Tip 1: Always use Page.IsValid before submitting data. Apart from the other benefits, using it prevents submitting data from old browsers that do not support javascript.

Tip 2: The display property for the ASP.NET validation controls has 3 settings: None, Static(default), and Dynamic. ‘Static’ outputs HTML code (related to error) at all times even when no error has occurred. So when there is more than one validation control placed next to the field, the first validation control occupies screen space even when there is no error. In case the second validation control fires an error message, the message is pushed away from the control since the first validation control is occupying screen space.
Set the ‘display’ property of a validation control to ‘Dynamic’. This property renders the error message with the attribute display:none; It helps you to display the error message next to the control .

Tip 3: To prevent validation to occur on the click of the Cancel button, set the ‘CausesValidation’ property to false
<asp:Button ID=”btnCancel” Runat=”server” CausesValidation=”False” Text=”Cancel” />

Tip 4: Use the ‘InitialValue’ property of the RequiredFieldValidator to validate controls like combobox which have a default value.
For eg: If your combobox has a default item called “–Select –“ and you want that the user should select a value other than the default value before submitting the form, then set the ‘InitialValue’ property of the RequiredFieldValidator to “–Select–“.
<asp:DropDownList ID=”DropDownList1″ runat=”server”>
<asp:ListItem Value=”–Select–” />
<asp:ListItem Value=”Item1″ />
<asp:ListItem Value=”Item2″ />
<asp:ListItem Value=”Item3″ />
</asp:DropDownList>
<asp:RequiredFieldValidator ID=”RequiredFieldValidator1″ runat=”server” ErrorMessage=”RequiredFieldValidator” ControlToValidate=”DropDownList1″ InitialValue=”–Select–”></asp:RequiredFieldValidator>

Tip 5: A RegularExpressionValidator can be used to handle string patterns. For eg: A Name textbox that should accept a maximum of 30 characters with only alphabets, space, fullstop(.) and a ‘(apostrophe). A regularexpression like ‘^([a-zA-z.'\s]{2,30})$’ does the trick.
However when you are using Localization and using a language like Arabic, you have to often provide for validating characters in a different dialect. You can solve this using the following technique:
- In the Resource.resx file, create a resourcekey called ValidName giving it a value of ^([a-zA-z.'\s]{2,30})$
- In the Resource.ar-EG.resx file, use the same key but with a diff value ^([\u0600-\u06FF.'\s]{2,30})$
Use it in your page using the following way. Observe the bold text:

<asp:RegularExpressionValidatorID=’regEVFname’ runat=’server’ControlToValidate=’txtName’
Display=’Dynamic’ErrorMessage=’Invalid’
ValidationExpression=’<%$ Resources:Resource, ValidName %>’SetFocusOnError=’True’></asp:RegularExpressionValidator>

When the user selects English, he can enter only A-Za-z. Similarly for Arabic, he can enter only the Arabic characters and not English.

Tip 6: The validation controls provide both Server and Client Side validation. To turn off client-side validation, set the ‘EnableClientScript = false’
<asp:RequiredFieldValidator ID=”RequiredFieldValidator2″ Runat=”server”
Text=”Error” ControlToValidate=”TextBox1″ EnableClientScript=”false”/>

Tip 7: Use CompareValidator to validate date with format of “dd/MM/yyyy”.
The validator uses the CultureInfo object of the thread to determine date format. So what you need to do is to set the desired culture format in the Page directive
<%@ Page culture=”your culture” %>
This tip was shared by PeterBlum in the asp.net forums. By the way, Peter has an amazing suite of data entry and validation controls on his site at a reasonable price.

Tip 8: Instead of the textual error message, you can even add an image or sound to your validator control. The Text and Error Message property accepts HTML tags.
<asp:TextBox ID=”TextBox1″ runat=”server”></asp:TextBox>
<br />
<asp:RequiredFieldValidator ControlToValidate=”TextBox1″ EnableClientScript=”false” ID=”RequiredFieldValidator1″ runat=”server” Text=”<bgsound src=’C:\Windows\Media\Windows Error.wav’>”></asp:RequiredFieldValidator>
Just make sure that the EnableClientScript=”false” when you want a sound instead of a text message.

Tip 9: If you have two set of forms (eg: Login and Registration) in a single page and want to keep the validation of the two groups separate, use ‘ValidationGroups’. All you need to do, is to specify a common group name for a set of controls that you want to validate separately.
<div>
<asp:TextBox ID=”TextBox1″ ValidationGroup=”Group1″ runat=”server”></asp:TextBox>
<br />
<asp:RequiredFieldValidator ControlToValidate=”TextBox1″ ValidationGroup=”Group1″ ID=”RequiredFieldValidator1″ runat=”server” Text=”Error”></asp:RequiredFieldValidator>
<asp:Button ID=”Button1″ runat=”server” ValidationGroup=”Group1″ Text=”Button” />
</div>
<br />
<br />
<div>
<asp:TextBox ID=”TextBox2″ ValidationGroup=”Group2″ runat=”server”></asp:TextBox>
<br />
<asp:RequiredFieldValidator ControlToValidate=”TextBox1″ ValidationGroup=”Group2″ EnableClientScript=”false” ID=”RequiredFieldValidator2″ runat=”server” Text=”Error”></asp:RequiredFieldValidator>
<asp:Button ID=”Button2″ runat=”server” ValidationGroup=”Group2″ Text=”Button” />
</div>

Tip 10: Other validator controls like CompareValidator, RangeValidator etc. do not provide a way to detect if the field is blank or required. The only way is to do this is to add a RequiredFieldValidator along with the other validator controls.
However one exception being the CustomValidator which provides a property called ‘ValidateEmptyText’. Just set it to true and it validates the field even if the user has kept the field blank.

Tip 11: If you want your validation error message to appear in the ‘ValidationSummary‘ control, then set the ‘ErrorMessage’ property on that validation control. Also, setting ‘ShowMessageBox = true’ on the ValidationSummary enables you to display a popup alert.

Tip 12: In order to create a CustomValidationControl you have to derive from the ‘BaseValidator’ class and implement the ‘EvaluateIsValid()’ method.

Tip 13: In case of an error, the validation controls allow you to set focus on a control in error using the ‘SetFocusOnError’ property.
<asp:RequiredFieldValidator SetFocusOnError=”true” ControlToValidate=”TextBox1″ ID=”RequiredFieldValidator1″ runat=”server” Text=”Error!!”></asp:RequiredFieldValidator>