In ASP.NET Web Forms, which validator control checks that an input control contains some data and is not left blank before the page is considered valid?

Introduction / Context:
ASP.NET Web Forms provides a set of validation controls that help enforce input rules on the server side and optionally on the client side. One of the most frequently used validators is the RequiredFieldValidator, which ensures that mandatory fields are not left empty. Understanding the role of each validator type is a standard interview topic for developers who work with Web Forms applications.

Given Data / Assumptions:

• We are using ASP.NET Web Forms validation controls.
• Some input fields must not be left blank by the user.
• The question asks which validator ensures that controls contain data.

Concept / Approach:
The RequiredFieldValidator is designed specifically to ensure that the associated control has a value. If the control is empty or contains only whitespace depending on configuration, the validator fails and displays an error message. It is commonly used for fields such as user names, passwords, required check boxes, or drop down lists where a non default selection is expected.

Step-by-Step Solution:
Step 1: Place the RequiredFieldValidator on the Web Forms page and set its ControlToValidate property to the ID of the control that should not be empty. Step 2: Configure the ErrorMessage property to display a user friendly message when validation fails. Step 3: Optionally set InitialValue if the control has a default value that should be treated as empty, such as a drop down list prompt like "Select one". Step 4: When the user submits the form, ASP.NET runs validation logic on both the client and server, and the RequiredFieldValidator checks whether the control has a value different from its initial value. Step 5: If the control is empty, the validator sets IsValid to false and the page validation fails, preventing server side processing unless you override the behaviour.

Verification / Alternative check:
You can verify this by creating a simple form with a TextBox and a RequiredFieldValidator, then attempting to submit the form without entering any text. The validator error should appear and Page.IsValid should be false. Filling in the text box and submitting again will cause the validator to pass. This clearly demonstrates that RequiredFieldValidator is responsible for enforcing non empty input.

Why Other Options Are Wrong:
Option A is wrong because CompareValidator is used to compare a control value against another control or a constant, for example to ensure equality or a particular relation. Option B is incorrect because RangeValidator checks whether a value falls within a specified range. Option D is wrong because CustomValidator is used when you need custom validation logic not covered by the built in validators; on its own, it does not necessarily enforce non empty values unless you implement that logic.

Common Pitfalls:
A common pitfall is forgetting to set the ControlToValidate property, which prevents the validator from functioning. Another issue is not handling the InitialValue for drop down lists, leading to cases where a default prompt is considered valid. It is also important to remember that client side validation can be bypassed, so always rely on server side validation results before trusting input. Proper use of RequiredFieldValidator ensures that mandatory fields are not accidentally skipped by users.

Final Answer:
The validator that checks whether an input control contains data and is not left blank is the RequiredFieldValidator control.