Cookie are one of several ways to store data about web site visitors during the time when web server and browser are not connected. Common use of cookies is to remember users between visits. Practically, cookie is a small text file sent by web server and saved by web browser on client machine.

To write a cookie in ASP.NET we can use a code like this:

[ VB.NET ]

Imports System.Web
 

' Use this line to save a cookie
Response.Cookies("MyCookieName").Value = "MyCookieValue"
' How long will cookie exist on client hard disk
Response.Cookies("MyCookieName").Expires = Now.AddDays(1)

[ C# ]

// Add this on the beginning of your .vb code file
using System;
 
 
// Use this line when you want to save a cookie
Response.Cookies["MyCookieName"].Value ="MyCookieValue";
// How long will cookie exist on client hard disk
Response.Cookies["MyCookieName"].Expires = DateTime.Now.AddDays(1);

// To add multiple key/value pairs in single cookie
Response.Cookies["VisitorData"]["FirstName"] = "Richard";
Response.Cookies["VisitorData"]["LastVisit"] = DateTime.Now.ToString();

To read a cookie value, use this:

[ VB.NET ]

Dim MyCookieValue As String
' We need to perform this check first, to avoid null exception
' if cookie not exists
IfNot Request.Cookies("MyCookieName") Is NothingThen
    MyCookieValue = Request.Cookies("MyCookieName").Value
EndIf

[ C# ]

string MyCookieValue;
// We need to perform this check first, to avoid null exception
// if cookie not exists
if(Request.Cookies["MyCookieName"] != null)
    MyCookieValue = Request.Cookies["MyCookieName"].Value;

To delete existing cookie we actually just set its expiration time to some time in the past. You can do it with code like this:

[ VB.NET ]

' First check if cookie exists
If Not Request.Cookies("MyCookieName") Is Nothing Then
    ' Set its expiration time somewhere in the past
    Response.Cookies("MyCookieName").Expires = Now.AddDays(-1)
End If

[ C# ]

// First check if cookie exists
if (Request.Cookies["MyCookieName"] != null)
{
   // Set its expiration time somewhere in the past
    Response.Cookies["MyCookieName"].Expires = DateTime.Now.AddDays(-1);
}

HttpCookie class is located in System.Web namespace. You can use HttpCookie class to create and manipulate cookies instead of using of Response and Request objects.

HttpCookie class have these properties:
- Domain - Gets or sets domain associated with a cookie. It is often used to limit cookie use to web site sub domain.
- Expires - Gets or sets time when cookie expires. After that time cookie is deleted by the browser. The maximum life time for cookie is 365 days. You can increase expiration time every time when visitor visits your web site, but if visitor don't comes for more than 365 days, the cookie will be deleted.
- HasKeys - Returns true if cookie has key pairs or false if not. Cookies are not limited to only simple data as strings, but could stores key/values pairs as well.
- HttpOnly - Gets or sets a true/false value if cookie is accesible by client side javascript. If value is true, cookie will be accessible only by server side ASP.NET code.
- Item - Not necessary, it exists only because it is used in old classic ASP.
- Name - A name of a cookie.
- Path - Similar like Domain property, path is used to limit a cookie scope to specific URL. For example, to limit using of a cookie to sub folder www.yourdomain.com/forum you need to set Path property to "/forum".
- Secure - Would cookies will transmit through HTTPS protocol by using SSL (secure socket layer) connection.
- Value - Gets or sets a cookie's value.
- Values - Used to get or set key/value pairs in individual cookie.

You can use HttpCookie class to create a cookie or set cookie's properties, like in this example code:

[ VB.NET ]

Dim MyGreatCookie As HttpCookie = New HttpCookie("MyCookieName")
MyGreatCookie.Value ="Some cookie value"
MyGreatCookie.Expires = Now.AddDays(100)
Response.Cookies.Add(MyGreatCookie)

[ C# ]

HttpCookie MyGreatCookie = new HttpCookie("MyCookieName");
MyGreatCookie.Value ="Some cookie value";
MyGreatCookie.Expires = DateTime.Now.AddDays(100);
Response.Cookies.Add(MyGreatCookie);

Cookie size is limited to 4096 bytes. It is not much, so cookies are used to store small amounts of data, often just user id.

Also, number of cookies is limited to 20 per website. If you make new cookie when you already have 20 cookies, browser will delete oldest one.

Your web site visitor can change browser settings to not accept cookies. In that case you are not able to save and retrieve data on this way! Because of this, it is good to check browser settings before saving a cookie.

If your visitor blocked cookies in web browser privacy settings, you need to decide do you still want to save that data on some other way (maybe with sessions) or to not save it at all. Anyway, you application must continue to work normally with any browser privacy settings. It is better to not store any sensitive or critical data to cookies. If using of cookies is necessary, you should inform your users with some message like: "Cookies must be enabled to use this application".

There are two possible cases when your client will not accept cookies:

- Web browser does not support cookies
- Web browser supports cookies, but user disabled that option through a browser's privacy settings.

[ VB.NET ]

If Request.Browser.Cookies Then
   ' Cookies supported
Else
   ' Web browser not supports cookies
EndIf

[ C# ]

if (Request.Browser.Cookies)
{
   // Cookies supported
}
else
{
   // Web browser not supports cookies
}

Code above will tell you does web browser supports cookie technology, but your visitor could disable cookies in web browser's privacy settings. In that case, Request.Browser.Cookies will still return true but your cookies will not be saved. Only way to check client's privacy settings is to try to save a cookie on the first page, and then redirect to second page that will try to read that cookie. You can eventually use the same page to save and read a cookie when perform a testing, but you must use Response.Redirect method after saving and before reading cookies.

Cookies are just plain text, so usually are not used to store sensitive informations like passwords without prior encryption. If you want to enable "Remember me" option on web site it is recommended to encrypt a password before it is stored in a cookie. Cookies are often used for data like: when visitor last time loged in, what site color she likes, to keep referer id if we offer affiliate program etc.

Because of security reasons, your web application can read only cookies related to your web domain. You can't read cookies related to other web sites. Web browser stores cookies from different sites separately.

Cookie is just a plain text file on client's hard disk so it could be changed on different ways outside of your application. Because of that, you need to treat cookie value as potentially dengerous input like any other input from the visitor, including prevention of cross site scripting attacks.