Accessing a struct collection property from within another collection

Question

I have a struct that I need to store in a collection. The struct has a property that returns a Dictionary.

public struct Item
{
    private IDictionary<string, string> values;
    public IDictionary<string, string> Values
    {
        get
        {
            return this.values ?? (this.values = new Dictionary<string, string>());
        }
    }
}

public class ItemCollection : Collection<Item> {}

When testing I've found that if I add the item to the collection and then try to access the dictionary the structs values property is never updated.

 var collection = new ItemCollection { new Item() }; // pre-loaded with an item
 collection[0].Values.Add("myKey", "myValue");
 Trace.WriteLine(collection[0].Values["myKey"]); // KeyNotFoundException here

However if I load up the item first and then add it to a collection the values field is maintained.

 var collection = new ItemCollection();
 var item = new Item();
 item.Values.Add("myKey", "myValue");
 collection.Add(item);
 Trace.WriteLine(collection[0].Values["myKey"]); // ok

I've already decided that a struct is the wrong option for this type, and when using a class the issue doesn't occur, but I'm curious what's different between the two methods. Can anybody explain what's happening?

msdn doc : Here is a msdn example http://msdn.microsoft.com/en-us/library/vstudio/8b0bdca4.aspx — Cybermaxs, Sep 19 '12 at 09:46

score 2 · Answer 1 · answered Sep 19 '12 at 09:31

2

Change your struct to be a class instead:

public class Item {}

Now I'm not that familiar with the innards, but a struct is a value type and a class is reference type, and I believe that there is logic in there which explains why you get an exception when running the first code example.

I'm hoping someone can step in an give you a more thorough answer, but for now, just change the struct to a class.

answered Sep 19 '12 at 09:31

Jason Evans

28,906
14
90
154

As you can see here (http://stackoverflow.com/questions/521298/when-to-use-struct-in-c), I think having a struct for this situration is not ideal. – Cybermaxs Sep 19 '12 at 09:46

score 1 · Answer 2 · answered Sep 19 '12 at 09:36

"Item" is a value item. that means, if you access the collection, a copy is made. all operations are done on that copy

 collection[0].Values.Add("myKey", "myValue");

here of the first item a copy is made, then the Value get accessor creates a new instance, which is stored in the copy, and then the item is added. then the copy is destroyed.

a workaround would be creating the Values dictionary direct on the creating of the struct

public struct Item
{
    private IDictionary<string, string> values = new Dictionary<string, string>();
    public IDictionary<string, string> Values
    {
        get
        {
            return this.values;
        }
    }
}

if you access the collection, a copy is made. but this copy contains a reference to the same values dictionary like the original one. so the original dictionary is modified

Thanks. You're correct about the copy it seems but the compiler won't allow the assignment to the values field - "cannot have instance field initializers in structs" — paddyb, Sep 19 '12 at 10:03

score 1 · Accepted Answer · answered Sep 19 '12 at 09:40

1

As it was mentioned before, change struct Item to class Item. When you use struct, an expression collection[0] returns not a reference to object created in new ItemCollection { new Item() } but rather creates a copy of it and gives it to you.

answered Sep 19 '12 at 09:40

Grzegorz Pływacz

166
1
4

Accessing a struct collection property from within another collection

3 Answers3