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