In ASP.NET Web Forms, which hidden field name is used internally on the page to implement view state across postbacks?

Introduction / Context:
ASP.NET Web Forms uses view state to preserve the state of server controls between postbacks. This state is stored in a hidden field rendered into the HTML form. Knowing the name of this hidden field helps developers debug, inspect, and tune page size. Interviewers sometimes ask this question to check familiarity with how Web Forms manages state under the hood.

Given Data / Assumptions:

• We are working with ASP.NET Web Forms pages that use runat="server" forms.
• View state is enabled for the page and controls by default.
• The framework serialises control state into a hidden field.

Concept / Approach:
When a Web Forms page renders, ASP.NET encodes the view state data into a Base64 string and places it in a hidden input element within the form. The name of this hidden field is a specific identifier that the framework recognises on postback so it can restore control state. The standard name for this hidden field in classic Web Forms is __VIEWSTATE, which includes two leading underscore characters.

Step-by-Step Solution:
Step 1: When a page is rendered with view state enabled, ASP.NET walks the control tree and serialises state information into a data structure. Step 2: This state is encoded into a string and placed in a hidden input element in the HTML form. Step 3: The hidden field uses a well known name, __VIEWSTATE, which ASP.NET looks for in incoming postback requests. Step 4: On postback, the framework reads the __VIEWSTATE value from the request, decodes it, and uses it to restore control state before raising events. Step 5: This process allows controls to remember properties such as selected values and text across postbacks without explicitly reloading them from another store.

Verification / Alternative check:
You can confirm this by viewing the page source of an ASP.NET Web Forms page in a browser. You will see a hidden input element with name="__VIEWSTATE" and a long encoded value. Tools such as browser developer tools or HTTP proxies also show this field in form submissions. None of the alternative names in the options match the actual field name, which always uses the double underscore prefix.

Why Other Options Are Wrong:
Option B is wrong because _VIEWSTATE has only a single underscore and is not the field name used by ASP.NET. Option C is incorrect because VIEW_STATE with an underscore in the middle is not recognised by the framework. Option D is wrong because VIEWSTATE_ has the underscore at the end, which again does not match the framework convention.

Common Pitfalls:
Developers sometimes ignore view state completely and then wonder why page sizes are large; understanding __VIEWSTATE helps them inspect and optimise state usage. Another pitfall is disabling view state globally without understanding which controls rely on it, which can break event handling and data binding. Knowing that __VIEWSTATE is the key mechanism for preserving state helps you decide when to use alternatives such as control state, session state, or model binding in newer frameworks.

Final Answer:
ASP.NET Web Forms uses a hidden field named __VIEWSTATE to implement view state across postbacks on a page.