C# Set property to an object in Dictionary list - Why NullReferenceException?

Question

This is my first question.

I have this code in my Forms App. I don't understand why i always get an NullReferenceException.

public partial class Form1 : Form
{
   Dictionary<string, Product> ProductList = new Dictionary<string, Product>();

   public Form1()
   {
      InitializeComponent();

      Product product = new Product();
      Position position = new Position();
      product.Name = "ACAD";
      position.Name = "Industry";

      ProductList.Add(product.Name, product);

      // NullReferenceException:
      ProductList["ACAD"].PositionList.Add(position.Name, position);

      // Following line works:
      //listBox1.Items.Add(ProductList["ACAD"]);
   }
}

class Product
{
   public string Name { get; set; }
   public Dictionary<string, Position> PositionList { get; set; }
}

class Position
{
   public string Name { get; set; }
}

Thank you =)

Someone has to initialize `Product.PositionList` (`= new Dictionary...`) — Ivan Stoev, Aug 19 '16 at 16:35
PositionList is null, so the NullReferenceException `Product product = new Product(); product.Name = "ACAD"; product.PositionList=new Dictionary();` — warrior, Aug 19 '16 at 16:36
change `class Product` so that `PositionList` is initialized. You could write `public var PositionList = new Dictionary()` — Charles Bretana, Aug 19 '16 at 16:37
If you [Use the debugger](http://www.codeproject.com/Articles/79508/Mastering-Debugging-in-Visual-Studio-A-Beginn), it is easy to tell what is null — Sam I am says Reinstate Monica, Aug 19 '16 at 16:41
Possible duplicate of [What is a NullReferenceException, and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Sam I am says Reinstate Monica, Aug 19 '16 at 16:42

score 1 · Accepted Answer · answered Aug 19 '16 at 16:43

Add a constructor for your product class:

class Product
{
   public string Name { get; set; }
   public Dictionary<string, Position> PositionList { get; set; }

   public Product()
   {
      PositionList = new Dictionary<string, Position>();
   }
}

And you can avoid the null reference exception from trying to add an entry to your null dictionary: ProductList["ACAD"].PositionList.Add(position.Name, position);

C# Set property to an object in Dictionary list - Why NullReferenceException?

1 Answers1