ASP.NET MVC3 - “对象引用未设置为对象的实例”错误

Question

I'm relatively new to .NET and MVC3. I'm having some trouble with the above error message when trying to add an instance of an object. Below is my code:

KeyDate class

public class KeyDate
{
    [Key]
    public int KeyID { get; set; }

    [StringLength(1000), Required]
    public string Title { get; set; }

    [Required]
    public string Description { get; set; }

    [Required]
    [Display(Name = "Event Date")]
    public DateTime EventDate { get; set; }

    [Display(Name = "Event End Date")]
    public DateTime? EventEndDate { get; set; }

    [Display(Name = "Course ID")]
    public int? CourseID { get; set; }

    [Display(Name = "Event ID")]
    public int? EventID { get; set; }

    public DateTime Created { get; set; }

    [Display(Name = "Created By")]
    public string CreatedBy { get; set; }

    public DateTime? Deleted { get; set; }

    [Display(Name = "Deleted By")]
    public string DeletedBy { get; set; }

    public virtual ICollection<Association> Associations { get; set; }
}

Association class

public class Association
{
    [Key, ForeignKey("KeyDate")]
    [Column(Order = 1)]
    public int KeyID { get; set; }

    [Key, ForeignKey("Category")]
    [Column(Order = 2)]
    public int CategoryID { get; set; }

    public virtual KeyDate KeyDate { get; set; }
    public virtual Category Category { get; set; }
}

I am getting the error in my Controller posted below

    [HttpPost]
    public ActionResult Create(KeyDate keyDate, int topic)
    {
        keyDate.Created = DateTime.Now;
        keyDate.CreatedBy = User.Identity.Name;

        // Create topic association
        Association keyDateTopic = new Association();
        keyDateTopic.CategoryID = topic;
        keyDate.Associations.Add(keyDateTopic); // <-- Object reference... error here

        repository.Add(keyDate);
        repository.Save();

        return RedirectToAction("Index");
    }

I have Googled this for ages, and have been following ScottGu's "Code First with Existing Database" example in which the code is quite similar, but I have had no luck. Any help appreciated. Thanks.

Answer 1

Associations is null. It has not been initialised.

Add a constructor for KeyDate and init it in there:

public KeyDate()
{
     Associations = new List<Association>();
}

Answer 2

Your Assosiactions List needs to be instantiated before adding items to it.

Best practice is to do this from within the constructor.

public class KeyDate
{
...
    public virtual ICollection<Association> Associations { get; set; }
    public KeyDate()
    {
       Associations = new List<Association>()
    }
}

Answer 3

Looks like keyDate.Associations is null.

Change

keyDate.Associations.Add(keyDateTopic);

to

if ( keyDate.Associations == null)
{
   keyDate.Associations = new Collection<Association>();
}

keyDate.Associations.Add(keyDateTopic);

Answer 4

Description

It looks like keyDate.Associations is null

You have several ways to solve this

Sample

• Create the ICollection in your ActionMethod.

   [HttpPost] public ActionResult Create(KeyDate keyDate, int topic) { keyDate.Created = DateTime.Now; keyDate.CreatedBy = User.Identity.Name; // Create topic association Association keyDateTopic = new Association(); keyDateTopic.CategoryID = topic; // create this list keyDate.Associations = new List<Association>(); keyDate.Associations.Add(keyDateTopic); // <-- Object reference... error here repository.Add(keyDate); repository.Save(); return RedirectToAction("Index"); } 

• OR Add a constructor to your KeyDate class like this

   public class KeyDate { public KeyDate() { // create this list this.Associations = new List<Association>(); } [Key] public int KeyID { get; set; } [StringLength(1000), Required] public string Title { get; set; } [Required] public string Description { get; set; } [Required] [Display(Name = "Event Date")] public DateTime EventDate { get; set; } [Display(Name = "Event End Date")] public DateTime? EventEndDate { get; set; } [Display(Name = "Course ID")] public int? CourseID { get; set; } [Display(Name = "Event ID")] public int? EventID { get; set; } public DateTime Created { get; set; } [Display(Name = "Created By")] public string CreatedBy { get; set; } public DateTime? Deleted { get; set; } [Display(Name = "Deleted By")] public string DeletedBy { get; set; } public virtual ICollection<Association> Associations { get; set; } }