Appropriate design/design pattern for this problem?

I previously posted this, but I guess it was too verbose and irrelevant. My question is also like this. One poster in the second link said the answer (of why you can't do the code below) was a problem of design, specifically "bad use of inheritance". So I'd like to check this issue again with the experts at StackOverflow and see if this is really an issue of "bad inheritance" - but more importantly, how to fix the design.

Like the poster, I'm also confused about the Factory method and how I can apply it. It seems the factory method is for multiple concrete classes that have the exact same implementation as the abstract base class and do not add their own properties. But, as you will see below, my concrete classes build upon the abstract base class and add extra properties.

The Base Class We Build Upon:

public abstract class FlatScreenTV
{
     public string Size { get; set; }
     public string ScreenType { get; set; }
}

Extension Class Examples:

public class PhillipsFlatScreenTV : FlatScreenTV
{
     // Specific to Phillips TVs. Controls the backlight intensity of the LCD screen.
     public double BackLightIntensity { get; set; }
}

public class SamsungFlatScreenTV : FlatScreenTV
{
     // Specific to Samsung TVs. Controls the time until the TV automatically turns off.
     public int AutoShutdownTime { get; set; }
}

Let's say there are more extension classes for more brands of flat screen TVs. And then, let's say we stick them all into a generic List:

public static void Main()
{
     List<FlatScreenTV> tvList = new List<FlatScreenTV>();

     tvList.Add(new PhillipsFlatScreenTV());
     tvList.Add(new SamsungFlatScreenTV());
     tvList.Add(new SharpFlatScreenTV());
     tvList.Add(new VizioFlatScreenTV());

     FlatScreenTV tv = tvList[9]; // Randomly get one TV out of our huge list
}

The Problem:

I want to access the specific properties of whatever 'original' brand TV this variable belongs to. I know the brand because if I call tv.GetType(), it returns the correct '开发者_开发知识库original' type - not FlatScreenTV. But I need to be able to cast tv from FlatScreenTV back to its original type to be able to access the specific properties of each brand of flat-screen TVs.

Question #1: How can I dynamically cast that, properly - without makeshift hacks and huge if-else chains to brute-guess the 'original' type?

After browsing around similar design issues, most answers are: you can't. Some people say to look at the Factory Pattern, and others say to revise the design using interfaces, but I don't know how to use either to solve this problem.

Question #2: So, how should I design these classes so that I can access the original type's specific properties in the context above?

Question #3: Is this really bad inheritance?

Your design violates the "Liskov Substitution Principle". In other words, the code that deals with items from your list of FlatScreenTV shouldn't know or care what derived type is.

Say your code needs to create a custom remote control GUI. It might be enough to simply know the names and types of the properties of each TV to auto-generate the UI. In which case you could do something like this to expose the custom properties from the base class:

public abstract class FlatScreenTV
{
    public FlatScreenTV()
    {
        CustomProperties = new Dictionary<string,object>();
    }

    public Dictionary<string,object> CustomProperties { get; private set; }
    public string Size { get; set; }
    public string ScreenType { get; set; }
}

public class PhillipsFlatScreenTV : FlatScreenTV
{
    public PhillipsFlatScreenTV()
    {
        BackLightIntensity = 0;
    }

    // Specific to Phillips TVs. Controls the backlight intensity of the LCD screen.
    public double BackLightIntensity 
    { 
        get { return (double)CustomProperties["BackLightIntensity"]; }
        set { CustomProperties["BackLightIntensity"] = value; }
    }
}

public class SamsungFlatScreenTV : FlatScreenTV
{
    public SamsungFlatScreenTV()
    {
        AutoShutdownTime = 0;
    }

    // Specific to Samsung TVs. Controls the time until the TV automatically turns off.
    public int AutoShutdownTime 
    {
        get { return (int)CustomProperties["AutoShutdownTime"]; }
        set { CustomProperties["AutoShutdownTime"] = value; }
    }
}

If you really do need to be working directly with the derived types, then you should instead consider moving to a plugin based architecture. For example, you might have a factory method like this:

IRemoteControlGUI GetRemoteControlGUIFor(FlatScreenTV tv)

which would scan your plugins and find the one that knew how to build the UI for the particular type of FlatScreenTV you passed in. This means that for every new FlatScreenTV you add, you also need to create a plugin that knows how to make its remote control GUI.

Factory Pattern would be the best way to go

I can offer a partial answer:

Firstly read up on Liskov's Substitution Principle.

Secondly you are creating objects that inherit from FlatScreenTV, but apparently for no purpose as you want to refer to them by their SubType (SpecificTVType) and not their SuperType (FlatScreenTV) - This is bad use of Inheritance as it is NOT using inheritance lol.

If your code wants to access properties particular to a given type, then you really want this code encapsulated within that type. Otherwise everytime you add a new TV type, all the code that handles the TV list would need to be updated to reflect that.

So you should include a method on FlatScreenTV that does x, and override this in TV's as required.

So basically in your Main method above, instead of thinking I want to be dealing with TVTypeX, you should always refer to the basetype, and let inheritance and method overriding handle the specific behaviour for the subtype you are actually dealing with.

Code eg.

  public abstract class FlatScreenTV
  {
      public virtual void SetOptimumDisplay()
      {
         //do nothing - base class has no implementation here
      }
  }


  public class PhilipsWD20TV
  {
      public int BackLightIntensity {get;set;}

      public override void SetOptimumDisplay()
      {
          //Do Something that uses BackLightIntensity
      }

  }

"the factory method is for multiple concrete classes that have the exact same implementation as the abstract base class [interface] and do not add their own properties."

No, speaking more practical, than theorical, the factory method can provide you with objects of concrete classes, in which the concrete classes, must have some common methods and interfaces, but, also some additional specific attributes.

Sometimes I use a method that creates the same class object every time I called, and I need to call it several times, and sometimes I use a method that create several different class objects, and that maybe be confusing, maybe another question.

And, your further comment about a switch sentence, with many options, when using the factory pattern, you usually provide an identifier for the concrete class / concrete object. This can be a string, an integer, an special type id, or an enumerated type.

You could use an integer / enum ID instead, and use a collection to lookup for the concrete class.

You can still leverage a factory. The point of a factory IMO is to put all the heavy lifting of constructing your various TVs in one place. To say categorically "a factory is for multiple concrete classes that have the exact same implementation as the abstract base class" is forgetting about polymorphism.

There is no law that says you cannot use a factory pattern because the sub classes declare unique properties and methods. But the more you can make use of polymorphism, the more a factory pattern makes sense. Also as a general guideline, IMHO, the more complexity that must go into constructing from the base the better off you are in the long run using a factory because you are "encapsulating change" - that is, constructing concrete classes is likely to change due to differing requirements and inherent construction complexity (a design analysis decision, to be sure) . And that change is in a single class - the factory.

Try this: Define everything in the abstract class and then for a given TV subclass either write concrete-specific code, and for those that don't apply write some standard "I don't do that" code.

Think about all the things your TVs do in generic terms: turn on, turn off, etc. Write a virtual method shell in the base class for all the generic things a TV does - this is a simple example of the template method pattern by the way. Then override these in the concrete classes as appropriate.

There are other things you can do in the base class to make it more fundgeable (that's a technical term meaning "reference subclasses as the base class, but do sub-classy things").

• Define delegate methods (very powerful yet under-utilized)
• use params[] for dynamic method parameter lists
• Make Property delegates
• Static methods
• Declare Properties and methods "abstract" - forces sub-class implementation, vis-a-vis "virtual"
• Hide inherited stuff in the sub class (generally using "new" keyword to communicate that it's on purpose)
• If construction parameters are numerous or complex, create a class specifically designed to pass configuration to the factory's build method.

    public class TVFactory {

    public TV BuildTV(Brands thisKind) {
        TV newSet;

        switch (thisKind) {
            case Brands.Samsung :
                Samsung aSamsungTV = new Samsung();
                aSamsungTV.BacklightIntensity = double.MinVal;
                aSamsungTV.AutoShutdownTime = 45;    //oops! I made a magic number. My bad
                aSamsungTV.SetAutoShutDownTime = new delegate (newSet.SetASDT);
                newSet = aSamsungTV;

                break;
            . . .
        } // switch
    }

    //more build methods for setting specific parameters
    public TV BuildTV (Brands thisKind, string Size) { ... }

    // maybe you can pass in a set of properties to exactly control the construction.
    // returning a concrete class reference violates the spirit of object oriented programming 
    public Sony BuildSonyTV (...) {}

    public TV BuildTV (Brands thisKind, Dictionary buildParameters) { ... }
}

public class TV {
    public string Size { get; set; }
    public string ScreenType { get; set; }
    public double BackLightIntensity { get; set; }
    public int AutoShutdownTime { get; set; }

    //define delegates to get/set properties
    public delegate  int GetAutoShutDownTime ();
    public delegate void SetAutoShutDownTime (object obj);

    public virtual TurnOn ();
    public virtural TurnOff();

    // this method implemented by more than one concrete class, so I use that
    // as an excuse to declare it in my base.
    public virtual SomeSonyPhillipsOnlything () { throw new NotImplementedException("I don't do SonyPhillips stuff"); }

}

public class Samsung : TV {
    public Samsung() {
        // set the properties, delegates, etc. in the factory
        // that way if we ever get new properties we don't open umpteen TV concrete classes
        // to add it. We're only altering the TVFactory.
        // This demonstrates how a factory isolates code changes for object construction.
    }

    public override void TurnOn() { // do stuff }
    public override void TurnOn() { // do stuff }

    public void SamsungUniqueThing () { // do samsung unique stuff }

    internal void  SetASDT (int i) {
        AutoShutDownTime = i;
    }
}

// I like enumerations. 
//   No worries about string gotchas
//   we get intellense in Visual Studio
//   has a documentation-y quality
enum Brands {
    Sony
    ,Samsung
    ,Phillips
}