using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
/// <summary>
/// Summary description for IGetUCable
/// </summary>
namespace Insus.NET
{
public interface IGetUCable
{
 UserControl GetUC();
}
}

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="Calculator.ascx.cs" Inherits="Calculator" %>
Number A: <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox> <br />
+ <br />
Number B: <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox><br />
<asp:Button ID="ButtonEqual" runat="server" Text="="
OnClick="ButtonEqual_Click1" />
<br />
Result: <asp:Label ID="LabelResult" runat="server" Text=""></asp:Label>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using Insus.NET;
public partial class Calculator : System.Web.UI.UserControl,IGetUCable
{
 protected void Page_Load(object sender, EventArgs e)
 {
 }
 protected void ButtonEqual_Click1(object sender, EventArgs e)
 {
 decimal a = decimal.Parse(this.TextBox1.Text.Trim());
 decimal b = decimal.Parse(this.TextBox2.Text.Trim());
 this.LabelResult.Text = (a + b)。ToString ();
 }
 public UserControl GetUC()
 {
 return this;
 }
}

protected void Page_Load(object sender, EventArgs e)
{
 IGetUCable uc1 = (IGetUCable)LoadControl("~/Calculator.ascx");
 this.form1.Controls.Add(uc1.GetUC());
}