securing view state using ViewstateUserKey property

Question

On reading about preventing cross-site request forgeries in webforms many authors use statements like this:

To use the ViewStateUserKey property within the Viewstate to protect against spoofed post backs. Add the following in the OnInit virtual method of the Page-derived class (This property must be set in the Page.Init event).

protected override OnInit(EventArgs e) {
 base.OnInit(e); 
 if (User.Identity.IsAuthenticated)
    ViewStateUserKey = Session.SessionID; }

The following keys the Viewstate to an individual using a unique value of your choice.

 (Page.ViewStateUserKey)

This must be applied in Page_Init because the key has to be provided to ASP.NET before Viewstate is loaded. This option has been available since ASP.NET 1.1

I have tried to put the above one in Base page, where all pages are deriving from that page But I am getting error like OnInit must return value

this is my code

public class BasePage : System.Web.UI.Page
{

    protected override OnInit(EventArgs e) {
 base.OnInit(e); 
 if (User.Identity.IsAuthenticated)
    ViewStateUserKey = Session.SessionID; }

 }

Many thanks in advance

Answer 1

The OnInit declaration is missing the return type void.

protected override void OnInit(EventArgs e)