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POOnly allow Factory method to instantiate objects (prevent instantiation of base class AND uninitialized objects)
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copied!<p>I have a base class for handling "jobs". A factory method creates derived "job handler" objects according to job type and ensures the job handler objects are initialized with all the job information.</p> <p><strong>Calling factory method to request a handler for Job and Person assigned:</strong></p> <pre><code>public enum Job { Clean, Cook, CookChicken }; // List of jobs. static void Main(string[] args) { HandlerBase handler; handler = HandlerBase.CreateJobHandler(Job.Cook, "Bob"); handler.DoJob(); handler = HandlerBase.CreateJobHandler(Job.Clean, "Alice"); handler.DoJob(); handler = HandlerBase.CreateJobHandler(Job.CookChicken, "Sue"); handler.DoJob(); } </code></pre> <p><strong>The Result:</strong></p> <pre><code>Bob is cooking. Alice is cleaning. Sue is cooking. Sue is cooking chicken. </code></pre> <p><strong>Job handler classes:</strong></p> <pre><code>public class CleanHandler : HandlerBase { protected CleanHandler(HandlerBase handler) : base(handler) { } public override void DoJob() { Console.WriteLine("{0} is cleaning.", Person); } } public class CookHandler : HandlerBase { protected CookHandler(HandlerBase handler) : base(handler) { } public override void DoJob() { Console.WriteLine("{0} is cooking.", Person); } } </code></pre> <p><strong>A sub-classed Job Handler:</strong></p> <pre><code>public class CookChickenHandler : CookHandler { protected CookChickenHandler(HandlerBase handler) : base(handler) { } public override void DoJob() { base.DoJob(); Console.WriteLine("{0} is cooking chicken.", Person); } } </code></pre> <p><strong>The best way of doing things? I have struggled with these issues:</strong></p> <ol> <li>Ensure that all derived objects have a fully initialized base object (Person assigned).</li> <li>Prevent instantiation of ANY objects other than through my factory method that performs all the initialization.</li> <li>Prevent instantiation of the base class object.</li> </ol> <h1>The Job handler <code>HandlerBase</code> base class:</h1> <ol> <li>A <code>Dictionary<Job,Type></code> maps Jobs to Handler classes.</li> <li>PRIVATE setter for job data (i.e., Person) prevents access except by factory method.</li> <li>NO default constructor and PRIVATE constructor prevents construction except by factory method.</li> <li>A protected "copy constructor" is the only non-private constructor. One must have an instantiated HandlerBase to create a new object, and only the base class factory can create a base HandlerBase object. The "copy constructor" throws an exception if one attempts to create new objects from a non-base object (again, preventing construction except by the factory method).</li> </ol> <p>A look at the base class:</p> <pre><code>public class HandlerBase { // Dictionary maps Job to proper HandlerBase type. private static Dictionary<Job, Type> registeredHandlers = new Dictionary<Job, Type>() { { Job.Clean, typeof(CleanHandler) }, { Job.Cook, typeof(CookHandler) }, { Job.CookChicken, typeof(CookChickenHandler) } }; // Person assigned to job. PRIVATE setter only accessible to factory method. public string Person { get; private set; } // PRIVATE constructor for data initialization only accessible to factory method. private HandlerBase(string name) { this.Person = name; } // Non-private "copy constructor" REQUIRES an initialized base object. // Only the factory method can make a HandlerBase object. protected HandlerBase(HandlerBase handler) { // Prevent creating new objects from non-base objects. if (handler.GetType() != typeof(HandlerBase)) throw new ArgumentException("THAT'S ILLEGAL, PAL!"); this.Person = handler.Person; // peform "copy" } // FACTORY METHOD. public static HandlerBase CreateJobHandler(Job job, string name) { // Look up job handler in dictionary. Type handlerType = registeredHandlers[job]; // Create "seed" base object to enable calling derived constructor. HandlerBase seed = new HandlerBase(name); object[] args = new object[] { seed }; BindingFlags flags = BindingFlags.Instance | BindingFlags.NonPublic; HandlerBase newInstance = (HandlerBase)Activator .CreateInstance(handlerType, flags, null, args, null); return newInstance; } public virtual void DoJob() { throw new NotImplementedException(); } } </code></pre> <p><strong>A look at the Factory Method:</strong></p> <p>Because I have made public construction of a new object impossible without already having an instantiated base object, the factory method first constructs a <code>HandlerBase</code> instance as a "seed" for calling the needed derived class "copy constructor".</p> <p>The factory method uses <code>Activator.CreateInstance()</code> to instantiate new objects. Activator.CreateInstance() looks for a constructor that matches the requested signature:</p> <p>The desired constructor is <code>DerivedHandler(HandlerBase handler)</code>, thus,</p> <ol> <li>My "seed" <code>HandlerBase</code> object is placed in <code>object[] args</code>.</li> <li>I combine <code>BindingFlags.Instance</code> and <code>BindingFlags.NonPublic</code> so that CreateInstance() searches for a non-public constructor (add <code>BindingFlags.Public</code> to find a public constructor).</li> <li>Activator.CreateInstance() instantiates the new object which is returned.</li> </ol> <h1>What I don't like...</h1> <p>Constructors are not enforced when implementing an interface or class. The constructor code in the derived classes is mandatory:</p> <pre><code>protected DerivedJobHandler(HandlerBase handler) : base(handler) { } </code></pre> <p>Yet, if the constructor is left out you don't get a friendly compiler error telling you the exact method signature needed: "'DerivedJobHandler' does not contain a constructor that takes 0 arguments".</p> <p>It is also possible to write a constructor that eliminates any compiler error, instead--WORSE!--resulting in a run-time error:</p> <pre><code>protected DerivedJobHandler() : base(null) { } </code></pre> <p>I do not like that there is no means of enforcing a required constructor in derived class implementations.</p>

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