C#

Company company1 = new Company();
            company1.CompanyId = 1;
            company1.Name = "abc";
            company1.Email = "abc@hotmail.com";
            company1.Address = "xyz street";
            company1.City = cityState[0];
            company1.State = sortedStates.Where(state => state.Abbreviation == cityState[1]).SingleOrDefault();
            company1.StateId = 1;
            company1.Zip = 2066;
            company1.Phone = "949494848";
            company1.Customers = new List<Customer>();

            Customer customer1 = new Customer();
            customer1.CustomerId = 1;
            customer1.FirstName = "bikash";
            customer1.LastName = "shrestha";
            customer1.Email = "bikash@hotmail.com";
            customer1.Address = "482 pacific hwy";
            customer1.City = "Phoenix,AZ";
            customer1.State = sortedStates.Where(state => state.Abbreviation == cityState[1]).SingleOrDefault();
            customer1.StateId = 1;
            customer1.Zip = 85230;
            customer1.Gender = Gender.Female;
            customer1.Orders = new List<Order>();

            Customer customer2 = new Customer();
            customer2.CustomerId = 2;
            customer2.FirstName = "prakash";
            customer2.LastName = "shrestha";
            customer2.Email = "prakash@hotmail.com";
            customer2.Address = "482 pacific hwy";
            customer2.City = "Phoenix,AZ";
            customer2.State = sortedStates.Where(state => state.Abbreviation == cityState[1]).SingleOrDefault();
            customer2.StateId = 2;
            customer2.Zip = 85231;
            customer2.Gender = Gender.Male;
            customer2.Orders = new List<Order>();

            Order order1 = new Order();
            order1.Id = 1;
            order1.CustomerId = 1;
            order1.Product = "apple";
            order1.Date = DateTime.Today;
            order1.Price = 5;
            order1.Quantity = 1;

            Order order2 = new Order();
            order2.Id = 2;
            order2.CustomerId = 1;
            order1.Product = "apple";
            order2.Date = DateTime.Today;
            order2.Price = 5;
            order2.Quantity = 1;


            customer1.Orders.Add(order1);
            customer2.Orders.Add(order2);
            company1.Customers.Add(customer1);
            company1.Customers.Add(customer2);
            

            using (var dbCtx = new  CustomerManagerContext())
            {               
               
                dbCtx.Companies.Add(company1); <-- here i get same error now.
                //call SaveChanges
                dbCtx.SaveChanges();
            }

using (var dbCtx = new CustomerManagerContext())
            {

                dbCtx.Companies.Add(company1);
                //call SaveChanges
                dbCtx.SaveChanges();
            }
                company1.Customers.Add(customer1);
                company1.Customers.Add(customer2);
                customer1.Orders.Add(order1);
                customer2.Orders.Add(order2);

using (var dbCtx = new CustomerManagerContext())
            {
                dbCtx.Customers.Add(customer1);
                dbCtx.SaveChanges();
                 dbCtx.Customers.Add(customer2);
                dbCtx.SaveChanges();
                dbCtx.Companies.Add(company1);
                //call SaveChanges
                dbCtx.SaveChanges();
               
                 dbCtx.Orders.Add(order1);
                 dbCtx.SaveChanges();
                dbCtx.Orders.Add(order2);
                 dbCtx.SaveChanges();
                
            }

protected override void OnModelCreating(DbModelBuilder modelBuilder)
       {

           modelBuilder.Entity<Customer>().
           HasMany(c => c.Companies).
           WithMany(p => p.Customers).
           Map(
           m =>
           {
               m.MapLeftKey("CustomerId");
               m.MapRightKey("CompanyId");
               m.ToTable("CustomerCompany");
           });
           modelBuilder.Entity<Company>().HasRequired(c => c.Customers).WithMany().WillCascadeOnDelete(false);
           modelBuilder.Entity<Customer>().HasRequired(c => c.Companies).WithMany().WillCascadeOnDelete(false);
           base.OnModelCreating(modelBuilder);
       }

public class MyContext : DbContext
{
   public DbSet<Customer> Customers { get; set; }
   public DbSet<Company> Companies { get; set; }
   public DbSet<Order> Orders { get; set; }
}

public class Company
{
[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)] [Display(AutoGenerateField = false)]
public int CompanyId { get; set; }

[StringLength(50)]
public string Name { get; set; }

[StringLength(200)]
public string Address { get; set; }

[StringLength(500)]
public string Email { get; set; }

[StringLength(1000)]
public string Phone { get; set; }

[StringLength(50)]
public string City { get; set; }

public State State { get; set; }
public int StateId { get; set; }
public int Zip { get; set; }

public virtual ICollection Orders{ get; set; } // Change for M:M mapping
}

public class Customer
{
[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)] [Display(AutoGenerateField = false)]
public int CustomerId { get; set; }

[StringLength(50)]
public string FirstName { get; set; }

[StringLength(50)]
public string LastName { get; set; }

[StringLength(100)]
public string Email { get; set; }

[StringLength(1000)]
public string Address { get; set; }

[StringLength(50)]
public string City { get; set; }
public int Zip { get; set; }

//State is not a valid class for the DB. If it's a Table do this
[Display(AutoGenerateField = false), ForeignKey("State")]
public int StateId { get; set; }
public virtual State State { get; set; } 

public virtual ICollection Orders{ get; set; } // Change for M:M mapping

// Gender is not a valid field class. Just use the enum string (or int) value
public string Gender { get; set; }

public class Order
{
[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)] [Display(AutoGenerateField = false)]
public int Id { get; set; }

/*
Order-specific fields here.
*/

[Display(AutoGenerateField = false), ForeignKey("Customer")]
public int CustomerId { get; set; }
[Display(AutoGenerateField = false), ForeignKey("Company")]
public int CompanyId { get; set; }

public virtual Customer Customer { get; set; }
public virtual Company Company { get; set; }

public Order(){}

public Order(Customer customer, Company company)
{
CustomerId = customer.CustomerId;
CompanyId = company.CompanyId;
}
}

public class MyContext : DbContext
{
   public DbSet<Customer> Customers { get; set; }
   public DbSet<Company> Companies { get; set; }
   public DbSet<Order> Orders { get; set; }

   public bool UpdateOrder(Order order)
   {
      if(Orders.Any(x => x.Id == order.Id))
         this.Entry(order).State = EntityState.Modified;
      else
         Orders.Add(order);
      return this.SaveChanges() != 0;
   }
   public bool UpdateCompany(Company company)
   {
      if(Companies.Any(x => x.CompanyId == company.CompanyId))
         this.Entry(company).State = EntityState.Modified;
      else
         Companies.Add(company);
      return this.SaveChanges() != 0;
   }
   public bool UpdateCustomer(Customer customer)
   {
      if(Customers.Any(x => x.CustomerId == customer.CustomerId))
         this.Entry(customer).State = EntityState.Modified;
      else
         Customers.Add(customer);
      return this.SaveChanges() != 0;
   }
   public bool DeleteOrder(Order order)
   {
      if(Orders.Any(x => x.Id == order.Id))
      {
         Orders.Remove(order);
         return this.SaveChanges() != 0;
      }
      return false;
   }
// etc...
}