C#

public class PopupWindowItem
{
	public Object WindowObject { get; set; } // wpf window object
	public UDPWindowType WindowType { get; set; } // wpf tabitem object
	public bool AllowMultipleInstances { get; set; }

	public PopupWindowItem(Object window, UDPWindowType windowType, bool allowMultiple)
	{
		this.WindowObject           = window;
		this.WindowType             = windowType;
		this.AllowMultipleInstances = allowMultiple;
	}
}

public class PopupWindowManager : List<PopupWindowItem>
{
	public PopupWindowManager()
	{
		this.Clear();
	}
}

WindowAccountManager accountManager = new WindowAccountManager();
accountManager.Closed += new EventHandler(accountManager_Closed);
m_popupWindowMgr.Add(new PopupWindowItem(accountManager, UDPWindowType.AccountManager, false));
accountManager.Show();

while (m_popupWindowMgr.Count > 0)
{
	switch (m_popupWindowMgr[0].WindowType)
	{
		case UDPWindowType.AccountManager :
			{
				// When you close the Window object inside the PopupWindowItem object, 
				// the PopupWindowItem object is removed from the collection, and the 
				// collection's count is reduced by 1.
				((WindowAccountManager)(m_popupWindowMgr[0].WindowObject)).Close();
			}
			break;
	}
}