How do you clear cookies using asp.net mvc 3 and c#?

You're close. You'll need to use the Response object to write back to the browser:

if ( Request.Cookies["MyCookie"] != null )
{
    var c = new HttpCookie( "MyCookie" );
    c.Expires = DateTime.Now.AddDays( -1 );
    Response.Cookies.Add( c );
}

More information on MSDN, How to: Delete a Cookie.

Cookies are stored on the client, not on the server, so Session.Clear won't affect them. Also, Request.Cookies is populated by IIS and given to your page with each request for a page; adding/removing a cookie from that collection does nothing.

Try performing a similar action against Response.Cookies. That should cause your client to overwrite the old cookie with the new one, causing it to be expired.

I did this and it worked for clearing (not deleting) a session cookie:

HttpContext.Response.Cookies.Set(new HttpCookie("cookie_name"){Value = string.Empty});

Based on Metro's response I created this extension method to make the code reusable in any controller.

/// <summary>
/// Deletes a cookie with specified name
/// </summary>
/// <param name="controller">extends the controller</param>
/// <param name="cookieName">cookie name</param>
public static void DeleteCookie(this Controller controller, string cookieName)
{
    if (controller.HttpContext.Request.Cookies[cookieName] == null)
            return; //cookie doesn't exist

    var c = new HttpCookie(cookieName)
                {
                    Expires = DateTime.Now.AddDays(-1)
                };
    controller.HttpContext.Response.Cookies.Add(c);
}