Linq to SQL

Can someone help me write out a linq query.  I am looking for the best way to optimize this.

Here is what I have so far..and what Im trying to do.  I am able to pull back a distinct record from the database by using the SessionId that is passed into this funtion and then used in the where clause.

I need to determine the following and then update based on conditions.

First I need to determine if their is a user.ConnectionId and that it is also = to Id.  If both is true then I just need to update the user.LastOn to lastOn and update the boolean field in db isActive to true.

otherwise I would also need to update the user.ConnectionId value to Id.

if var user is null I need to handle that condition as well.

Any other suggestions for good linq programming is welcome.

and anything else to make this method better formatting is welcome as well!

public class DirectModeHub : Hub
    {

        private string _userId = null;
        private string _sessionId = null;

        // Public hub methods
        public void RegisterICD(string userId, string SessionId)
        {
            _userId = userId;
            _sessionId = SessionId;

            var id = Context.ConnectionId;
            var lastOn = DateTime.Now.ToString("dd-mm-yyyy hh:MM:ss");

            using (SqlDbContext sqlDb = new SqlDbContext("SqlDbContext", null))
            {
                var user = (from u in sqlDb.UserSessions
                            where u.SessionId.ToString() == SessionId
                            select u).FirstOrDefault();

                if (user.ConnectionId != id )
                {

                    
                }

            }

        }



    }

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