In ASP.NET Web Forms, which type of events are raised when a web page posts data back to the same page on the server for processing, for example when a Button control is clicked?

Introduction / Context:
ASP.NET Web Forms uses a postback model in which pages send data back to themselves on the server to process user actions such as button clicks. Understanding postback events is fundamental when working with server controls, event handlers, and view state. Interviewers often ask this question to ensure that candidates understand the basic life cycle and interaction model of Web Forms applications.

Given Data / Assumptions:

• We are using ASP.NET Web Forms, not MVC.
• A user interacts with a form that contains server controls such as Button or DropDownList.
• On certain actions, the page sends data back to the server for processing and then re renders.

Concept / Approach:
In Web Forms, a postback occurs when the page is submitted back to the same URL on the server, carrying form data and view state. Server controls can raise postback events to signal that a user action needs server side handling. For example, clicking a Button causes a Click event that is treated as a postback event. These events are processed during the page life cycle after view state and control state have been restored, allowing developers to react based on current state and user input.

Step-by-Step Solution:
Step 1: A user loads an ASP.NET Web Forms page, which renders HTML containing form elements and hidden fields such as view state. Step 2: The user interacts with a server control, such as clicking a Button, and the browser sends a form submission back to the same page URL. Step 3: ASP.NET recognises that this is a postback, recreates the page and control hierarchy on the server, and restores view state values. Step 4: The framework detects which control caused the postback and raises the corresponding postback event, such as Button.Click or SelectedIndexChanged for a list control. Step 5: After the event handler runs, the page continues through the life cycle to rendering, and the updated HTML is sent back to the browser.

Verification / Alternative check:
Developers can verify this by examining the __EVENTTARGET and __EVENTARGUMENT fields in the rendered HTML and by stepping through the page life cycle in a debugger. Documentation describes these events as postback events because they are tied to the form posting back to the same page. Validation events also occur during the life cycle but are focused on validating input, not on defining the act of posting back.

Why Other Options Are Wrong:
Option A is wrong because validation events refer to input validation processes that can run during postback but are not what causes a page to post back. Option B is incorrect because cached events are not a standard concept in Web Forms life cycle terminology. Option D is wrong because postback events are the recognised term for this behaviour.

Common Pitfalls:
A common pitfall is misunderstanding the difference between initial requests and postbacks, which can lead to running heavy initialisation code on every request instead of only when IsPostBack is false. Another issue is performing logic before view state is restored, leading to unexpected values. Knowing that postback events occur after control state restoration helps you place logic in the correct life cycle stages and handle user interactions reliably.

Final Answer:
The events described are postback events, which are raised when a Web Forms page posts data back to the same page on the server for processing user actions.