ASP.NET MVC3 - “Object reference not set to an instance of an object” error

问题

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.

回答1:

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>()
    }
}

回答2:

Associations is null. It has not been initialised.

Add a constructor for KeyDate and init it in there:

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

回答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);

回答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; }
  }
  

来源：https://stackoverflow.com/questions/8740877/asp-net-mvc3-object-reference-not-set-to-an-instance-of-an-object-error