Question

The .NET Framework contains since version 3.0 the ObservableCollection<T>, but why isn´t there a ObservableKeyedCollection<TKey, TValue>.

Okay i could implement my own collection by deriving from KeyedCollection<TKey,TValue> and implementing the INotifyCollectionChanged interface, but whouldn´t it be a good addition to the .NET Framework.

Answer 1

The reason that there is no ObservableKeyedCollection (or any other such type which is merely a combination of other generic types) is because ObservableCollection is generic, and that makes implementation of an “ObservableKeyedCollection” as easy as this:

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Collections.Specialized;

public class DictionaryWatcher : ObservableCollection<KeyValuePair<string, object>>, IDisposable
{
    private NotifyCollectionChangedEventHandler watcher;
    private bool watching = false;

    public DictionaryWatcher()
    {
        watcher = new NotifyCollectionChangedEventHandler( ReportChange );
        CollectionChanged += watcher;
        Watched = true;
    }

    public bool Watched
    {
        get
        {
            return watching;
        }

        set
        {
            if (watching)
            {
                lock (this)
                {
                    CollectionChanged -= watcher;
                    watching = false;
                }
            }
        }
    }

public void Dispose()
{
    Dispose( true );
    GC.SuppressFinalize( this );
}

    public void Initialize()
    {
        this.Add( new KeyValuePair<string, object>( "First", 1 ) );
        this.Add( new KeyValuePair<string, object>( "Second", 2 ) );
        this.Add( new KeyValuePair<string, object>( "Turd", 3 ) );
        KeyValuePair<string, object> badValue = this[2];
        this.Remove( badValue );
    }

protected virtual void Dispose( bool disposing )
{
    if (disposing && Watched)
    {
        Watched = false;
    }
}

    private void ReportChange( object sender, NotifyCollectionChangedEventArgs e )
    {
        Console.WriteLine( "Change made: {0}", e.Action );
    }
}

While that is certainly not a one-liner program, most of it is boilerplate. Most importantly, it doesn’t re-implement the ObservableCollection as you were suggesting; instead it fully utilizes it.

The reason that it “whouldn’t be a good addition to the .NET Framework” is because when there’s already one way to do something, creating another way to do it is a bad idea. The fewer ways there are to get some particular task done, the fewer ways there are to do it poorly. 8 )

The tools are provided, it’s now all about how you use them.

Hope that helps!