Vb.net - Page Not Loading In Asp While Debugging / Error Could Not Load Type 'a1_main'?

i have a small 2 aspx page vb.net application. its has main.aspx with main.aspx.vb in back.
first line on aspx reads -

<%@ Page Language="VB"
AutoEventWireup="false"
codebehind="main.aspx.vb"
Inherits="a1_main" %>

first lines of main.aspx.vb read -

Partial Public Class a1_main
Inherits System.Web.UI.Page
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs)
Handles Me.Load
End Sub
End Class

its completely blank. but when i try to debug it just to see design on aspx page it gives me this error -

Could not load type 'a1_main'.Line 1: <%@ Page Language="VB" AutoEventWireup="false" codebehind="main.aspx.vb" Inherits="a1_main" %>