In ASP.NET Web Forms, which validator control allows you to check whether the value entered in one control matches the value of another control, for example when confirming a password?

Introduction / Context:
Many forms require users to enter the same value twice, such as a password and a confirm password field. ASP.NET Web Forms provides a validator specifically designed to compare two inputs and ensure they match. Understanding which validator to use in this scenario is a straightforward but important part of working with Web Forms validation controls.

Given Data / Assumptions:

• We have at least two input controls on the page, such as TextBoxes.
• We need to ensure that the value of one control matches the value of another.
• The question asks which validator is suitable for this task.

Concept / Approach:
The CompareValidator control compares the value of one control with either another control or a constant value. It can perform equality checks as well as relational comparisons, but the most common use in user interfaces is to verify that two fields contain identical text. The validator is declared in markup and configured by setting ControlToValidate and ControlToCompare properties.

Step-by-Step Solution:
Step 1: Place two input controls on the page, such as TextBoxPassword and TextBoxConfirmPassword. Step 2: Add a CompareValidator control to the page. Step 3: Set the ControlToValidate property of the CompareValidator to TextBoxConfirmPassword and the ControlToCompare property to TextBoxPassword. Step 4: Set the Operator property to Equal and specify an appropriate data type if needed. Step 5: When the form is submitted, the validator compares the values in the two text boxes and fails validation if they do not match, displaying the configured error message.

Verification / Alternative check:
To verify, you can test the form by entering different values in the password and confirm password fields and clicking submit. The CompareValidator should display an error message and Page.IsValid should be false. Entering matching values should cause the validator to pass. The official documentation and examples show CompareValidator used in exactly this way, confirming that it is the correct control for matching values.

Why Other Options Are Wrong:
Option A is wrong because RangeValidator checks whether a value lies within a specified numeric, date, or string range, not whether two controls match exactly. Option C is incorrect because RequiredFieldValidator only ensures that a control is not empty; it does not compare two values. Option D is wrong because CustomValidator is used for custom logic; while you could implement comparison logic with it, the dedicated CompareValidator is simpler and more appropriate for direct comparisons.

Common Pitfalls:
One pitfall is forgetting to validate both that fields are not empty and that they match; you often need a RequiredFieldValidator on each control as well as a CompareValidator. Another issue is misconfiguring ControlToValidate or ControlToCompare, which leads to validators that silently do nothing. Testing edge cases, such as whitespace differences and case sensitivity, helps confirm that the validator behaves as expected for your scenario.

Final Answer:
The control used to verify that the data in one control matches the data in another is the CompareValidator.