Enable null values when assigning one collection to other in asp.net mvc

Question

I wrote to following controller to assign one collection to another

    public ActionResult Discussion_List() {

        var discussions = (from D in db.AB_Discussion
                           select new
                           {

                           Discussion_ID = D.Discussion_ID,
                           Discussion_Name = D.Discussion_Name,
                           CreatedBy = D.CreatedBy,
                           CreatedDate = D.CreatedDate,
                           UpdatedBy  =  D.UpdatedBy,
                           UpdatedDate =  D.UpdatedDate

                           }).ToList();

        var models = discussions.Select(b => new Discussion_Dashboard()

        {
            Discussion_ID = b.Discussion_ID,
            Discussion_Name = b.Discussion_Name,
            CreatedBy = db.AspNetUsers.Find(b.CreatedBy).UserName,
            CreatedDate = b.CreatedDate,
            UpdatedBy = db.AspNetUsers.Find(b.UpdatedBy).UserName,
            UpdatedDate = b.UpdatedDate

        });

        return View(models);
    }

but when I have null values in any one of above field in database table AB_Discussion I'm getting following error

Object reference not set to an instance of an object.

How to avoid this error

this is my related model classes

   public partial class Discussion_Dashboard
    {
        public int Discussion_ID { get; set; }
        public string Discussion_Name { get; set; }
        public string Discription { get; set; }
        public string CreatedBy { get; set; }
        public Nullable<System.DateTime> CreatedDate { get; set; }
        public string UpdatedBy { get; set; }
        public Nullable<System.DateTime> UpdatedDate { get; set; }
        public int Views { get; set; }

        public int replys { get; set; }

    }

   public partial class AB_Discussion
    {
        [Key]
        [DatabaseGenerated(System.ComponentModel.DataAnnotations.Schema.DatabaseGeneratedOption.Identity)]
        public int Discussion_ID { get; set; }
        public string Discussion_Name { get; set; }
        public string Discription { get; set; }
        public string CreatedBy { get; set; }
        public Nullable<System.DateTime> CreatedDate { get; set; }
        public string UpdatedBy { get; set; }
        public Nullable<System.DateTime> UpdatedDate { get; set; }
    }

Why are you creating the first collection of anonymous objects? - it can just be var discussions = (from D in db.AB_Discussion select D).ToList(); — user3559349
– user3559349, Commented Sep 1, 2015 at 9:34
Your just creating another collection which is identical to your existing collection (its pointless) — user3559349
– user3559349, Commented Sep 1, 2015 at 9:37
UpdatedBy = b.UpdatedBy == null ? null : db.AspNetUsers.Find(b.UpdatedBy).UserName, — user3559349
– user3559349, Commented Sep 1, 2015 at 10:37

Patrik · Accepted Answer · 2015-09-01 09:41:34Z

0

I would advice to use a mapper, for example the AutoMapper that you can find on GitHub or install via NuGet. The AutoMapper helps you map one class to another in a relatively easy way with reasonable defaults for fields with the same name and so on.

If you want to allow null values you can just define that using the ? operator right after the type in your variable declaration, for example:

public int? views { get; set; }

This will give your variable a public readonly property HasValue that you can use to see whether your variable contains a value or not.

answered Sep 1, 2015 at 9:41

Patrik

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2 Comments

Chathz Over a year ago

what the syntax for strings ?

Patrik Over a year ago

The syntax is the same, just replace 'int?' with 'string?'

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