What does Convert to Web Application do?

In VS2008, I go to File/New/Project, select ASP.NET Web Application and create a new project. 开发者_StackOverflow中文版

If I right-click on the newly created project, there is a menu item to Convert to Web Application.

I thought I just picked a Web Application. What exactly does the Convert to Web Application menu item do?

It will generate .designer file with controls from aspx/ascx file. If you have e.g. in you page control

<asp:Label ID="lbId" runat="server" />

in designer file it generates member

 protected global::System.Web.UI.WebControls.Label lbId;

if you haven't this .designer file you cannot use controls from aspx page in code behind.