I'm a beginer of ASP.net
If I define a usercontrol in the main.aspx file like this:
[u]<% @Page Inherits="CodeBehind" Language="c#" src="code.cs" %>
<% @Register tagprefix="usercontrol" Tagname="footer" src="footer.ascx" %>
<asp:Label id="theLabel" runat="server"/>
<usercontrol:footer id="footer1" runat="server"/></u>
And i write the codebehind file(code.cs) like this:
[u]public class CodeBehind:Page
{
  public Label theLabel;
  ??;
}</u>
What should I write at the place of "??" if I what to use the usercontrol "footer" in the codebehind file?
and what should i write at the top of the codebehind file?

If you have a usercontrol called "footer" than I'm going to assume it's codebehind class is "footer" so...

If you aspx code behind you should be able to do this...
Then you can use "footer1" in your codebehind methods. For any control that is defined in an ascx or aspx file (the HTML), you put a line in the codebehind to declare the corresponding control ID. Just remember not to use the "new" keyword. The code for the control in the HTML causes the control to be instantiated. You just need the control ID declared in the code behind.

Also, you should use something like this in the code behind:
This way you get the control's events to fire in the codebehind.
(Please excuse any C# typos, I work in VB.)

------------------------
Peter
Somewhere outside
Boston, MA USA Earth