How to Use Delegate in ASP.NET using C#

using System;

using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq; 

public partial class delegatewith2arg : System.Web.UI.Page
{ 

    public delegate void calculation(double a, double b);  //  define delegate
    protected void Page_Load(object sender, EventArgs e)
    {
 
    }
    protected void btnok_Click(object sender, EventArgs e)
    {
        calculation calc = sum;        // invoke first method
        calc += minus;                   //invoke second
        calc += multiply;                // third
        calc += devide;                 // forth
        calc.Invoke(double.Parse(txtfirstvalue.Text), double.Parse(txtsecondvalue.Text));   // pass value as arguemet
    } 

    void sum(double x, double y)                 // first method
    {
        lblsum.Text = (x + y).ToString();
    }
    void minus(double x, double y)            // second method
    {
        lblminus.Text = (x - y).ToString();
    }
    void multiply(double x, double y)        // third method
    {
        lblmultiply.Text = (x * y).ToString();
    }
    void devide(double x, double y)        //forth method
    {
        lbldevide.Text = (x / y).ToString();
    }
}