What methods are executed in which order in this ASP.NET webforms page

Question

I was refactoring some old website the other day and stumbled upon this scenario. I have an ASP.NET 3.5 C# WebForms page. In the code behind I have an event handler like so:

protected override void OnPreRender(EventArgs e) { }

On the other hand, in the markup, I also have:

<script language="C#" runat="server">
    void Page_PreRender()
    {

    }
</script>

Question is: what does the lifecycle look like? What is executed first? Is one of them even executed?

Answer 1

Two points upfront:

• The OnPreRender method is not an event handler. It is an override of the method Page.OnPreRender (which is the method that will raise the PreRender event).
• The method Page_PreRender is automatically wired-up to the PreRender event. So this is an event handler (for the PreRender event).

Order of invocation

The following code sample shows the order of execution:

// code-behind
protected override void OnPreRender(EventArgs e)
{
    // 1. code put here will be executed first

    // now we call the base class' version, which will then raise the
    // PreRender event
    base.OnPreRender(e);

    // 3. code put here will be executed last
}

// markup       
<script language="C#" runat="server">
    void Page_PreRender()
    {
        // 2. code put here will be executed second
    }
</script>

Note that inside OnPreRender() the base class is invoked: base.OnPreRender(). As written above, if this is missing then the PreRenderevent will not be raised and event handlers will therefore not be called.

Your code sample

In your sample code, the call to base.OnPreRender is missing. This means, that PreRender event is not raised and therefore the (event handler) method Page_PreRender() will not be called.

Some more points to consider

• If you have a Page_PreRender() event handler in your markup and in your code-behind, then only the one in the code-behind will be called.

• If you disable the auto-wiring of events in the Page directive (e.g. < % @ Page AutoEventWireup="false" ... % >), then the Page_PreRender() event handler will not be wired/connected with the PreRender event and will therefore not be called.

• You can also manually attach handlers to events, e.g:

---

 protected void Page_Load(object sender, EventArgs e)
 {
     PreRender += PreRenderEventHandler;
 }
 void PreRenderEventHandler(object sender, EventArgs e)
 {
 }