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Co-variance Delegates in .NET

Posted by Vijay Prativadi Articles | C# Language October 24, 2012
Today, in this article let’s play around with one of the interesting and most useful concepts in C#.
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Introduction

Today, in this article let's play around with one of the interesting and most useful concepts in C#.

Question: What is Co-variance delegate?

In simple terms "It provides flexibility to encapsulate a method with a return type (i.e. derived class) that is directly or indirectly derived from the delegate's return type (i.e. base class).".

Step 1: Create a "ASP.NET Web Application", as in:

Co-variance-Delegates.gif

Step 2: The complete code of Addition.cs looks like this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
namespace ContravarianceDelegateApp
{
    public class Addition
    {
        public double Add(double a, double b)
        {
            return a + b;
        }
    }
}

Step 3: The complete code of Substraction.cs looks like this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
namespace ContravarianceDelegateApp
{
    public class Substraction : Addition
    {
        public double Sub(double a, double b)
        {
            return a - b;
        }
    }
}

Step 4: The complete code of Multiplication.cs looks like this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
namespace ContravarianceDelegateApp
{
    public class Multiplication : Substraction
    {
        public double Mul(double a, double b)
        {
            return a * b;
        }
    }
}

Step 5: The complete code of Division.cs looks like this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
namespace ContravarianceDelegateApp
{
    public class Division : Multiplication
    {
        public double Div(double a, double b)
        {
            return a / b;
        }
    }
}

Step 6: The complete code of WebForm1.aspx looks like this:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="CovarienceDelegateApp.WebForm1" %>
 
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title></title>
</head>
<
body>
    <form id="form1" runat="server">
    <center>
        <div>
            <table>
                <tr>
                    <td colspan="2">
                        <asp:Label ID="Label1" runat="server" Text="Co-Variance Delegates" Font-Bold="true"
                            Font-Size="Large" Font-Names="Verdana" ForeColor="Maroon"></asp:Label>
                    </td>
                </tr>
                <tr>
                    <td>
                        <asp:Label ID="Label3" runat="server" Text="Please Enter First Number" Font-Size="Large"
                            Font-Names="Verdana" Font-Italic="true"></asp:Label>
                    </td>
                    <td>
                        <asp:TextBox ID="TextBox1" runat="server" Width="120px"></asp:TextBox>
                    </td>
                </tr>
                <tr>
                    <td>
                        <asp:Label ID="Label2" runat="server" Text="Please Enter Second Number" Font-Size="Large"
                            Font-Names="Verdana" Font-Italic="true"></asp:Label>
                    </td>
                    <td>
                        <asp:TextBox ID="TextBox2" runat="server" Width="120px"></asp:TextBox>
                    </td>
                </tr>
                <tr>
                    <td colspan="2" align="center">
                        <asp:Button ID="Button1" runat="server" Text="Addition" Font-Names="Verdana" Width="213px"
                            BackColor="Orange" Font-Bold="True" OnClick="Button1_Click" />
                    </td>
                </tr>
                <tr>
                    <td colspan="2" align="center">
                        <asp:Button ID="Button2" runat="server" Text="Substraction" Font-Names="Verdana"
                            Width="213px" BackColor="Orange" Font-Bold="True" OnClick="Button2_Click" />
                    </td>
                </tr>
                <tr>
                    <td colspan="2" align="center">
                        <asp:Button ID="Button3" runat="server" Text="Multiplication" Font-Names="Verdana"
                            Width="213px" BackColor="Orange" Font-Bold="True" OnClick="Button3_Click" />
                    </td>
                </tr>
                <tr>
                    <td colspan="2" align="center">
                        <asp:Button ID="Button4" runat="server" Text="Division" Font-Names="Verdana" Width="213px"
                            BackColor="Orange" Font-Bold="True" OnClick="Button4_Click" />
                    </td>
                </tr>
                <tr>
                    <td colspan="2" align="center">
                        <asp:Label ID="Label5" runat="server" Font-Bold="true" Font-Names="Verdana" ForeColor="Maroon"></asp:Label>
                    </td>
                </tr>
            </table>
        </div>
    </center>
    </form>
</body>
</
html>

Step 7: The complete code of WebForm1.aspx.cs looks like this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace CovarienceDelegateApp
{
    public partial class WebForm1 : System.Web.UI.Page
    {
        public delegate Addition AdditionDelegate();
        protected void Page_Load(object sender, EventArgs e)
        {
            TextBox1.Focus();
        }
        protected void Button1_Click(object sender, EventArgs e)
        {
            if (string.IsNullOrEmpty(TextBox1.Text) || string.IsNullOrEmpty(TextBox2.Text))
            {
                Label5.Text = "Please Enter Some Values";
                Label5.ForeColor = System.Drawing.Color.Red;
            }
            else
            {
                AdditionDelegate objAddDelegate = new AdditionDelegate(DoAddition);
                Addition objAddition = objAddDelegate();
                Label5.Text = "Addition Result is: " + objAddition.Add(double.Parse(TextBox1.Text),
                    double.Parse(TextBox2.Text));
                Label5.ForeColor = System.Drawing.Color.Green;
                TextBox1.Text = string.Empty;
                TextBox2.Text = string.Empty;
            }
        }
        protected void Button2_Click(object sender, EventArgs e)
        {
            if (string.IsNullOrEmpty(TextBox1.Text) || string.IsNullOrEmpty(TextBox2.Text))
            {
                Label5.Text = "Please Enter Some Values";
                Label5.ForeColor = System.Drawing.Color.Red;
            }
            else
            {
                AdditionDelegate objSubDelegate = new AdditionDelegate(DoSubstraction);
                Substraction objSubstraction = (Substraction)objSubDelegate();
                Label5.Text = "Substraction Result is: " + objSubstraction.Sub(double.Parse(TextBox1.Text),
                    double.Parse(TextBox2.Text));
                Label5.ForeColor = System.Drawing.Color.Green;
                TextBox1.Text = string.Empty; TextBox2.Text = string.Empty;
           }
        }
        protected void Button3_Click(object sender, EventArgs e)
        {
            if (string.IsNullOrEmpty(TextBox1.Text) || string.IsNullOrEmpty(TextBox2.Text))
            {
                Label5.Text = "Please Enter Some Values";
                Label5.ForeColor = System.Drawing.Color.Red;
            }
            else
            {
                AdditionDelegate objMulDelegate = new AdditionDelegate(DoMultiplication);
                Multiplication objMultiplication = (Multiplication)objMulDelegate();
                Label5.Text = "Multiplication Result is: " + objMultiplication.Mul(double.Parse(TextBox1.Text),
                    double.Parse(TextBox2.Text)); Label5.ForeColor = System.Drawing.Color.Green;
                TextBox1.Text = string.Empty; TextBox2.Text = string.Empty;
            }
        }
        protected void Button4_Click(object sender, EventArgs e)
        {
            if (string.IsNullOrEmpty(TextBox1.Text) || string.IsNullOrEmpty(TextBox2.Text))
            {
                Label5.Text = "Please Enter Some Values"; Label5.ForeColor = System.Drawing.Color.Red;
            }
            else
            {
                AdditionDelegate objDivDelegate = new AdditionDelegate(DoDivision);
                Division objDivision = (Division)objDivDelegate();
                Label5.Text = "Division Result is: " + objDivision.Div(double.Parse(TextBox1.Text),
                    double.Parse(TextBox2.Text));
                Label5.ForeColor = System.Drawing.Color.Green;
                TextBox1.Text = string.Empty; TextBox2.Text = string.Empty;
            }
        }
        public Addition DoAddition()
        {
            return new Addition();
        }
        public Substraction DoSubstraction()
        {
            return new Substraction();
        }
        public Multiplication DoMultiplication()
        {
            return new Multiplication();
        }
        public Division DoDivision()
        {
            return new Division();
        }
    }
}

Step 8: The output of the application looks like this:

Co-variance-Delegates1.png

Step 9: The addition operation output of the application looks like this:

Co-variance-Delegates2.png

I hope this article is useful for you.. I look forward to your comments and feedback.. Thanks Vijay Prativadi

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