How to run custom code when List.Add or List.Remove is running

Chris Richner

Hi, Before generics I used something like this to connect childs with parent classes

interface IMyItem
{
CollectionOwner Owner{get;set;}
}
class MyCollection : System.Collections.CollectionBase
{
// ctor
public MyCollection(CollectionOwner owner)
{
_owner = owner;
}
// internal interface
protected override OnAdd(object item)
{
if(item is IMyItem) ((IMyItem)item).Owner = _owner;
}
}

this way one was able to add items to a owner class owner.Items.Add(new Item()) and the owner had the item instance in his list while the item had a reference to his owner, which made a call like item.Remove() possible instead of calling owner.Items.Remove(item) Now, .net 2.0 introduced generics, way cool. But how do I implement this behaviour with generics? There is no overrideable method instead of using the brute force method with new

class MyCollection<T> : List<T>
where T : IMyItem

{
// ctor
MyCollection(CollectionOwner owner)
{
_owner = owner;
}
new public void Add(T item)
{
item.Owner = _owner;
base.Add(item);
}
}

this way if one is casting MyCollection back to List<> my custom add method isn't run. bad design how can I extend generic collection base classes with such logic to do some extra work on each add or remove?

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Judah Gabriel Himango

Rather from inheriting from List<T>, have your class implement IList<T>, and keep a List<T> field in your class. Something like this:

public class MyCollection<T> : IList<T>
{
readonly List<T> list = new List<T>();

public void Add(T item)
{
    item.Owner = \_owner;
    list.Add(item);
}

....

}

Of course, for you to set the Owner property of the item, there must be a constraint on your T class type. You could accomplish this like this:

interface IOwnable
{
CollectionOwner Owner { get; set; }
}

The tell your MyCollection object that only IOwnable objects can be stored in it.

class MyCollection<T> : IList<T>
where T : IOwnable
{
...
}

*edit* alternately, as Daniel said, inheriting from the System.Collections.ObjectModel.Collections would require less work. You'll still need the IOwnable constraint if you want to set the owner on the items.

Tech, life, family, faith: Give me a visit. I'm currently blogging about: God-as-Judge, God-as-Forgiver The apostle Paul, modernly speaking: Epistles of Paul Judah Himango

Daniel Grunwald

Instead of inheriting from List, inherit from System.Collections.ObjectModel.Collection. There you can override ClearItems, RemoveItem and InsertItem, which will be called by all normal Add/Clear/Insert/Remove/RemoveAt methods.

Chris Richner

Thanks guys, This way round things look like they did in System.Collections.xx namespace. Seems like I just got the wrong base class ;(

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