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; }
}