Dynamically add items in DropDownList on Button click in Asp.Net C# and Vb.Net

A DropDownList control in Asp.Net, as we know, holds a list of data in it. I have shared few interesting articles with examples before on my blog on how to dynamically bind a DropDownList control in Asp.Net to a database table. However, here in this article, I’ll share a simple and yet useful example on how to add items to a DropDownList dynamically with just a click of a button, using a simple code behind procedure.

I am not binding the control to a database table or an Asp.Net DataTable. All I’ll have on my web page is a textbox control, which will take values from the user and add the entered value in the DropDownList after the user clicks a button. Its just filling an empty DropDownList with some values in it.

Along with adding items to the list, we’ll also see how to check for duplicates using a code behind procedure, before actually adding the value to the list.

The Markup

<!DOCTYPE html>
<html>
<head runat="server">
    <title>DropDownList Example in Asp.Net</title>

    <style>
        #tbBooks, #ddlBooks, #lblMessage
        {
            font:14px Verdana;
        }
    </style>
</head>
<body>
    <form runat="server">
    <div>
        <div>
            <%--ADD A TEXTBOX AND BUTTON.--%>
            <asp:TextBox ID="tbBooks" runat="server"></asp:TextBox>
                <asp:Button ID="btAdd" Text="Add Book to the List" 
                    OnClick="Add_Books" runat="server"/>

            <p>
                <%--YOUR DROPDOWNLIST CONTROL WITH A LABEL CONTROL (TO MESSAGE).--%>
                <asp:DropDownList ID="ddlBooks" runat="server">
                    <asp:ListItem Value="--Select a Book--"></asp:ListItem>
                </asp:DropDownList>
                
                <p>
                    <asp:Label ID="lblMessage" runat="server"></asp:Label>
                </p>
            </p>
        </div>
    </div>
    </form>
</body>
</html>

Code Behind (C#)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class SiteMaster : System.Web.UI.MasterPage
{
    protected void Add_Books(object sender, EventArgs e)
    {
        string sBooks = null;
        sBooks = tbBooks.Text.Trim();

        // CHECK IF TEXTBOX HAS ANY VALUE BEFORE ADDING.

        if (!string.IsNullOrEmpty(sBooks.Trim()))
        {
            // IN-ADDITION, CHECK IF THE ENTERED VALUE (BOOK) ALREADY EXISTS IN THE LIST,
                // TO AVOID DUPLICATE ENTRIES.   
            if ((ddlBooks.Items.FindByText(sBooks) == null))
            {
                ddlBooks.Items.Add(new ListItem(sBooks, sBooks));

                tbBooks.Text = "";

                int iBooksCount = ddlBooks.Items.Count - 1;
                lblMessage.Text = "A new book added to the list. <br />" +
                    "Now you have <b>" + iBooksCount  + "</b> item(s) in the list.";
            }
            else
            {
                lblMessage.Text = "Item <b>" + sBooks.Trim() + 
                    "</b> already exists in the list. <br />" + "Add another book.";
            }

            tbBooks.Text = "";
            tbBooks.Focus();
        }
    }
}

The code up there is very simple. Let me explain. All I am doing is checking the contents of the textbox control, before filling the value in the DropDownList. This is important, as there can be other functions related the value in the textbox.

Next, I am validating or checking if the value that is entered by user, already exists in the list. This is an important functionality to avoid adding duplicates in the list. To check if value exists, I am using the FindByText() method.

if ((ddlBooks.Items.FindByText(sBooks) == null))

The method will search the collection for a ListItem with a Text property that equals the text specified (the text entered by a user in the textbox). The method takes a parameter text (the text or value to search for).

Code Behind (Vb.Net)

Option Explicit On

Partial Class Site
    Inherits System.Web.UI.MasterPage

    Protected Sub Add_Books(sender As Object, e As EventArgs)
        Dim sBooks As String
        sBooks = Trim(tbBooks.Text)

        ' CHECK IF TEXTBOX HAS ANY VALUE BEFORE ADDING.
        If Trim(sBooks) <> "" Then

            ' IN-ADDITION, CHECK IF THE ENTERED VALUE (BOOK) ALREADY EXISTS IN THE LIST,
                ' TO AVOID DUPLICATE ENTRIES.
            If IsNothing(ddlBooks.Items.FindByText(sBooks)) Then
                ddlBooks.Items.Add(New ListItem(sBooks, sBooks))

                tbBooks.Text = ""
                lblMessage.Text = "A new book added to the list. <br />" & _
                    "Now you have <b>" & ddlBooks.Items.Count - 1 & _
                        "</b> item(s) in the list."
            Else
                lblMessage.Text = "Item <b>" & Trim(sBooks) & _
                    "</b> already exists in the list. <br />" & _
                        "Add another book."
            End If

            tbBooks.Text = ""
            tbBooks.Focus()
        End If
    End Sub
End Class

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