How to Split(using Partial class ) Code behid .CS?

I requirement is my code Behind file contains more than 5 thousand line, Now i want to split the code into two file ...
The other file also accept the web controls events

i tried with partial class

This is my Code Behind File


public partial class _Default : System.Web.UI.Page { protected void Page_Load(object sender, EventArgs e) { } }
this my class added to App_code Folder


public partial class _Default : System.Web.UI.Page { protected void Button1_Click(object sender, EventArgs e) { } }
here the Button1_Click method is web control
 
But it's giving compilation error
...