Life cycle of Custom Control within ASP.NET page

There are a lot of articles about life-cycle of the page and custom controls. The problem is they are all looked at separately. I had to do a bit of research on the order of control events within the events of the web page and I would like to share my findings.

To see what happens I created two items:

• TestCustomControl.cs — custom composite control that gets included on Default.aspx. Adds a line to the log for the following events/methods:

OnInit

LoadViewState

OnLoad

CreateChildControls

SaveViewState

OnPreRender

Render

• Default.aspx page — contains TestCustomControl and submit button. Adds line to the log for the following events:

Page_Init

Page_Load

Page_PreRender

Then I opened Default.aspx page. Here is the log of events:

Control: OnInit
Page: Page_Init
Page: Page_Load
Control: OnLoad
Page: Page_PreRender
Control: CreateChildControls
Control: PreRender
Control: SaveViewState
Control: Render

Looks pretty much what you expected. Here is what happens when I clicked on submit button:

Control: OnInit
Page: Page_Init
Control: LoadViewState
Control: CreateChildControls
Control: LoadPostData
Page: Page_Load
Control: OnLoad
Page: Page_PreRender
Control: PreRender
Control: SaveViewState
Control: Render

There are a couple of differences. LoadViewState and LoadPostData are now included in the cycle. The biggest difference (that has caused many headaches) is that 'CreateChildControls' now gets called before "Page_Load/OnLoad". Let's say you have a simple composite control that contains a text box and some other controls:

public class TextBoxPlus : CompositeControl
{

private TextBox _txt;
…

#region Class Properties

public string TextBoxValue
{

get {return (_txt.Text);}
set {_txt.Text=value;}

}

#endregion

protected override void CreateChildControls()
{

_txt = new TextBox();
_txt.ID = "txtTemp ";
_txt.CssClass = this.CssClass;
_txt.Columns = 3;
_txt.MaxLength = 3;

Controls.Add(_txt);

…

}

protected override void Render(HtmlTextWriter output)
{

…

}

}

The issue with CreateChildControls is that if you set 'TextBoxValue' property of the control in 'Page_Load' event of the page – you will get an exception that '_txt' doesn't exist. It does work without any problems if you set that value after the first form submit.

So:

protected void Page_Load(object sender, EventArgs e)

{

if (!this.isPostBack)

textBoxPlus.Value="SomeValue"; //Throws exception

else

textBoxPlus.Value="SomeValue"; //Works without any problems

}

To fix this problem modify TextBoxValue property as follows

public string Value
{

get
{

EnsureChildControls();

return _txt.Text;

}

set
{

EnsureChildControls();

_txt.Text=value;

}

}

EnsureChildControls() checks if CreateChildControls() has been called and if it hasn't — calls it.