DropDownList Example in ASP.NET using C#

DropDownList.aspx (source code):

 <%@ Page Title="" Language="C#" MasterPageFile="~/RoseindiaMaster.master"  
 AutoEventWireup="true" CodeFile="DropDownList.aspx.cs" Inherits="DropDownList" %>  
 <asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">  
 <style type="text/css">  
 .style3  
 {  
 color: #800000;  
 }  
 </style>  
 </asp:Content>  
 <asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">  
 <div>  
 <h2 style="color:Green">DropDownList in ASP.NET 4 , C#</h2>  
 <strong><span class="style3">Enter first number:</span>  
 </strong>  
 <br />  
 <asp:TextBox ID="txt1" runat="server" Text="12"/>  
 <br />  
 <br />  
 <span class="style3">  
 <strong>Enter second number:  
 </strong>  
 </span>  
 <br />  
 <asp:TextBox ID="txt2" runat="server" Text="3" />  
 <br />  
 <br />  
 <span class="style3">  
 <strong>Select operation:  
 </strong>  
 </span>  
 <br />  
 <asp:DropDownList ID="drp1" runat="server" Width="145px"   
 Font-Bold="True" ForeColor="#006666"   
 onselectedindexchanged="drp1_SelectedIndexChanged" AutoPostBack="True">  
 <asp:ListItem>----Select----</asp:ListItem>  
 <asp:ListItem>Addition</asp:ListItem>  
 <asp:ListItem>Subtraction</asp:ListItem>  
 <asp:ListItem>Multiplication</asp:ListItem>  
 <asp:ListItem>Division</asp:ListItem>  
 </asp:DropDownList>  
 </div>  
 <br />  
 <asp:Label ID="label1" runat="server" Font-Bold="True" ForeColor="#000099" />  
 </asp:Content>  

DropDownList.aspx.cs (C# code file):

 using System;  
 using System.Collections.Generic;  
 using System.Linq;  
 using System.Web;  
 using System.Web.UI;  
 using System.Web.UI.WebControls;  
 public partial class DropDownList : System.Web.UI.Page  
 {  
 protected void Page_Load(object sender, EventArgs e)  
 {  
 }  
 protected void drp1_SelectedIndexChanged(object sender, EventArgs e)  
 {  
 double firstno = Convert.ToDouble(txt1.Text);  
 double secondno = Convert.ToDouble(txt2.Text);  
 if(drp1.SelectedIndex == 1)  
 {  
 double add = firstno + secondno;  
 label1.Text = "Addition is :" + add;  
 }  
 if (drp1.SelectedIndex == 2)  
 {  
 double sub = firstno - secondno;  
 label1.Text = "Subtraction is :" + sub;  
 }  
 if (drp1.SelectedIndex == 3)  
 {  
 double mul = firstno * secondno;  
 label1.Text = "Multiplication is :" + mul;  
 }  
 if (drp1.SelectedIndex == 4)  
 {  
 double div = firstno / secondno;  
 label1.Text = "Division is :" + div;  
 }  
 }  
 }  

OUTPUT