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Validate ListBox and DropDown List using Required Field Validator

Validate ListBox and DropDown List using Required Field Validator

27 Mar 2024
Intermediate
7.63K Views
5 min read

Validate ListBox and DropDown List using Required Field Validator: An Overview

To validate ListBox and DropDown List using Required field validator is sometimes a need, while we are working in Asp.net. It is tricky and avoids the use of JavaScript function to validate ListBox and DropDown List. Through this Tutorial, we will learn How to validate DropDown List and ListBox with Example Program, Its Implementation Steps and much more. Get to explore more about other ASP.NET features and concepts through our ASP.NET Certification Training Program.

Read More: Top 50 ASP.NET Core Interview Questions and Answers for 2024

Validate DropDown List and ListBox

Here is an example to show how you can validate DropDown List and ListBox:

 <form id="form1" runat="server">
<div>
 <asp:ListBox ID="lstboxCity" runat="server" Width="200px" SelectionMode="Multiple">
 <asp:ListItem Value="1">Delhi</asp:ListItem>
 <asp:ListItem Value="2">Noida</asp:ListItem>
 <asp:ListItem Value="3">Gurgaon</asp:ListItem>
 <asp:ListItem Value="4">Mumbai</asp:ListItem>
 <asp:ListItem Value="5">Pune</asp:ListItem>
 <asp:ListItem Value="6">Banglore</asp:ListItem>
 <asp:ListItem Value="7">Lucknow</asp:ListItem>
 </asp:ListBox> <asp:RequiredFieldValidator ID="RequiredFieldValidator1" ControlToValidate="lstboxCity" InitialValue="" runat="server" ErrorMessage="Please select atleast one city"></asp:RequiredFieldValidator>
</div>
<br />
<div>
 <asp:DropDownList ID="ddlCity" runat="server" Width="200px">
 <asp:ListItem Value="0">--Select--</asp:ListItem>
 <asp:ListItem Value="1">Delhi</asp:ListItem>
 <asp:ListItem Value="2">Noida</asp:ListItem>
 <asp:ListItem Value="3">Gurgaon</asp:ListItem>
 <asp:ListItem Value="4">Mumbai</asp:ListItem>
 <asp:ListItem Value="5">Pune</asp:ListItem>
 <asp:ListItem Value="6">Banglore</asp:ListItem>
 <asp:ListItem Value="7">Lucknow</asp:ListItem>
</asp:DropDownList>
<asp:RequiredFieldValidator ID="RequiredFieldValidator2" ControlToValidate="ddlCity" InitialValue="0" runat="server" ErrorMessage="Please select city"></asp:RequiredFieldValidator>
 </div>
 <br />
 <asp:Button ID="btnSubmit" runat="server" Text="Submit" />
 </form> 

Note

For validating DropDown List, make sure you have "--Select--" option value 0 other wise validation on it will not work.

Read More: Why you need ASP.NET Core in your tech stack?

Implementation Steps

You can follow these steps to validate a ListBox and a DropDown List using Required field Validator:
  1. Set up the Web Form-Create a new ASP.NET Web Form or if you have one, open it.
  2. Add ListBox and DropDownList controls-Add a ListBox and a DropDownList control where ever you want them in your form.
  3. Add RequiredFieldValidator controls-Next, you need to add a RequiredFieldValidator control to each control you want to validate and assign the 'ControlToValidate property to each of them.
  4. Set the error message-Set an error message with the help of 'ErrorMessage' property so if the validation fails, the message will appear. You can also customize the appearance of the message if you want to.
  5. Handle Form Submission-At last, you can handle form submission by subscribing to the click event of a submit button. Check the page if it is valid with the help of 'Page.IsValid' before processing the form.

Handling Validation Errors

Sometimes you may encounter Validation errors, so to handle them, here are some tips:
  1. Display Error Messages- To each validated control, you can attach a error message using validation controls such as 'RequiredFieldValidator' or custom error messages with code-behind so that it displays whenever there is a validation error.
  2. Prevent Form Submission- You can also prevent the form from submitting if there is any validation errors. For doing that, you need to check if the page is valid using 'Page.IsValid' property before processing the form.
  3. Style Invalid Controls- Sometimes, what you can do is just style the invalid controls so that they stand out visually and you can do that using CSS or setting properties like 'ForeColor' and 'BackColor'.
Summary

In this article I try to explain how to validate DropDownList and ListBox using Required Field Validator. I hope after reading this article you will be able to use this trick in your code. I would like to have feedback from my blog readers. Please post your feedback, question, or comments about this article. Also, Consider enrolling in our ASP.NET Certification Course to delve deep into core concepts of ASP.NET with our comprehensive step by step guide.

FAQs

Q1. What is the purpose of the Required Field Validator in ASP.NET?

The purpose of the RequiredFieldValidator in ASP.NET is that it makes sure that a particular field in a form has been filled out by the user by checking if that field contains an output or not.

Q2. What types of validation errors can occur with ListBox and DropDown List inputs?

Validation errors with ListBox and DropDown List can be like when a user has not selected anything in the form or if the selected item does not meet the specified criteria. 

Q3. How do you add validation to a dropdown in HTML?

To add validation to a DropDown in HTML, follow these steps:
  1. Create the dropdown with the help of '<select>' element.
  2. Ensure that a selection is made before submitting the form by implementing client-side validation using JavaScript.
  3. You can, additionally, use HTML5 attributes such as 'required' on '<select>' element for built-in browser validation.

Q4. Are there any security risks associated with ListBox and DropDown List validation?

Yes, there may occur security risks such as client-side validation bypassing or by posing only client-side validation, there is a risk that it user can disable it or manipulate.

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About Author
Shailendra Chauhan (Microsoft MVP, Founder & CEO at Scholarhat by DotNetTricks)

Shailendra Chauhan is the Founder and CEO at ScholarHat by DotNetTricks which is a brand when it comes to e-Learning. He provides training and consultation over an array of technologies like Cloud, .NET, Angular, React, Node, Microservices, Containers and Mobile Apps development. He has been awarded Microsoft MVP 8th time in a row (2016-2023). He has changed many lives with his writings and unique training programs. He has a number of most sought-after books to his name which has helped job aspirants in cracking tough interviews with ease.
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