Derive user control from custom user control base class --> namespace error?

See more:

C#

.NET

WebForms

.NET4.5

Dear all,
I'm stuck with this problem: in VS 2012 I've created this basic User Control, derived from System.Web.UI.UserControl:

File test.ascx.cs:

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

namespace B2CInterface.UC
{
    public partial class UC_Base_test : System.Web.UI.UserControl
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            int a = 1; // some code here...
        }
    }
}

and the corresponding test.ascx:

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="test.ascx.cs" Inherits="B2CInterface.UC.UC_Base_test" %>
<p>test</p>

Now, I've a second User Control that should be derived from the first one (UC_Base_test):

File test2.ascx.cs:

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

namespace B2CInterface.UC
{
    public partial class UC_Base_test2 : UC_Base_test // System.Web.UI.UserControl
    {
        protected void Page_Load(object sender, EventArgs e)
        {
           //other code here...
        }
    }
}

and the corresponding test2.ascx:

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="test2.ascx.cs" Inherits="B2CInterface.UC.UC_Base_test2" %>

Now, when I compile I get this error:

Error	12	The type or namespace name 'UC_Base_test' could not be found (are you missing a using directive or an assembly reference?)	C:\Progetti\B2CInterface\UC\Base\test2.ascx.cs	10	42	B2CInterface

I thank in advance anyone who will be able to solve this!!
PJ