If you have worked with HTML forms in the past, you are probably used to creating forms that post to a new page. To take advantage of Validation controls in a Web Form Page, however, you cannot post the form information to a new page. Instead, you must post the Web Form Page back to the same page on the Web server.

For example, suppose that you want to add two Web Form Pages to a project—a page named Register.aspx and a page named ThankYou.aspx. The Register.aspx page contains a registration form and the ThankYou.aspx page contains a thank you message for registering.

When designing your project, you might be tempted to have the Register.aspx page post the form to the ThankYou.aspx page. The user fills out the registration form, submits it, and receives a thank you message. However, you can’t take advantage of Validation controls in the Register.aspx page when you post a form to a new page.

Instead, the Register.aspx page should post back to itself. This way, you can display error messages with any Validation controls that you have added to the form. When there are no validation errors in the Register.aspx page, you can explicitly redirect the user to the ThankYou.aspx page.

By default, Web Form Pages automatically post back to themselves. However, you must explicitly add the code to the Web Form to redirect the user to a new page when there are no validation errors in the page. You’ll see several examples of how to do this when we discuss the individual Validation controls.

When you use the Validation controls with Microsoft Internet Explorer 4.0 or later, the Validation controls will automatically use client-side JavaScript. This means that the Validation controls can immediately display error messages on the browser while the user is in the process of completing a form. If there are any validation errors in a page, the client-side JavaScript will prevent the user from submitting the form back to the server.

If you are not using Microsoft Internet Explorer 4.0 or later, you can still use the Validation controls. The Validation controls will “fall back” to performing validation on the server after the form is submitted to the Web server. Validation error messages won’t be displayed immediately, but they will appear after the page is posted back to the Web server.

You can use the RequiredFieldValidator control to automatically display an error message when you submit a form with an empty form field. For example, you can use the RequiredFieldValidator control with a login form to prevent users from submitting the form when values have not been entered for the username and password form fields.

The RequiredFieldValidator has two important properties that you need to set:

To create a login form with a required username form field, complete the following steps.

First, you need to create the necessary form elements:

Next, you need to add the code that executes when you click the Button control:

Finally, you need to add the success page to your project and compile the pages:

If you attempt to submit the Login.aspx page by clicking the Button control and you don’t enter a username, the error message in Figure 4.1 is displayed. However, if you enter a value into the username text box and submit the form, you are automatically redirected to the Success.aspx page.

Figure 4.1. Using the RequiredFieldValidator control.

When you click the Button control, the Button1_Click event handler executes. This subroutine contains the following code:

C#. 

private void Button1_Click(object sender, System.EventArgs e)
{
  if (Page.IsValid)
    Response.Redirect( "success.aspx" );
}

VB.NET. 

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)
 Handles Button1.Click
  If IsValid Then
    Response.Redirect("Success.aspx")
  End If
End Sub

The IsValid property has the value True when there are no validation errors in the form. If there are no validation errors, the Response.Redirect method is used to automatically redirect the user to the Success.aspx page. Otherwise, the Login.aspx page is redisplayed and the user can see any validation error messages.

You can use the RangeValidator control to specify a minimum and maximum value for a TextBox control. You can use this control to check for a range of integers, currency amounts, dates, strings, or doubles.

The RangeValidator control has five important properties:

For example, suppose that you want to add an Age text box to a Web Form Page. In that case, you might want to prevent a user from entering an age less than 2 or greater than 120.

To use the RangeValidator control with an Age text box, perform the following steps.

First, you need to add the necessary controls to the page:

Next, you need to add the code that executes when you click the Button control:

If you enter a value less than 2 or greater than 120 into the Age text box and submit the form, an error message will be displayed by the RangeValidator (see Figure 4.2). Otherwise, the Button1_Click event handler will execute and redirect you to the Success.aspx page (we created the Success.aspx page in the previous section).

Figure 4.2. Using the RangeValidator control.

You can use the CompareValidator to compare the value of one control to another control or to compare the value of one control to a fixed value. The CompareValidator has six important properties:

For example, suppose that you are building a Web application to schedule meetings. The form might contain a field for entering the starting time of the meeting and a second field for entering the ending time of the meeting. You could use the CompareValidator to ensure that the time entered into the second field occurs after the time entered into the first field.

Or, suppose that you have only one form field for entering a meeting date. In that case, you might want to check that the date entered into the field occurs after the current date and time (you can’t retroactively schedule meetings).

To use the CompareValidator to check whether a value entered into a field occurs after the current date and time, perform the following steps.

First, you need to add the necessary controls to the page:

Next, you need to add the necessary code for both the Page_Load and Button_Click event handlers:

The current date is assigned to the CompareValidator control’s ValueToCompare property within the Page_Load event handler. If you enter a date that is not greater than the current date into the text box and submit the form, you’ll receive the error message displayed in Figure 4.3.

Figure 4.3. Comparing values with the CompareValidator control.

You can use the RegularExpressionValidator to check whether the value of a form field matches a specified pattern. For example, you can use this Validation control to validate Social Security numbers, postal codes, phone numbers, and email addresses.

Regular expressions are a complex topic that falls outside the scope of this book. Fortunately, however, you don’t need to understand regular expressions to take advantage of the RegularExpressionValidator. Visual Studio .NET provides you with several prewritten regular expressions.

The RegularExpressionValidator has three important properties:

You can supply any regular expression pattern that you want for the ValidationExpression property. Alternatively, you can use one of the predefined regular expression patterns supplied with Visual Studio .NET. If you add a RegularExpressionValidator control to a page and click the ellipses next to the ValidationExpression property, the Regular Expression Editor dialog box in Figure 4.5 appears.

Figure 4.5. The Regular Expression Editor.

For example, suppose that you need to validate a Phone Number form field by using the RegularExpressionValidator. To do so, perform the following steps:

Next, you need to add the code that executes when you click the Button control:

If you attempt to enter a phone number that has an invalid pattern into the text box, you’ll receive the error displayed in Figure 4.6.

Figure 4.6. Using the RegularExpressionValidator control.

If you are creating a long form that uses several Validation controls, it might be difficult for someone completing the form to see all the validation error messages. For example, if the form has 50 form fields and there is a single validation error that appears next to the 48th form field, the person completing the form may never see it.

You can display a summary of all the validation error messages for a Web Form Page in a single location by using the ValidationSummary control.

To take advantage of the ValidationSummary control, you need to provide a value for the ErrorMessage property for each Validation control on a page. The ErrorMessage property contains the error that the ValidationSummary control displays when there is an error.

It is important to not confuse a Validation control’s Text property with its ErrorMessage property. The message assigned to the Text property appears at the same location on the Web Form Page as the Validation control. On the other hand, the message assigned to the ErrorMessage property is displayed as part of the summary displayed by the ValidationSummary control.

Typically, you’ll want to supply a different error message for the Text and ErrorMessage properties. For example, for the text property, you might assign the message “First name is required!” for the ErrorMessage property.

For example, suppose that you need to create a simple pizza order form with two required form fields—the customer name and customer phone number. Additionally, suppose that you want to display a summary of validation errors at the top of the pizza order form. To do so, perform the following steps:

If you attempt to submit the form in the PizzaOrder.aspx page without entering a value for the Customer Name and Customer Phone number fields, validation errors will appear in two places. Each of the two RequiredFieldValidator controls will display the error message Required!. Furthermore, the ValidationSummary control will display the error messages Customer Name is Required! and Customer Phone Number is Required! (see Figure 4.8).

Figure 4.8. Using the ValidationSummary control.