How to set a DLL class as a code behind file for a project..?

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C#

CSS

HTML

I have Page with 2 text boxes and 4 buttons

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="DllController.ascx.cs" Inherits="SampleDll.DllController" %>
<div>
    <asp:TextBox ID="txtFirst" runat="server">0</asp:TextBox>
</div>
<div>
    <asp:TextBox ID="txtSecond" runat="server">0</asp:TextBox>
</div>
<div>
    <asp:Button ID="btnAdd" runat="server" Text="Add" OnClick="btnAdd_Click"  OnClientClick="btnAdd_Click" Width="42px" />
    
    <asp:Button ID="btnSubtract" runat="server" Text="Sub" OnClick="btnSubtract_Click" Width="42px" />
    
    <asp:Button ID="btnMultiple" runat="server" Text="Mul" OnClick="btnMult_Click"  Width ="42px" />
    
    <asp:Button ID="btnDivide" runat="server" Text="Div"  OnClick="btnDivide_Click" Width="42px" />
    
</div>

And I have a DLL class

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace WrappesrSample
{
        public class WrapSample
        {
            public WrapSample()
            {

            }
            public int Add(int First, int Second)
            {
                WrapSample.inside obj = new WrapSample.inside();
                int sum = obj.add(First, Second);
                return sum;
            }
            public int Sub(int First, int Second)
            {
                WrapSample.inside obj = new WrapSample.inside();
                int sum = obj.Sub(First, Second);
                return sum;
            }
            public int Mul(int First, int Second)
            {
                WrapSample.inside obj = new WrapSample.inside();
                int sum = obj.Mul(First, Second);
                return sum;
            }
            public void Div(int First, int Second, out int res, out int rem)
            {
                WrapSample.inside obj = new WrapSample.inside();
                obj.Div(First, Second, out res, out rem);
            }

            private class inside
            {

                public int add(int x, int y)
                {

                    return x + y;
                }
                public int Sub(int x, int y)
                {

                    return x - y;
                }
                public int Mul(int x, int y)
                {

                    return x * y;
                }
                public void Div(int x, int y, out int Res, out int rem)
                {
                    Res = x / y;
                    rem = x % y;
                }
            }
        }
    }

Name of the Dll class is wrapper sample.
Now I want to set My Dll class as the Code behind of the Page.
How can I add that