Question

Previously, I had a class that wrapped an internal System.Collections.Generic.List<Item> (where Item is a class I created). The wrapper class provided several collection-level properties that provided totals, averages, and other computations on items in the list. I was creating a BindingSource around this wrapped List<> and another BindingSource around my class and was able to get at the Items in the wrapped list through the first BindingSource and the collection-level properties of the wrapper class using the second.

A simplified example looks like:

public class OldClass() {   private List<Item> _Items;    public OldClass()   {     _Items = new List<Item>();   }    public List<Item> Items { get { return _Items; } }    // collection-level properties   public float AverageValue { get { return Average() } }   public float TotalValue { get { return Total() } }   // ... other properties like this  } 

With the binding sources created in this way:

_itemsBindingSource = new BindingSource(oldClass.Items); _summaryBindingSource = new BindingSource(oldClass); 

Recently, I tried to change this class to be derived from System.Collections.Generic.List<Item> instead of keeping a wrapped List<> member. My hope was to get rid of the extra wrapper layer and use only one BindingSource instead of two. However, now I find that I cannot get at the properties that apply to all items in the list (such as AverageValue) when I do data binding. Only the properties of list items are available.

Am I forced to go back to using a wrapped List<> of Items? Or is there a way that I can get at both the properties of Items stored my new class as well as the properties that apply to the collection itself?

Answer 1

The system treats anything that implements IList (or IListSource) as a container, rather than an item. As such, you cannot bind to properties of anything that implements IList. As such, encapsulation (i.e. what you already have) is the best approach if you want to be able to bind to properties of the container.

However, you should note that many bindings support dot-notation in the source – i.e. either binding to ‘Items.SomeProperty’, or setting the auxiliary property (typically DataMember) to specify sub-lists.

This allows you to have a single BindingSource, and have different controls bound to different levels in the hierarchy – i.e. you might have a TextBox bound to AverageValue, and a DataGridView (with the same DataSource) that has DataMember='Items'.