We Keep Coding

Should I use a private field of a public property inside a class where I declare that property?

Let say I have a class like this:
public class A
{
private BaseSettings fieldA;
public ISettings PropertyA
{
get {return fieldA;}
set {fieldA= value as BaseSettings;}
}
}
where BaseSettings implements ISettings. Inside class A, if I want to access BaseSettings' property called PropertyB, which of this is a good practice:
fieldA.PropertyB;
or
((BaseSettings)PropertyA).PropertyB;
One may say the first approach may hide the hint to when a property changed. For example, a code may listen to PropertyChangedEvent and then the value for property changed without raising the event.
Another one may say the second approach may expose a risk that when a person who is not familiar with current code modify it, he may cast the PropertyA to different type that implements ISettings.
Both approaches have its downside. In a good programming practice, which one should be more preferable?
EDIT:
Added based on the comments belows:
I agree that setting the backing-field as ISettings makes absolute sense. But what should I do to make sure that the PropertyA is always type of BaseSettings. That will raise a question: "Then why don't you set both property and backing-field to BaseSettings?".
The reason behind why property and its backing field are different is that class A also implement an interface like this
public interface IControlWithSettings
{
ISettings OwnerSettings
{
get;
set;
}
ISettings Settings
{
get;
set;
}
}
So the actual classA would look like this
public class BaseForm: Form, IControlWithSettings
{
private BaseFormSettings settings;
public ISettings Settings
{
get {return settings;}
set {settings= value as BaseFormSettings;}
}
}
And I have another class B would also implement IControlWithSettings
public class BaseUserControl: UserControl, IControlWithSettings
{
private BaseUserControlSettings settings;
public ISettings Settings
{
get {return settings;}
set {settings= value as BaseUserControlSettings ;}
}
}
Both BaseFormSettings : ISettings and BaseUserControlSettings : ISettings . This is actual ISettings interface
public interface ISettings
{
Dictionary<string, ISettings> Children { get; set; }
}
the 'as' casting is a side effect I put into the setter so that it will ignore and return null if the setting is set to wrong one. I read somewhere saying I shouldn't throw exception in a setter. So making it null is my way to inform there is something wrong has been done.
So what is the better approach. Did I design it wrong?

As you stated, both approaches have their downsides, and it also depends on whether the property setter may contain some additional logic (e.g. validation), and you may use or circumvent this additional logic from inside the class.
If there's nothing that speaks against it, I'll use direct access to the field. It's neater and it avoids all this typecasting stuff.
But generally: Why would you back your property with the derived type, while the property itself has the interface type? This doesn't make much sense. Why not just:
public class A
{
public ISettings PropertyA { get; set; }
}
This would be much cleaner, and your question wouldn't even arise.
Edit (based on the answer's edit)
In case of the 'double use' of the backing field the typecasting makes sense. But I don't think (and never heard that before) that it's a bad thing to throw an exception from a property setter. On the contrary: Validating a value and throwing an exception if it doesn't pass is a very common pattern.
So, in your concrete case, I would validate the value for the correct type, throw if it's not correct, and use the backing field internally to bypass this type check.

Creating ViewModels WIth Common Properties with OnPropertyChanged?

I relised I have lots of models view models with those two properties
public Visibility OkButtonVisibility
{
get{ return _OkButtonVisibility;}
set{
_OkButtonVisibility = value;
RaisePropertyChanged("OkButtonVisibility");
}
}
public Visibility CancelButtonVisibility
{
get{ return _CancelButtonVisibility;}
set{
_CancelButtonVisibility = value;
RaisePropertyChanged("CancelButtonVisibility");
}
}
I wanted to create attachable interface for them like this:
Interface IOKandCancelButtonsVM
{
public Visibility OkButtonVisibility
{
get{ return _OkButtonVisibility;}
set{
_OkButtonVisibility = value;
RaisePropertyChanged("OkButtonVisibility");
}
}
public Visibility CancelButtonVisibility
{
get{ return _CancelButtonVisibility;}
set{
_CancelButtonVisibility = value;
RaisePropertyChanged("CancelButtonVisibility");
}
}
and have my viewmodels that use this to inherite them and another interfaces with proxy properties like this
class VM1:BaseVM,IOKandCancelButtonsVM,IOtherCommonPropertyVM
{
}
but then I relaised my new interfaces don't impliment INotifyChanged.
would it be a bad idea to haveIOKandCancelButtonsVM impliment INotifyChanged
and have VM1 explicitly impliment BaseVM?
I never dealt with class inheriting same interface twice and not sure what to do.
this is only an example less than half of the VMs use those I have a few hundred properties, many shared over dozens of screens. i need a more elegant solution than sticking all on thos into the BaseViewModel

You cannot have an implementation in your interface definition. This means that your IOKandCancelButtonsVM definition is incorrect.
would it be a bad idea to have IOKandCancelButtonsVM impliment BaseVM
Yes it would be a very bad idea - impossible in fact - interfaces cannot implement or extend classes.
Since you asked for opinion on how to structure your code, I will also throw this out there: remove any reference to 'visibility' from your viewmodel. Do not return a System.Visibility value from your viewmodel.
A viewmodel shouldn't know anything about the view. The semantically correct way to do this is to simply return a flag from the viewmodel, and use a converter to change it to a Visibility value in the binding - there is even a ready made converter in the framework for doing this: BooleanToVisibilityConverter. Try to avoid any mention of UI related terms in your VM even if it is returning a bool - it is a good practice which leads to tighter and more disciplined code, using those names willy nilly will eventually lead to smelly code.
I have a blog post which illustrates a nice approach to property change notifications: Streamlining property notifications in MVVM.
Just to make it clear, I am advising that you get rid of any ideas you have about IOKandCancelButtonsVM interfaces, by all means include simple boolean properties on a base viewmodel that will be extended by many other viewmodels that will be bound to dialogs with OK/Cancel buttons.
To diagram this in text it would look like this:
YourBaseVm <-- BaseVmUsedByDialogs <-- SpecificDialogVm
(note the italicising which indicates the class is abstract)

It seems you already have a base viewmodel class. Just add the two properties there. You might want to create a second base inheriting from the first, which you use when those two properties are required, so in case their not, there is no overhead.

Why not to go for virtual properties in your base class BaseVM with default functionality implemented in it. In case other derived classes want to extend it they can always override it as per funcitonality required.
Make sure your BaseVM is implementing INotifyPropertyChanged.

I'd just create an abstract base class, and have the ViewModels that need those properties inherit from it. Keep it simple.
abstract class BaseDialogVM : BaseVM
{
private Visibility _OkButtonVisibility;
public Visibility OkButtonVisibility {
get { return _OkButtonVisibility; }
set {
_OkButtonVisibility = value;
RaisePropertyChanged("OkButtonVisibility");
}
}
private Visibility _CancelButtonVisibility;
public Visibility CancelButtonVisibility {
get { return _CancelButtonVisibility; }
set {
_CancelButtonVisibility = value;
RaisePropertyChanged("CancelButtonVisibility");
}
}
}
class VM1 : BaseDialogVM { /*...*/ }

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 '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
}

Class extending - best practice/best solution

First thing to note - I KNOW DELEGATION AND DECORATOR PATTERNS!
Second - I am using C# .NET 4.0, so if you come up with a solution that is specific for it, that's fine. But if solution will work for any OOP language and platform, that would be great.
And here the question goes...
I have a partial class (lets name it Class1), which I cannot modify. Thus, I can just extend it or/and inherit from it. This class provides a perfect data model for me, the only thing I need is to add some attributes to its properties (for validation, defining label text value in MVC etc - for now, I do not need answers like 'you can do what you need without attributes', that's not the matter of my question).
It is not a problem to use another class as a data model, so I can, say, create Class2 : Class1 and use Class2 as a model. Properties that need attributes would be defined as public new <type> <propertyname>. This will limit me to rewriting only the properties that need attributes, leaving all other untouched.
The smaller problem is that I do not what to redefine getters and setters for the properties, as all they gonna contain is return base.<propertyname> and base.<propertyname> = value, and if there are lots of such properties, this means lots of "stupid" coding. Is there a way to avoid this?
The bigger problem is that I have to parametrize my Class2 with Class1 instance and make something like class2.<propertyname> = class1.<propertyname> for each single property I have - too much of "stupid" coding. I can avoid it using reflection - find all properties with public getters and setters in Class1 and call prop.SetValue(child, prop.GetValue(parent, null), null); in the loop. This provides a generic function for simple cases, which is quite fine, as I mostly have simple models - lots of properties with public getters and setters without body and another logic. But I want more generic solution, and I do not like reflection. Any ideas?
Here goes the full code of the extension method that creates Class2 basing on Class1
public static Child ToExtendedChild<Parent, Child>(this Parent parent)
where Child : Parent, new()
{
Child child = new Child();
var props = typeof(Parent).GetProperties().Where(p => p.GetAccessors().Count() >= 2);
foreach (var prop in props)
{
prop.SetValue(child, prop.GetValue(parent, null), null);
}
return child;
}
(by the way, this method may not ideally implement my solution, so any corrections would also be appreciated)
Thanks in advance!

The smaller problem doesn't seem to be much of a problem. Maybe I'm misunderstanding the question, but assuming you're simply deriving a subclass, there should be no reason to redefine either the properties or their associated getters/setters.
The bigger problem might be resolved using something a little simpler. Using reflection for a lot of your object initialization seems a little expensive. If you're dealing with a class that is primarily a big bag or properties, maybe you should as if you need access to all of those properties in any given situation. You mention MVC and validation, is the entire model being used in the controller method you're validation is taking place in? If not, why not look at using a viewmodel that only exposes those pieces you need in that method?
Your reflection initializer is interesting, but if you're going to be doing a lot of this then you might consider investing a little time with Automapper. Otherwise maybe consider moving away from a generic solution to something that just tackles the problem at hand, i.e. mapping properties from an instance of an object to another instance of a derived object. Maybe you can create a copy constructor in the parent class and use that in your derived class?
public class Foo {
public string PropOne { get; set; }
public string PropTwo { get; set; }
public Foo(string propOne, string propTwo) {
PropOne = propOne;
PropTwo = propTwo;
}
public Foo(Foo foo) {
PropOne = foo.PropOne;
PropTwo = foo.PropTwo;
}
}
public class Pho : Foo {
// if you have additional properties then handle them here
// and let the base class take care of the rest.
public string PropThree { get; set; }
public Pho(string propOne, string propTwo, string propThree)
: base(propOne, propTwo) {
PropThree = propThree;
}
public Pho(Pho pho) : base(pho) {
PropThree = pho.PropThree;
}
// otherwise you can just rely on a copy constructor
// to handle the initialization.
public Pho(Foo foo) : base(foo) {}
}

I assume the partial class is generated code, it makes the most sense given your scenario.
I know of one way to do this, but depending on how the attribute gets crawled, it may not work.
// Generated Code
public partial Class1
{
public string Foo { get { ... } }
}
// Your Code
public interface IClass1
{
[MyAttribute]
public string Foo { get; }
}
public partial Class1 : IClass1
{
}
If someone were to look at attributes by using GetCustomAttributes with inheritance, then I think they would get this attribute.
As an aside, whenever I see generated code that doesn't have virtual properties it makes me cry a little bit inside.
To address your bigger question, why don't you just make Class2 a wrapper for Class1. Instead of copying all of the properties you can just give Class2 an instance of Class1 in the constructor, store it locally and make all of your properties pass-throughs. It means some hand coding, but if you're building a Class2 by hand anyway and want to decorate it with a bunch of attributes, well, you're hand coding Class2 anyway.

Tracking instances with generics and supporting subclasses

I've defined the following generic class
public class ManagedClass<T> where T : ManagedClass<T>
{
static ManagedClass()
{
Manager = new ObjectManager<T>();
}
public static ObjectManager<T> Manager { get; protected set; }
public ManagedClass()
{
Manager.Add( (T)this );
}
}
The idea is that I can use it like so:
class Product : ManagedClass<Product> {}
Now I can do something to the 7th product created like so:
Product.Manager.GetById(7).DoSomething();
The problem comes in if i try to use a derived class:
class ExtendedProduct : Product {}
now ExtendedProduct.Manager has a list of 'Products', and if i want to use a new function that I have added to ExtendedProduct (DoSomethingElse), I have to cast the object I get back like so:
((ExtendedProduct)ExtendedProduct.Manager.GetById(7)).DoSomethingElse();
This is a bit ugly, and the whole point of using generics for this is to avoid casting. I suppose I could add a static constructor to the derived class to set Manager = new ObjectManager() and add a new Manager.addObject( this ) in the derived class constructor, but It seems like there should be some better way of doing this using generics. Any suggestions?

The problem is that ExtendedProduct.Manager is the same thing as Product.Manager; the manager object can't act differently depending on where it's accessed from.
A couple of possibilities I can think of:
Hide the typecast inside the GetById method by making it generic:
Product.Manager.GetById<ExtendedProduct>(7).DoSomethingElse();
Use one ObjectManager instance per subclass, connecting them privately if needed
Option 1 reminds me of NHibernate's ICriteria interface. It's effectively the same as a typecast, but a little harder to accidentally break.

Really what you're running into is a weakness with Generics. Once your class has resolved what type it's using for generics, you're somewhat restricted in what you can do.
Normally, I'd say Dependency Injection would be a savior here, but since the problematic method is static, that muddies up the waters.
I'd say the best thing is to have the ObjectManager class do the work for you:
static public class ObjectManager<T>
{
... the code that already exists in ObjectManager ...
static public U GetById<U>(long id)
{
object obj = GetById(id);
if (obj is U)
return (U)obj;
return default(U);
}
}
Then, in your code:
ExtendedProduct.Manager.GetById<ExtendedProduct>(7).DoSomethingElse();
It's not really tons more elegant than casting, but may be one of the only solutions using Generics.