why my new Class is not visible in default.aspx.cs?

Hi,
I use VS2008 to create a Asp.net web application. Then I create a App_Code new folder. After that I add a new item, Class1.cs, under App_Code.
When I come back to default.aspx.cs and type Class1 within Page_Load, I found it is not recognized. If I compile, compiler error says "Class1" could not be found.
Any idea?
Thx
Tao
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