eCompanies/eCompanies/App.xaml.cs

using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Data;
using System.Linq;
using System.Windows;

namespace eCompanies
{
    /// <summary>
    /// Логика взаимодействия для App.xaml
    /// </summary>
    public partial class App : Application
    {
        private void addCompany(Company c)
        {
            using (var db = new CompaniesDBContext())
            {
                db.Company.Add(c);
                var count = db.SaveChanges();
            }
        }

        public static ObservableCollection<Company> getCompanies()
        {
            using (var db = new CompaniesDBContext())
                return new ObservableCollection<Company>(db.Company.AsEnumerable()); 
        }

        public static HashSet<Users> getUsers(int id)
        {
            using (var db = new CompaniesDBContext())
                return new HashSet<Users>(db.Users.Where(b => b.CompanyId.Equals(id)));
        }

        public static void updateCompany(Company c)
        {
            using (var db = new CompaniesDBContext())
            {
                db.Company.Update(c);
                db.SaveChanges();
            }
        }

    }
}