CheckedItemCollection for wrapping a collection of checkable items

CheckedItemCollection.cs

using System.Collections.ObjectModel;
using System.Collections.Generic;
using System.Linq;
using System;

/// <summary>
/// A collection of checked items.
/// </summary>
/// <typeparam name="TItem">The type of the item.</typeparam>
public class CheckedItemCollection<TItem> : ObservableCollection<CheckedItemWrapper<TItem>>
{
    public CheckedItemCollection()
        : base()
    {

    }

    public CheckedItemCollection(IEnumerable<TItem> items)
        : base(items.Select(x => new CheckedItemWrapper<TItem>(x)))
    {

    }

    public List<TItem> CheckedItems
    {
        get
        {
            return this.Items == null ? new List<TItem>() 
                                            : this.Items.Where(x => x.Checked)
                                                        .Select(y => y.Item)
                                                        .ToList();
        }
    }

    public void Add(TItem item)
    {
        this.Add(new CheckedItemWrapper<TItem>(item));
    }

    public void CheckAll()
    {
        SetChecked(true);
    }

    public void UncheckAll()
    {
        SetChecked(false);
    }

    public void CheckWhere(Predicate<TItem> filterAction, bool unCheckAllFirst = false)
    {
        if (unCheckAllFirst)
            UncheckAll();

        SetChecked(true, this.Items.Where(x => filterAction(x.Item)));
    }

    public void UnCheckWhere(Predicate<TItem> filterAction, bool unCheckAllFirst = false)
    {
        if (unCheckAllFirst)
            UncheckAll();

        SetChecked(false, this.Items.Where(x => filterAction(x.Item)));
    }

    private void SetChecked(bool isChecked, IEnumerable<CheckedItemWrapper<TItem>> items = null)
    {
        if (items == null)
            items = this.Items;

        foreach (var item in items)
        {
            if (item.Checked != isChecked)
                item.Checked = isChecked;
        }
    }
}

public class CheckedItemWrapper<TItem> : MVVM4u2.Common.ViewModelBase
{
    private bool chk;
    public bool Checked
    {
        get { return chk; }
        set
        {
            if (chk != value)
            {
                chk = value;
                OnPropertyChanged("Checked");
            }
        }
    }

    private TItem item;
    public TItem Item
    {
        get { return item; }
        set
        {
            item = value;
            OnPropertyChanged("Item");
        }
    }

    public CheckedItemWrapper()
        : this(default(TItem))
    {

    }

    public CheckedItemWrapper(TItem item)
    {
        this.Item = item;
    }
}