IExtension : extensible object pattern : How does it work?

Question

I was exploring about Extensible object pattern using (IExtension, IExtensibleObject) interfaces in C#, I came with a very simple example to convert dates to UTC from local and vice-versa:

public class BaseObject : IExtensibleObject<BaseObject> 
{
    private DateTime _startDate;
    private ExtensionCollection<BaseObject> _extensions;

    public DateTime StartDate
    {
        get { return _startDate; }
        set { _startDate = value; }
    }

    public BaseObject()
    {
        StartDate = DateTime.Now;
        _extensions = new ExtensionCollection<BaseObject>(this); 
    }

    #region IExtensibleObject<BaseObject> Members

    public IExtensionCollection<BaseObject> Extensions
    {
        get 
        {
            return _extensions;
        }
    }

    #endregion
}

public class DateTimeConverterExtension : IExtension<BaseObject>
{
    private BaseObject _owner;

    #region IExtension<BaseObject> Members

    public void Attach(BaseObject owner)
    {
        _owner = owner;
        _owner.StartDate = owner.StartDate.ToUniversalTime();
    }

    public void Detach(BaseObject owner)
    {
        _owner.StartDate = _owner.StartDate.ToLocalTime();
    }

    #endregion
}

class Program
{
    static void Main(string[] args)
    {
        BaseObject obj = new BaseObject();
        Console.WriteLine("Local Time: "+obj.StartDate);

        obj.Extensions.Add(new DateTimeConverterExtension());


        Console.WriteLine("UTC: "+obj.StartDate);

        DateTimeConverterExtension ext = obj.Extensions.Find<DateTimeConverterExtension>();
        obj.Extensions.Remove(ext);

        Console.WriteLine("Local Time: "+obj.StartDate);
    }
}

Output:

Local Time: 4/13/2009 11:09:19 AM
UTC: 4/13/2009 5:39:19 AM
Local Time: 4/13/2009 11:09:19 AM
Press any key to continue . . .

So it works, the question is:

How does .Net framework invoke "Attach" and "Detach" when IExtensionCollection.Add and IExtensionCollection.Detach methods are invoked by the program? Fill in the internal details which I am missing here.

Answer 1

The Attach and Detach methods are called by ExtensionCollection<T>.InsertItem and ExtensionsCollection<T>.RemoveItem, respectively. These methods, in turn, are virtual methods called by SynchronizedCollection<T>.Add and SynchronizedCollection<T>.Remove, which is what your code ends up calling into. This can be verified by using Reflector. If you created your own implementation of IExtensionCollection<T>, there's no guarantee that Attach and Detach would be called correctly.