IntroductionToday, 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: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:Step 9: The addition operation output of the application looks like this:I hope this article is useful for you.. I look forward to your comments and feedback.. Thanks Vijay Prativadi
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