Build Your Own ASP.NET 2.0 Web Site Using C# & VB, Second Edition

CompareValidator

One of the most useful validation controls is the CompareValidator control,

which performs a comparison between the data entered into a given control and

some other value. That other value can be a fixed value, such as a number, or a

value entered into another control.

Let’s look at an example that builds on the login example from the previous sec-

tion. Here, we’ll validate that the data entered into both the password fields is

identical. Make the following changes to Login.aspx:

File: Login.aspx (excerpt)

<asp:TextBox id="passwordTextBox" runat="server"

TextMode="Password" />

⋮

<asp:TextBox id="confirmPasswordTextBox" runat="server"

TextMode="Password" />

⋮

<asp:CompareValidator id="comparePasswords" runat="server"

ControlToCompare="passwordTextBox"

ControlToValidate="confirmPasswordTextBox"

ErrorMessage="Your passwords do not match up!"

Display="Dynamic" />

</p>

Run the page and enter different passwords into the two fields. The CompareVal-

idator control will appear as soon as you move on from two fields whose data

doesn’t match, as Figure 6.5 shows.

Figure 6.5. A CompareValidator in action

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CompareValidator

As you’ve probably noticed, the CompareValidator control differs very little from

the RequiredFieldValidator control:

File: Login.aspx (excerpt)

<asp:RequiredFieldValidator id="confirmPasswordReq" runat="server"

ControlToValidate="confirmPasswordTextBox"

ErrorMessage="Password confirmation is required!"

SetFocusOnError="True" Display="Dynamic" />

<asp:CompareValidator id="comparePasswords" runat="server"

ControlToCompare="passwordTextBox"

ControlToValidate="confirmPasswordTextBox"

ErrorMessage="Your passwords do not match up!"

Display="Dynamic" />

The only difference is that in addition to a ControlToValidate property, the

CompareValidator has a ControlToCompare property. We set these two properties

to the IDs of the controls we want to compare. So, in our example, the

ControlToValidate property is set to the confirmPasswordTextBox, and the

ControlToCompare property is set to the passwordTextBox.

The CompareValidator can be used to compare the value of a control to a fixed

value, too. CompareValidator can check whether the entered value is equal to,

less than, or greater than, any given value. As an example, let’s add an age field

to our login form:

File: Login.aspx (excerpt)

<!-- Age -->

<p>

Age:<br />

<asp:TextBox id="ageTextBox" runat="server" />

<asp:RequiredFieldValidator id="ageReq" runat="server"

ControlToValidate="ageTextBox"

ErrorMessage="Age is required!"

SetFocusOnError="True" Display="Dynamic" />

<asp:CompareValidator id="ageCheck" runat="server"

Operator="GreaterThan" Type="Integer"

ControlToValidate="ageTextBox" ValueToCompare="15"

ErrorMessage="You must be 16 years or older to log in" />

</p>

In this case, the CompareValidator control is used to check that the user is old

enough to log in to our fictitious web application. Here, we set the Operator

property of the CompareValidator to GreaterThan. This property can take on

any of the values Equal, NotEqual, GreaterThan, GreaterThanEqual, LessThan,

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Chapter 6: Using the Validation Controls