How to implement Custom Validator in ASP.NET

Introduction

Firstly, this article is for beginners, in this article we will see how to use ‘ASP.NET validation Custom Validator’ in a very simpler way and this will be very helpful to those who has never used custom validator in their projects. I’m sure after reading my article, you will be in that situation where you can easily go with custom validator control. Please give your suggestions and feedback in the comments.

Why we choose Custom Validator ?

If anyone of the other ASP.NET validation controls do not suit your requirements, then you can define a custom server-side validation function and call it using Custom Validator control.

How to implement Custom Validator in ASP.NET project?

<asp:TextBox ID="txtLicenseNo" runat="server" MaxLength="10" Font-Bold="true"></asp:TextBox>
<br />
<asp:RequiredFieldValidator ID="rfvLicenseNo" runat="server" 
ControlToValidate="txtLicenseNo"
ErrorMessage="Required" 
Display="Dynamic" 
SetFocusOnError="true" 
ValidationGroup="vgLicense"
EnableClientScript="true">
</asp:RequiredFieldValidator>
 <asp:CustomValidator ID="cmvalLicneseNumber" 
SetFocusOnError="true" 
ValidationGroup="vgLicense"
ControlToValidate="txtLicenseNo" 
ClientValidationFunction="ValLicenseLength"                        
Display="Static" 
runat="server" 
ErrorMessage="License number should have at least 4 characters!">
</asp:CustomValidator>

<script type="text/javascript">
function ValLicenseLength(sender, e) {
var LicenseLength = (document.getElementById('<%=txtLicenseNo.ClientID %>').value).length;
e.IsValid = true;
if (LicenseLength < 4) {
e.IsValid = false;
}
else {
e.IsValid = true;
}
}
</script>

In above example there, is one TextBox, one RequiredFieldValidator and one CustomValidator whereas RequiredFieldValidator is used to make sure the textbox having text or not and by using CustomValidator we are going to make sure the entered text would be more than of 4 characters. 

CustomValidator having same properties as other Validators except ‘ClientValidationFunction’. This property having JavaScript method which is defined by us. Just give a look.

Review the attached project for more and better understanding.

Conclusion

Reference