Create object with many-to-many relationship in MVC 4 - asp.net-mvc

Question

I am using MVC 4 with EF code first approach. I have two simple objects. These are their POCO classes:
public class Activity
{
//Primitive Properties
[HiddenInput]
public int Id { get; set; }
[Required]
public int LengthInMinutes { get; set; }
public string AdditionalInfo { get; set; }
[Required]
public bool Archive { get; set; }
//Navigation Properties
public virtual User User { get; set; }
public virtual ActivitySet ActivitySet { get; set; }
public virtual ICollection<Company> Companies { get; set; }
public virtual ICollection<Description> Descriptions { get; set; }
}
public class Company
{
//Primitive Properties
[HiddenInput]
public int Id { get; set; }
[Required]
public string Title { get; set; }
[Required]
public bool Archive { get; set; }
//Navigation Properties
public virtual ICollection<Activity> Activities { get; set; }
}
So there is a many-to-many relationship between Activity and Company entities. I am creating new Activity in my repository class, but when I assign a Company for the Activity like so:
activity.Companies.Add(company);
I get NullReference exception. I had a look around but according to this link:
BuildStarted.com
my approach seems to be right.
Why doesn't it work???

Answer 1

Before you can use Add() off Companies, it needs to be initialized.
activity.Companies = new List<Company>();
You could also initialize that in the contructor of Activity.
public class Activity
{
public Activity()
{
Companies = new List<Company>();
Descriptions = new List<Description>();
}
//Primitive Properties
[HiddenInput]
public int Id { get; set; }
[Required]
public int LengthInMinutes { get; set; }
public string AdditionalInfo { get; set; }
[Required]
public bool Archive { get; set; }
//Navigation Properties
public virtual User User { get; set; }
public virtual ActivitySet ActivitySet { get; set; }
public virtual ICollection<Company> Companies { get; set; }
public virtual ICollection<Description> Descriptions { get; set; }
}