Null Reference when calling an internal void function inside a class from controller ASP MVC

Question

I am looking for a savior to this my problem.

I am trying to follow this tutorial about Nested Collection:

Nested collection in MVC add multiple phone number

Nevertheless, I ran into a problem with this:

internal void CreatePart(int count = 1)
    {
        for (int i = 0; i < count; i++)
        {
            Parts.Add(new Part()); // ====>>>>>> it throws null reference
        }
    }

I could not find the source of the null. Can you please spot it for me?

here is my codes:

entity for product

public class Product
{
    public int Id { get; set; }

    [StringLength(50, MinimumLength = 2)]
    public string Name { get; set; }

    public virtual ICollection<Part> Parts { get; set; }
    public virtual ICollection<Version> Versions { get; set; }

    internal void CreatePart(int count = 1)
    {
        for (int i = 0; i < count; i++)
        {
            Parts.Add(new Part());
        }
    }
}

entity for parts

public class Part
{
    public int Id { get; set; }

    [StringLength(50, MinimumLength = 2)]
    public string Name { get; set; }

    public int ProductId { get; set; }
    public virtual Product Product { get; set; }

    public string DeletePart { get; set; }
}

Create controller:

public ActionResult Create()
    {
        var product = new Product();
        product.CreatePart(2);
        return View(product);
    }

product.Parts property is null? – scheien May 28 '14 at 11:41 — scheien, May 28 '14 at 11:41

score 3 · Accepted Answer · answered May 28 '14 at 11:41

3

You haven't actually instantiated Parts, so you get an exception when trying to add elements to it.

If you look at the example you linked to, you're missing a constructor in your Product class.

Try adding this to your class:

public Product()
{
    this.Parts = new HashSet<Part>();
}

answered May 28 '14 at 11:41

Grant Winney

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Just for my learning, can anybody pls explain what does HashSet mean? – Ace Supriatna May 28 '14 at 11:56
Mr Grant, too bad your website is down. Should you in any way come to Indonesia, I will gladly assist you here. – Ace Supriatna May 28 '14 at 12:02

score 0 · Answer 2 · answered May 28 '14 at 11:45

Not entirely sure on this but!

Change this:

for (int i = 0; i < count; i++)
{
   Parts.Add(new Part());
}

To this:

for (int i = 0; i < count; i++)
{
   Part p = new Part(); 
   Parts.Add(p);
}

If the error is now moved to:

Part p = new Part(); << Here

Then I guess the problem is how it is getting initialised.

Could you try defining a constructor for the Part class and give it dummy values see what happens then.

Null Reference when calling an internal void function inside a class from controller ASP MVC

2 Answers2