which Update Panels to activate when each tab is selected.

Your markup will look something like this:

    <cc2:TabContainerExtender ID="TabContainerExtender1" 
    runat="server" 
    TargetControlID="TabContainer1" OnActiveTabChanged="ActiveTabChanged">
    <RegisterUpdatePanels>
    <cc2:UpdatePanelInfo TabIndex="0" UpdatePanelID="UpdatePanel1" />
    <cc2:UpdatePanelInfo TabIndex="1" UpdatePanelID="UpdatePanel2" />
    <cc2:UpdatePanelInfo TabIndex="2" UpdatePanelID="UpdatePanel3" />
    </RegisterUpdatePanels>
    </cc2:TabContainerExtender> 

4. If you want to add some code-behind to your active tab changed event, add set the OnActiveTabChanged property of the Extender to the name of your handler.  The thrown event will pass the correct Index number for the active Tab, as well as the ID of the mapped Update Panel.  The handler's signature looks like this:

        protected void ActiveTabChanged(int index, string panelID)

        {

            ...

        }

I highly encourage you to read Matt Berseth's blog entry (which I have to admit is pretty good) to get a clear idea of the techniques being applied in this ajax extender.  If you just need a quick solution, however, feel free to download this code from the link at the top and use it any way you like with no strings attached.  There is a sample project attached to the solution that will demonstrate how to use the Tab Container Extender, in case you run into any problems with lazy loading your panels.

For reference, here is the code for the sample implementation, which loads controls on the fly based on the tab selected:

    <cc1:TabContainer ID="TabContainer1" runat="server">
    <cc1:TabPanel ID="TabPanel1" runat="server" HeaderText="Tab Panel 1">
    <ContentTemplate>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
        Content 1 ...
        <br />
            <asp:PlaceHolder ID="PlaceHolder1" runat="server"></asp:PlaceHolder>      
        </ContentTemplate>
        </asp:UpdatePanel>    
    </ContentTemplate>
    </cc1:TabPanel>
        <cc1:TabPanel ID="TabPanel2" runat="server" HeaderText="Tab Panel 2">
    <ContentTemplate>
        <asp:UpdatePanel ID="UpdatePanel2" runat="server">
        <ContentTemplate>
        Content 2 ...
        <br />
            <asp:PlaceHolder ID="PlaceHolder2" runat="server"></asp:PlaceHolder>      
        </ContentTemplate>
        </asp:UpdatePanel>    
    </ContentTemplate>
    </cc1:TabPanel>
        <cc1:TabPanel ID="TabPanel3" runat="server" HeaderText="Tab Panel 3">
    <ContentTemplate>
        <asp:UpdatePanel ID="UpdatePanel3" runat="server">
        <ContentTemplate>
        Content 3 ...
        <br />
            <asp:PlaceHolder ID="PlaceHolder3" runat="server"></asp:PlaceHolder>      
        </ContentTemplate>
        </asp:UpdatePanel>    
    </ContentTemplate>
    </cc1:TabPanel>
    </cc1:TabContainer>
    <cc2:TabContainerExtender ID="TabContainerExtender1" 
    runat="server" 
    TargetControlID="TabContainer1" OnActiveTabChanged="ActiveTabChanged">
    <RegisterUpdatePanels>
    <cc2:UpdatePanelInfo TabIndex="0" UpdatePanelID="UpdatePanel1" />
    <cc2:UpdatePanelInfo TabIndex="1" UpdatePanelID="UpdatePanel2" />
    <cc2:UpdatePanelInfo TabIndex="2" UpdatePanelID="UpdatePanel3" />
    </RegisterUpdatePanels>
    </cc2:TabContainerExtender>