How to access facebook account information when login with facebook in my asp.net page

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C#

ASP.NET

Hi All....

In my web application i want a functionality like signup/login with facebook. My conditions are as follows:

1. If user is already registered in my web application then simply login with facebook.

2. If user is not registered with me and clicks on Login with Facebook button then user should be registered with my application and logged in.

Also guide me about :

1. How many informations we can retrieve from facebook account for any user and which informations...
2. Is it necessary that any one field like "mailid" or anything else should be common at facebook account and at my web application account when user tries to login with facebook.

if not than how should i match the informations for a particular user...

3. if user is not registered with my web application and tries to login with facebook i want to register that user at my application first than logged in but how i don't know, so please provide me proper code for it...