If you’ve read my previous post explaining a common pitfall with view state, I’d hope you’re preparing all your controls in the Init event of the page/control lifecycle.

Even if I’m not reusing them through my application much, I like to factor elements like a drop down list of countries into their own control. This centralizes their logic and allows us to write clear, succinct markup like this:

<tat:CountriesDropDownList ID="AddressCountry" runat="server" />

The code for a control like this is quite simple:

[ToolboxData("<{0}:CountriesDropDownList runat=\"server\" />")]
public class CountriesDropDownList : DropDownList
{
    protected override void OnInit(EventArgs e)
    {
        DataSource = Countries;
        DataBind();

        base.OnInit(e);
    }
}

The Problem

Once you start using this encapsulation technique, it won’t be long until you want to pass in a parameter that affects the data you load. Before we do, we need to be aware that the Init event is fired in reverse order. That is, the child controls have their Init event fired before that event is fired at the parent. As such, the Page.Init event is too late for us to set any properties on the controls.

The natural solution is to try and use the Page.PreInit event, however when you do you’ll often find that your control references are all null. This happens when your page is implemented using a master page, and it relates to how master pages are implemented. The <asp:ContentPlaceHolder /> controls in a master page use the ITemplate interface to build their contents. This content (child controls) is not usually prepared until the Init event is called, which means the control references are not available. For us, this represents a problem.

The Solution

The fix is remarkably simple; all we need to do is touch the Master property on our Page and it will cause the controls to become available. If we are using nested master pages, we need to touch each master page in the chain.

I often create a file called PageExtensions.cs in my web project and add this code:

public static class PageExtensions
{
    /// <summary>
    /// Can be called during the Page.PreInit stage to make child controls available.
    /// Needed when a master page is applied.
    /// </summary>
    /// <remarks>
    /// This is needed to fire the getter on the top level Master property, which in turn
    /// causes the ITemplates to be instantiated for the content placeholders, which
    /// in turn makes our controls accessible so that we can make the calls below.
    /// </remarks>
    public static void PrepareChildControlsDuringPreInit(this Page page)
    {
        // Walk up the master page chain and tickle the getter on each one
        MasterPage master = page.Master;
        while (master != null) master = master.Master;
    }
}

This adds an extension method to the Page class, which then allows us to write code like the following:

protected override void OnPreInit(EventArgs e)
{
    this.PrepareChildControlsDuringPreInit();

    MyCustomDropDown.MyProperty = "my value";

    base.OnPreInit(e);
}

Without the call to the extension method, we would have received a NullReferenceException when trying to set the property value on the MyCustomDropDown control.

You now have one less excuse for preparing your controls during the Load event. 🙂

(If you don’t get the title reference, Wikipedia can explain. A more direct title could be: Understanding and Respecting the ASP.NET Page Lifecycle.)

This whole article needs a technical review. Parts of it are misleading. I’ll get back to you Barry.

Page lifecycle in ASP.NET is a finicky and rarely understood beast. Unfortunately, it’s something that we all need to get a handle on.

A common mishap that I see is code like this:

protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        AddressCountryDropDown.DataSource = CountriesList;
        AddressCountryDropDown.DataBind();
    }
}

The problem here is that we’re clogging our page’s view state. Think of view state as one of a page’s core arteries, then think of data like cholesterol. A little bit is all right, but too much is crippling.

To understand the problem, lets investigate the lifecycle that’s in play here:

1. The Page.Init event is being fired, however we are not subscribed to that.
2. Immediately after the Init event has fired, view state starts tracking. This means that any changes me make from now on will be saved down to the browser and re-uploaded on the next post back.
3. The Page.Load event is being fired in which we are setting the contents of the drop down list. Because we are doing this after the view state has started tracking, every single entry in the drop down is being written to both the HTML and the view state.

There’s yet another problem here as well. By the time the Page.Load event is fired, all of the post back data has been loaded and processed.

To investigate the second problem, let’s investigate the lifecycle that’s in play during a post back of this same page:

1. The user triggers the post back from their browser and all of the post back data and view state is uploaded to the server.
2. The Page.Init event is fired, however we are not subscribed to that.
3. Immediately after the Init event has fired, view state starts tracking. This means that any changes me make from now on will be saved down to the browser and re-uploaded on the next post back.
4. The view state data is loaded for all controls. For our drop down list example, this means the Items collection is refilled using the view state that was uploaded from the browser.
5. Post back data is processed. In our example, this means the selected item is set on the drop down list.
6. The Page.Load event is fired however nothing happens because the developer is checking the Page.IsPostBack property. Usually, they say this is a “performance improvement” however it is also required in this scenario otherwise we would lose the selected item when we rebound the list.
7. The contents of the drop down list are once again written to both the HTML and the view state.

How do we do this better? Removing the IsPostBack check and placing the binding code into the Init event is all we need to do:

protected override void OnInit(EventArgs e)
{
    AddressCountryDropDown.DataSource = CountriesList;
    AddressCountryDropDown.DataBind();

    base.OnInit(e);
}

What does this achieve?

• We are filling the contents of the drop down before the Init event is fired; therefore a redundant copy of its contents is not written to the view state.
• We are filling the contents of the drop down before the postback data is processed, so our item selection is successfully loaded without it being overridden later.
• We have significantly reduced the size of the page’s view state.

This simple change is something that all ASP.NET developers need to be aware of. Unfortunately so many developers jumped in and wrote their first ASP.NET page using the Page_Load event (including myself). I think this is largely because it’s the one and only event handler presented to us when we create a new ASPX page in Visual Studio. While this makes the platform appear to work straight away, it produces appalling results.