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Singleton (Update)


I recently came across a new way of implementing the Singleton pattern, detailed in this article. The article details, (1) the use of the volatile keyword, and (2) a new simpler way of implementing a Singleton by using the Lazy<T> class, something that was not available when I first wrote the section on the Singleton.

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Multiton

A fairly simple example of the multiton pattern, with a private constructor, and a tracker for created objects, the name being an integer identifier.

Salient Chacteristic(s)
A private keyed list for tracking objectsA private constructorNamed objectsCode

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

namespace DesignPatterns
{
    public class Multiton
    {
        //read-only dictionary to track multitons
        private static IDictionary<int, Multiton> _Tracker = new Dictionary<int, Multiton> { };

        private Multiton()
        {
        }

        public static Multiton GetInstance(int key)
        {
            //value to return
            Multiton item = null;

            //lock collection to prevent changes during operation
            lock (_Tracker)
            { 
                //if value not found, create and add
                if(!_Tracker.TryGetValue(key, out item))
                {
                    item = new Multiton();

                    //calculate next key
       …

Decorator Pattern

This is an example of the Decorator pattern, in this case a decorator for ObservableCollection.  Working with WPF and ObservableCollection using threads, one will run into the problem whereby the ObservableCollection cannot be updated from outside the owning thread; Delegates and Invoke will not work. A solution is to contain and expand the class, as is done in this example on michIG's Blog.

This linked file contains the original code in C#, as well as the same code converted to VB.NET.

Salient Characteristic(s)
Sets an internal pointer to the decorated object, sending method calls and property actions to the internal objectExtends the object by wrapping it and adding some aspect handled by the decorator
Code

using System;
using System.Collections.ObjectModel;
using System.Windows.Threading;
using System.Collections.Specialized;
using System.ComponentModel;

namespace DesignPatterns
{
    /// <summary>  
    /// This class is an observablecollection which invokes automatically.  
    /// T…