ASP.NET

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class _Default : System.Web.UI.Page 

{
    private IfaceIt firstInterface;
//Error: Error	1	The type or namespace name 'IfaceIt' could not be found (are you missing a //using directive or an assembly reference?)	C:\inetpub\wwwroot\InterfaceTest\IFace.aspx.cs	//11	13	http://localhost/InterfaceTest/


    private String showMe;
    
    protected void Page_Load(object sender, EventArgs e)
    {
        Label1.Text = firstInterface.Show();
    }
}
App_Code/IfaceIt.cs
using System;

/// <summary>
/// Summary description for IfaceIt
/// </summary>
namespace hub.interfaces {
public interface IfaceIt
{
	String Show();
}

}