Entity framework question - add entity

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C#

entity-framework

User table:
UserID (PK)
UserName
CompanyID (FK NOT NULL)

Company table:
CompanyID (PK)
CompanyName

entity framework model (v3.5) generated like :

public class User
{
  public string UserID;
  public string UserName;
  public Company company;
}

public class Company
{
   public string CompanyID;
   public string CompanyName;
}

You can see that there is no CompanyID field within User class.Then how to add new user to the database.

What I have tried:

Company c = new Company();
c.CompanyID = "...";

User u = new User();
...
u.company = c;

DBContext.AddUsers(u);
DBContext.SaveChanges();

this will cause the EF to add a new company to the database first, which will end up with a duplicate key exception.

i tried to set up a "foreign key" property like versions of Entity Framework, but failed.