What is the difference between a Session and a Cookie?

Question

Sessions

Sessions are stored per-user in memory(or an alternative Session-State) on the server. Sessions use a cookie(session key) to tie the user to the session. This means no "sensitive" data is stored in the cookie on the users machine.

Sessions are generally used to maintain state when you navigate through a website. However, they can also be used to hold commonly accessed objects. Only if the Session-state is set to InProc, if set to another Session-State mode the object must also serializable.

Session["userName"] = "EvilBoy";

if(Session["userName"] != null)
  lblUserName.Text = Session["userName"].ToString();

Cookies

Cookies are stored per-user on the users machine. A cookie is usually just a bit of information. Cookies are usually used for simple user settings colours preferences ect. No sensitive information should ever be stored in a cookie.

You can never fully trust that a cookie has not been tampered with by a user or outside source however if security is a big concern and you must use cookies then you can either encrypt your cookies or set them to only be transmitted over SSL. A user can clear his cookies at any time or not allow cookies altogether so you cannot count on them being there just because a user has visited your site in the past.

//add a username Cookie
Response.Cookies["userName"].Value = "EvilBoy";
Response.Cookies["userName"].Expires = DateTime.Now.AddDays(10);
//Can Limit a cookie to a certain Domain
Response.Cookies["userName"].Domain = "Stackoverflow.com";

//request a username cookie
if(Request.Cookies["userName"] != null)
   lblUserName.Text = Server.HtmlEncode(Request.Cookies["userName"].Value);

sidenote

It is worth mentioning that ASP.NET also supports cookieless state-management

Cookie is a client side storage of your variables. It stored on client machine by browser physically. It's scope is machine wide. Different users at same machine can read same cookie.

Because of this :

1. You should not store sensitive data on cookie.
2. You should not store data that belongs to one user account.
3. Cookie has no effect on server resources.
4. Cookie expires at specified date by you.

Session is a server side storage of your variables. Default, it stored on server's memory. But you can configure it to store at SqlServer. It's scope is browser wide. Same user can run two or more browsers and each browser has it's own session.

Because of this :

1. You can save sensitive data in session.
2. You should not save everything in session. it's waste of server resources.
3. After user closes browser, session timeout clears all information. (default is 20 minutes)

A cookie is an identifaction string stored by a server (who has a domain) in the browser of the user who visits the server/domain.

A session is a unit of maybe variables, state, settings while a certain user is accessing a server/domain in a specific time frame. All the session information is in the traditional model stored on the server (!)

Because many concurrent users can visit a server/domain at the same time the server needs to be able to distinguish many different concurrent sessions and always assign the right session to the right user. (And no user may "steal" another uses's session)

This is done through the cookie. The cookie which is stored in the browser and which should in this case be a random combination like s73jsd74df4fdf (so it cannot be guessed) is sent on each request from the browser to the server, and the server can assign and use the correct session for its answers (page views)

The cookie allows the server to recognize the browser/user. The session allows the server to remember information between different page views.

Sessions are not reliant on the user allowing a cookie. They work instead like a token allowing access and passing information while the user has their browser open. The problem with sessions is that when you close your browser you also lose the session. So, if you had a site requiring a login, this couldn't be saved as a session like it could as a cookie, and the user would be forced to re-login every time they visit.

Its possible to have both: a database primary key is hashed and stored in a lookup table: then the hash is stored on the client as a cookie. Once the hash cookie (hahhahaha :) is submitted, its corresponding primary key is looked up, and the rest of the details are associated with it in another table on the server database.