public class DAO
{
    static void Main()
    {
        Edit("ALFKI", "Plamen");
        Add("GRIVICA", "T233I");
    }

    static void Add(string name, string id)
    {
        Customer newCustomer = new Customer()
        {
            CompanyName = name,
            CustomerID = id
        };

        using (NORTHWNDEntities db = new NORTHWNDEntities())
        {
            bool isInDB = IsInDataBase(db, id);

            if (!isInDB)
            {
                db.Customers.Add(newCustomer);
                db.SaveChanges();
                Console.WriteLine("Added Successful.");
            }
            else
            {
                throw new ArgumentException("Such customer already exists");
            }
        }
    }

    static void Edit(string id, string newContactName)
    {
        using (NORTHWNDEntities db = new NORTHWNDEntities())
        {
            var customer = db.Customers.Where(x => x.CustomerID == id).FirstOrDefault();
            customer.ContactName = newContactName;
            db.SaveChanges();
        }
    }

    static void Delete(string id)
    {
        using (NORTHWNDEntities db = new NORTHWNDEntities())
        {
            var customer = db.Customers.Where(x => x.CustomerID == id).FirstOrDefault();
            db.Customers.Remove(customer);
            db.SaveChanges();
        }
    }

    static bool IsInDataBase(NORTHWNDEntities db, string id)
    {
        bool alreadyInDB = db.Customers.Where(a => a.CustomerID == id).Any();
        return alreadyInDB;
    }
}