public class Users
    {
        public int UserId { get; set; }
        public String Username { get; set; }
        public virtual ICollection<Role> Roles { get; set; }
}
public class Role
    {
        public int RoleId { get; set; }
        public string RoleName { get; set; }
        public virtual ICollection<Users> Users { get; set; }
    }

public Users GetUsers(string Uname, string psw)
            {
                Role role1 = new Role { RoleName = "Admin" ,RoleId=100,Description="Admin"};

                var x = (from n in db.Users
                        where n.Username == Uname && n.Password == psw
                        select n).FirstOrDefault();
                if(x!=null)
                 x.Roles.Add(role1); return x;