How to delete cookies on an ASP.NET website

Try something like that:

if (Request.Cookies["userId"] != null)
{
    Response.Cookies["userId"].Expires = DateTime.Now.AddDays(-1);   
}

But it also makes sense to use

Session.Abandon();

besides in many scenarios.

No, Cookies can be cleaned only by setting the Expiry date for each of them.

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

At the moment of Session.Clear():

• All the key-value pairs from Session collection are removed. Session_End event is not happen.

If you use this method during logout, you should also use the Session.Abandon method to Session_End event:

• Cookie with Session ID (if your application uses cookies for session id store, which is by default) is deleted

This is what I use:

    private void ExpireAllCookies()
    {
        if (HttpContext.Current != null)
        {
            int cookieCount = HttpContext.Current.Request.Cookies.Count;
            for (var i = 0; i < cookieCount; i++)
            {
                var cookie = HttpContext.Current.Request.Cookies[i];
                if (cookie != null)
                {
                    var expiredCookie = new HttpCookie(cookie.Name) {
                        Expires = DateTime.Now.AddDays(-1),
                        Domain = cookie.Domain
                    };
                    HttpContext.Current.Response.Cookies.Add(expiredCookie); // overwrite it
                }
            }

            // clear cookies server side
            HttpContext.Current.Request.Cookies.Clear();
        }
    }

Unfortunately, for me, setting "Expires" did not always work. The cookie was unaffected.

This code did work for me:

HttpContext.Current.Session.Abandon();
HttpContext.Current.Response.Cookies.Add(new HttpCookie("ASP.NET_SessionId", ""));

where "ASP.NET_SessionId" is the name of the cookie. This does not really delete the cookie, but overrides it with a blank cookie, which was close enough for me.