Question

I am using Facebook API how i can get userid in facebook from its mail ?

I am using Facebook.dll

Answer 1

Using a Search API, you can query all publicly available information. That means, you can get user ID if the user allowed his e-mail address to be public.
You would issue a query something like:

 https://graph.facebook.com/search?q=[e-mail]&type=user&access_token=... 

The JSON response would be:

{
      "data": [
        {
         "name": "Firstname Lastname"
         "id": "123456799"
        }
   ]
} 

If the user’s address is not public, you will get an empty data.

I’m not aware if there is an implementation for this in C#, but you can easily make one.