Chris's coding blog

ASP.NET control event order/lifecycle

March 24, 2010

This is a very concise reference for the order which events are fired when you subclass a control. The longer more detailed MSDN page on the lifecycle can be found here. The particular control this came from was a subclassed CompositeControl, but this could be any control, or a page.

OnInit: 1
LoadViewState: 2
CreateChildControls: 3
OnLoad: 4
ShowButton: 5
OnPreRender: 6
Render: 7
OnUnload: 8

The code that produced this is below. The list above was produced after a button press on the form. This button pressed called the ShowButton() method below.

int count = 1;
protected override void RecreateChildControls()
{
System.Diagnostics.Debug.WriteLine(string.Format("RecreateChildControls: {0}", count++));
base.RecreateChildControls();
}
protected override void OnInit(EventArgs e)
{
System.Diagnostics.Debug.WriteLine(string.Format("OnInit: {0}", count++));
base.OnInit(e);
}
protected override void OnLoad(EventArgs e)
{
System.Diagnostics.Debug.WriteLine(string.Format("OnLoad: {0}", count++));
base.OnLoad(e);
}
protected override void OnPreRender(EventArgs e)
{
System.Diagnostics.Debug.WriteLine(string.Format("OnPreRender: {0}", count++));
base.OnPreRender(e);
}
public void ShowButton()
{
System.Diagnostics.Debug.WriteLine(string.Format("ShowButton: {0}", count++));
}
protected override void OnUnload(EventArgs e)
{
System.Diagnostics.Debug.WriteLine(string.Format("OnUnload: {0}", count++));
base.OnUnload(e);
}
protected override void Render(HtmlTextWriter writer)
{
System.Diagnostics.Debug.WriteLine(string.Format("Render: {0}", count++));
base.Render(writer);
}
protected override object SaveControlState()
{
System.Diagnostics.Debug.WriteLine(string.Format("SaveControlState: {0}", count++));
return base.SaveControlState();
}
protected override void LoadViewState(object savedState)
{
System.Diagnostics.Debug.WriteLine(string.Format("LoadViewState: {0}", count++));
base.LoadViewState(savedState);
}
view raw gistfile1.cs hosted with ❤ by GitHub

There is also an equivalent lifecycle chart for AJAX.

Here’s a few gotchas that have gotcha’d me:

  • OnInit() is called before the ViewState is loaded, so if you need the values of the controls after postback, this isn’t the event.
  • OnLoad() is called before your control events on the page are fired. So if you have a button with an event handler and that changes your control, any OnLoad logic for properties of the control will be overwritten.
  • CreateChildControls() is not guaranteed to be 3rd as show above. It may be called at any stage as the MSDN docs state.
  • If you put rendering code in PreRender() and a method in your control that is called by a button event (or any event) relies on a control created in PreRender(), you’ll get an exception. The control won’t have been created until after that event has fired.
  • Put the properties of the control that need persitance inside the ViewState, not just a backing field (example below). It might seem an obvious point but can cause headaches.

public int CurrentPage
{
get
{
int val;
int.TryParse(ViewState["CurrentPage"].ToString(), out val);
return val;
}
set
{
if (value < 0)
value = 0;
else if (value <= SomeObject.Pages.Count)
value = MandarinForm.Pages.Count - 1;
ViewState["CurrentPage"] = value;
}
}
view raw gistfile1.cs hosted with ❤ by GitHub

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Chris Small

I'm Chris Small, a software engineer working in London. This is my tech blog. Find out more about me via GithubStackoverflowResume