I have a factory object ChallengeManager to generate instances of a Challenge object for a game I'm building. There are many challenges. The constructors for each Challenge class derivation are different, however there is a common interface among them, defined in the base class.
When I call manager.CreateChallenge(), it returns an instance of Challenge, which is one of the derived types.
Ideally, I would like to keep the code for the object construction inside the derived class itself, so all the code related to that object is co-located. Example:
class Challenge {}
class ChallengeA : Challenge {
public static Challenge MakeChallenge() {
return new ChallengeA();
}
}
class ChallengeB : Challenge {
public static Challenge MakeChallenge() {
return new ChallengeB();
}
}
Now, my ChallengeManager.CreateChallenge() call only needs to decide the class to call MakeChallenge() on. The implementation of the construction is contained by the class itself.
Using this paradigm, every derived class must define a static MakeChallenge() method. However, since the method is a static one, I am not able to make use of an Interface here, requiring it.
It's not a big deal, since I can easily remember to add the correct method signature to each derived class. However, I am wondering if there is a more elegant design I should consider.
I really like the pattern you are describing and use it often. The way I like to do it is:
abstract class Challenge
{
private Challenge() {}
private class ChallengeA : Challenge
{
public ChallengeA() { ... }
}
private class ChallengeB : Challenge
{
public ChallengeB() { ... }
}
public static Challenge MakeA()
{
return new ChallengeA();
}
public static Challenge MakeB()
{
return new ChallengeB();
}
}
This pattern has many nice properties. No one can make a new Challenge because it is abstract. No one can make a derived class because Challenge's default ctor is private. No one can get at ChallengeA or ChallengeB because they are private. You define the interface to Challenge and that is the only interface that the client needs to understand.
When the client wants an A, they ask Challenge for one, and they get it. They don't need to worry about the fact that behind the scenes, A is implemented by ChallengeA. They just get a Challenge that they can use.
You're "decentralizing" the factory, such that each subclass is responsible for creating itself.
More commonly you would have a central factory that would know about the possible subtypes and how to construct them (often enough, simply by creating a new instance and returning that instance typed as a common interface or common base class). That approach avoids the issue you currently have. I also see no benefit to your current approach. You are currently gaining no encapsulation or code reuse over the more typical implementation of a factory.
For additional reference, have a look at
http://www.oodesign.com/factory-pattern.html
Not necessarily the answer you are looking for but...
You can use following implementation, if you can move away from static method per class.
using System;
public class Test
{
public static void Main()
{
var c1 = ChallengeManager.CreateChallenge();
var c2 = ChallengeManager.CreateChallenge();
//var c = ChallengeManager.CreateChallenge<Challenage>(); // This statement won't compile
}
}
public class ChallengeManager
{
public static Challenage CreateChallenge()
{
// identify which challenge to instantiate. e.g. Challenage1
var c = CreateChallenge<Challenage1>();
return c;
}
private static Challenage CreateChallenge<T>() where T: Challenage, new()
{
return new T();
}
}
public abstract class Challenage{}
public class Challenage1: Challenage{}
public class Challenage2: Challenage{}
Related
I faced with the code below
XmlReader xmlreader =
XmlReader.Create("http://www.ecb.int/stats/eurofxref/eurofxref-daily.xml");
here to make a new object of XmlReader, it just used calling method of XmlReader.
I know here Create is a static method, but it is a little odd for me.
I used to exploit new word to command making new instance.
Can anyone please tell me how does this line works?
The method you're calling does it for you:
public class XmlReader {
public static XmlReader Create(String url) {
// There's probably a lot of fancy code in this method, but it'll use new somewhere
return new XmlReader(...);
}
}
(It's possible to avoid new altogether by using a technique called reflection, but that's not what's going on here.)
This is an example of a factory method. (Which can often be a step toward using a separate factory object.)
Somewhere in XmlReader.Create it is internally making use of the new keyword. In order to instantiate a new instance of an object, you need to use new to call a constructor. To reduce coupling between objects, however, you may abstract that behind a factory method or factory object.
For example, these two implementations do roughly the same thing:
public class Widget
{
public Widget() { }
}
//... elsewhere ...
var widget = new Widget();
And:
public class Widget
{
private Widget() { }
public static Widget Create()
{
return new Widget();
}
}
//... elsewhere ...
var widget = Widget.Create();
In an example this simple, there's little difference between the two. As code evolves and becomes more complex, there can be a number of compelling reasons to choose one over the other. For example:
There are complex constructors and you want to expose a simple interface for building an instance.
The constructors are likely to change often and you want consumers of your library to have a single unchanging interface.
There is significant logic invoked when building an object and you want to move that logic to its own object.
You want to use a separate object for mockability in automated testing.
You have a more complex inheritance structure and want to expose a top-level abstract factory.
etc.
It's a static method that, in it's body, creates a new object (using new) and returns it.
You can emulate the pattern like so:
public class Foo
{
public static Foo Create()
{
return new Foo();
}
}
public class Foo
{
public string Prop { get;set; }
public Foo(string prop)
{
Prop = prop;
}
public static Foo Create(string prop)
{
return new Foo(prop);
}
}
This is how it can look like under.
There are several reasons to create factory Methods. Do you want control about all the Instances created of a Type? For example you could do something like that:
public class MyClass {
private MyClass() // private constructor optional
{}
public void Create()
{
return new MyClass();
}
}
(private constructors are often used to implement the Singleton Pattern)
Factory Methods could also be in a seperate Class.
public class MyClass {
internal MyClass() // means only Classes of same assembly may access this
{}
}
public class MyClassFactory {
public void NewMyClass()
{
// Do some license checking here or whatever
return new MyClass();
}
}
Factory Methods define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
Read more about Factory Methods here.
I need to define a static property or method in certain classes of my bussiness logic, to explicity determine which classes are cacheables in Session or Cache of ASP.NET service. I'm thinking, static property or method in the interface would be perfect, but C# 4.0 doesn't support this.
All a need is be able to evaluate in a generic manager which classes are cacheables and, if they are, at what level: session (user) or cache (application).
Now I'm trying with a empty interface with T parameter to evaluate, but, maybe exists a better approach?? Thanks.
public interface ICacheable<T>
{
}
public class Country : ICacheable<CacheApplication>
{
}
public class Department : ICacheable<CacheUser>
{
}
public class Gestor<T>
{
// ...
if (typeof(T) is ICacheable<CacheApplication>)
{
}
// ...
}
How about using a custom attribute? Your classes then would look something like this:
[Cacheable(Level = CacheLevels.Application)]
public class Country { }
[Cacheable(Level = CacheLevels.User)]
public class Department { }
You can read here on how to create your own custom attribute and then access its value by using reflection.
You cant define static interfaces, for one thing, you cant make instances of static classes so you cant substitute them for others with the same base class.
You might be better off having a singleton instance of one class and using interfaces as normal. You could enforce one and one-only instance through a factory pattern too.
We define interface as below:
interface IMyInterface
{
void MethodToImplement();
}
And impliments as below:
class InterfaceImplementer : IMyInterface
{
static void Main()
{
InterfaceImplementer iImp = new InterfaceImplementer();
iImp.MethodToImplement();
}
public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}
}
instead of creating a interface , why can we use the function directly like below :-)
class InterfaceImplementer
{
static void Main()
{
InterfaceImplementer iImp = new InterfaceImplementer();
iImp.MethodToImplement();
}
public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}
}
Any thoughts?
You are not implementing the interface in the bottom example, you are simply creating an object of InterfaceImplementer
EDIT: In this example an interface is not needed. However, they are extremely useful when trying to write loosely coupled code where you don't have to depend on concrete objects. They are also used to define contracts where anything implementing them has to also implement each method that it defines.
There is lots of information out there, here is just a brief intro http://www.csharp-station.com/Tutorials/Lesson13.aspx
If you really want to understand more about interfaces and how they can help to write good code, I would recommend the Head First Design Patterns book. Amazon Link
instead of creating a interface , why
can we use the function directly like
below
Are you asking what the point of the interface is?
Creating an interface allows you to decouple your program from a specific class, and instead code against an abstraction.
When your class is coded against an interface, classes that use your class can inject whichever class they want that implements this interface. This facilitates unit testing since not-easily-testable modules can be substituted with mocks and stubs.
The purpose of the interface is for some other class to be able to use the type without knowing the specific implementation, so long as that type conforms to a set of methods and properties defined in the interface contract.
public class SomeOtherClass
{
public void DoSomething(IMyInterface something)
{
something.MethodToImplement();
}
}
public class Program
{
public static void Main(string[] args)
{
if(args != null)
new SomeOtherClass().DoSomething(new ImplementationOne());
else
new SomeOtherClass().DoSomething(new ImplementationTwo());
}
}
Your example doesn't really follow that pattern, however; if one that one class implements the interface, then there really isn't much of a point. You can call it either way; it just depends on what kind of object hierarchy you have and what you intend to do for us to say whether using an interface is a good choice or not.
To sum: Both snippets you provide are valid code options. We'd need context to determine which is a 'better' solution.
Interfaces are not required, there is nothing wrong with the last section of code you posted. It is simply a class and you call one of it's public methods. It has no knowledge that an interface exists that this class happens to satisfy.
However, there are advantages:
Multiple Inheritance - A class can only extend one parent class, but can implement any number of interfaces.
Freedom of class use - If your code is written so that it only cares that it has an instance of SomethingI, you are not tied to a specific Something class. If tomorrow you decide that your method should return a class that works differently, it can return SomethingA and any calling code will not need to be changed.
The purpose of interfaces isn't found in instantiating objects, but in referencing them. Consider if your example is changed to this:
static void Main()
{
IMyInterface iImp = new InterfaceImplementer();
iImp.MethodToImplement();
}
Now the iTmp object is of the type IMyInterface. Its specific implementation is InterfaceImplementer, but there may be times where the implementation is unimportant (or unwanted). Consider something like this:
interface IVehicle
{
void MoveForward();
}
class Car : IVehicle
{
public void MoveForward()
{
ApplyGasPedal();
}
private void ApplyGasPedal()
{
// some stuff
}
}
class Bike : IVehicle
{
public void MoveForward()
{
CrankPedals();
}
private void CrankPedals()
{
// some stuff
}
}
Now say you have a method like this somewhere:
void DoSomething(IVehicle)
{
IVehicle.MoveForward();
}
The purpose of the interface becomes more clear here. You can pass any implementation of IVehicle to that method. The implementation doesn't matter, only that it can be referenced by the interface. Otherwise, you'd need a DoSomething() method for each possible implementation, which can get messy fast.
Interfaces make it possible for an object to work with a variety of objects that have no common base type but have certain common abilities. If a number of classes implement IDoSomething, a method can accept a parameter of type IDoSomething, and an object of any of those classes can be passed to it. The method can then use all of the methods and properties applicable to an IDoSomething without having to worry about the actual underlying type of the object.
The point of the interface is to define a contract that your implementing class abides by.
This allows you to program to a specification rather than an implementation.
Imagine we have the following:
public class Dog
{
public string Speak()
{
return "woof!";
}
}
And want to see what he says:
public string MakeSomeNoise(Dog dog)
{
return dog.Speak();
}
We really don't benefit from the Interface, however if we also wanted to be able to see what kind of noise a Cat makes, we would need another MakeSomeNoise() overload that could accept a Cat, however with an interface we can have the following:
public interface IAnimal
{
public string Speak();
}
public class Dog : IAnimal
{
public string Speak()
{
return "woof!";
}
}
public class Cat : IAnimal
{
public string Speak()
{
return "meow!";
}
}
And run them both through:
public string MakeSomeNoise(IAnimal animal)
{
return animal.Speak();
}
Sounds like silly idea but I was wondering if it's possible somehow.
Is it possible to change the base class of a derived class at runtime? Of course, there are lot of ifs and buts and the the question of why would someone do it and its a bad design probably and all that.
Keeping all those aside (even though they might be perfectly valid), let's say, just for kicks or to show your nerdiness, is it possible in C# or any language for that matter?
So something like:
public class baseOriginal {
public string justAProperty;
}
public class baseSwapped {
public int sillyNumber;
}
public class derivedClass : baseOriginal {
public bool iAmDumb;
}
void Main() {
baseOriginal derived = new derivedClass ();
Console.WriteLine(derived.justAProperty);
baseSwapped derivedSwapped = (??);
Console.WriteLine(derivedSwapped.sillyNumber);
}
It isn't possible in C#. Probably what you want is more of a prototype-based solution commonly found in dynamic languages like JavaScript where you can "extend" the functionality of the object by adding to how it's defined.
But to accomplish what your code hints at doing, you can have the swappable classes inherit from a common ancestor class. That way you can assign instances of each to their decendents.
public class baseClassAncestor{
}
public class baseOriginal:baseClassAncestor {
public string justAProperty;
}
public class baseSwapped:baseClassAncestor {
public int sillyNumber;
}
public class derivedClass : baseOriginal {
public bool iAmDumb;
}
You can do one time base class swap by loading different assemblies that implement base class BEFORE using derived class. But this approach will not make your exact code working as you will not be able to compile that - but moving access to methods of different base classes to separate functions could be made working.
You add UnionBase class that contains all possible methods/properties from all base classes so you can compile your Main code against the assembly with this class. Than at run time you load assembly that has contains your particular base class.
Usual warning: You need to have very good reasons and understanding for going this route. I.e. existing external code is a reason to consider such approach.
"Don't do it at home, performed on a closed course by trained professional".
One more possible workaround could be implemented using some AOP solution that is based on compile-time weaving, i.e. PostSharp, which is able to seamlessly inject new methods and interfaces to existing types as well as modify (intercept) existing ones.
There is actually a good reason where you may want to swap the base class. Let say you want to modify the base class but you don't wan't to perturb the current code base as it is shared among other teams. Let say there are 10+ derived class that inherits from base. You could create 10+ custom derived classes to override the base class but that is a lot of work. Here is what you do. The key to the problem is to create an interface and a base proxy class.
class Program
{
static void Main(string[] args)
{
IActionable action = new Derived<Base1>();
action.open();
action = new Derived<Base2>();
action.open();
}
}
// Proxybase is a fake base class. ProxyBase will point to a real base1 or
// base2
public class Derived<T>:ProxyBase,IActionable
{
public Derived():base(typeof(T))
// the open function is not overriden in this case allowing
// the base implementation to be used
}
// this looks like the real base class but it is a fake
// The proxy simply points to the implementation of base1 or base2 instead
public abstract class ProxyBase: IActionable
{
IActionable obj;
public ProxyBase(Type type,params object[] args)
{
obj = (IActionable)Activator.CreateInstance(type,args);
}
public virtual void open()
{
obj.open();
}
}
// notice base1 and base2 are NOT abstract in this case
// consider this the original implementation of the base class
public class Base1: IActionable
{
public virtual void open()
{
Console.WriteLine("base1 open");
}
}
// here base2 acquired the functionality of base1 and hides base1's open
function
// consider this implementation the new one to replace the original one
public class Base2: Base1, IActionable
{
public new virtual void open()
{
Console.WriteLine("base2 open");
}
}
public interface IActionable
{
void open();
}
The result would be as follows
base1 open
base2 open
UPDATE:
Although this answer works, the reality is that inheritance introduces coupling which makes this exercise difficult at best. Also, in a practical scenario, your requirements may lead you to want to derive from multiple base class which is not possible in c#. If you want to interchange the base class you are best to use the bridge design pattern (which in fact avoids inheritance altogether thus avoiding the coupling).
The closest thing I can think of is the following:
http://msdn.microsoft.com/en-us/library/dd264736.aspx
static void Main(string[] args)
{
ExampleClass ec = new ExampleClass();
// The following line causes a compiler error if exampleMethod1 has only
// one parameter.
//ec.exampleMethod1(10, 4);
dynamic dynamic_ec = new ExampleClass();
// The following line is not identified as an error by the
// compiler, but it causes a run-time exception.
dynamic_ec.exampleMethod1(10, 4);
// The following calls also do not cause compiler errors, whether
// appropriate methods exist or not.
dynamic_ec.someMethod("some argument", 7, null);
dynamic_ec.nonexistentMethod();
}
class ExampleClass
{
public ExampleClass() { }
public ExampleClass(int v) { }
public void exampleMethod1(int i) { }
public void exampleMethod2(string str) { }
}
I have no idea if the dynamic language runtime can do what you want it to do.
Closest you could get would be to
derive from both types by defining at
least one as an interface, then
casting derived from one to the other.
I would have to agree, based on the example this suggestion would satisfy what he wants to do, it also is a better design then what he actually wants to do.
Closest you could get would be to derive from both types by defining at least one as an interface, then casting derived from one to the other.
If a class has a private constructor then it can't be instantiated.
So, if I don't want my class to be instantiated and still use it, then I can make it static.
What is the use of a private constructor?
Also, it's used in the singleton class, but except for that, is there any other use?
(Note: The reason I am excluding the singleton case above is that I don't understand why we need a singleton at all when there is a static class available. You may not answer this for my confusion in the question. )
Factory
Private constructors can be useful when using a factory pattern (in other words, a static function that's used to obtain an instance of the class rather than explicit instantiation).
public class MyClass
{
private static Dictionary<object, MyClass> cache =
new Dictionary<object, MyClass>();
private MyClass() { }
public static MyClass GetInstance(object data)
{
MyClass output;
if(!cache.TryGetValue(data, out output))
cache.Add(data, output = new MyClass());
return output;
}
}
Pseudo-Sealed with Nested Children
Any nested classes that inherit from the outer class can access the private constructor.
For instance, you can use this to create an abstract class that you can inherit from, but no one else (an internal constructor would also work here to restrict inheritance to a single assembly, but the private constructor forces all implementations to be nested classes.)
public abstract class BaseClass
{
private BaseClass() { }
public class SubClass1 : BaseClass
{
public SubClass1() : base() { }
}
public class SubClass2 : BaseClass
{
public SubClass2() : base() { }
}
}
Base Constructor
They can also be used to create "base" constructors that are called from different, more accessible constructors.
public class MyClass
{
private MyClass(object data1, string data2) { }
public MyClass(object data1) : this(data1, null) { }
public MyClass(string data2) : this(null, data2) { }
public MyClass() : this(null, null) { }
}
As Stefan, Adam and other have pointed out, private constructors are useful in cases where it is undesirable for a class to be created by code outside of the class. Singletons, factories, static method objects are examples of where being able to restrict constructon of a type is useful to enforce a particular pattern.
To respond to the second part of your question about why singletons are needed if static classes exist: singletons and static classes are not equivalent.
For example, a singleton class can implement an interface, a static class cannot. A singleton object may be passed to methods as a parameter - this is not so easy to do with static classes without resorting to wrapper objects or reflection. There are also cases where you may want to create an inheritance hierarchy in which one (or more) of the leaf classes are singleton - this is not possible with static classes either. As another example, you may have several different singletons and you may want to instantiate one of them at runtime based on environmental or configurational parameters - this is also not possible with static classes.
It is important to understand the language features and choose the right one for the job - they're there for a reason.
Sometimes you shouldn't be able to instantiate a class. This makes this explicit and enforces this at the compiler level.
Singletons are just one use case. Constants classes, static methods classes, and other types of patterns dictate that a class should not be instantiable.
Purpose to create the private constructor within a class
To restrict a class being inherited.
Restrict a class being instantiate or creating multiple instance/object.
To achieve the singleton design pattern.
public class TestPrivateConstructor
{
private TestPrivateConstructor()
{ }
public static int sum(int a , int b)
{
return a + b;
}
}
class Program
{
static void Main(string[] args)
{
// calling the private constructor using class name directly
int result = TestPrivateConstructor.sum(10, 15);
// TestPrivateConstructor objClass = new TestPrivateConstructor(); // Will throw the error. We cann't create object of this class
}
}
You can use it to force a singleton instance or create a factory class.
A static method can call the private constructor to create a new instance of that class.
For example a singleton instance:
public class Foo
{
private Foo (){}
private Foo FooInstance {get;set;}
public static Foo GetFooInstance ()
{
if(FooInstance == null){
FooInstance = new Foo();
}
return FooInstance;
}
}
This allows only one instance of the class to be created.
Well if your only objective is that you don't want it to be instantiated, then making it static is sufficient.
If, otoh, you simply don't want it to be instaniated frm outside the class, (maybe you only want users to get one by using a static factory on the class) - then you need a private ctor to allow those publicly accessible static factories to instantiate it.
Historically, remember that making a class static has not always been around... Making the ctor private was a way to make it not-instantiatable (is this a word? ) before the static keyword could be applied to a class...
Regarding singletons - singleton is a design pattern used when the environment and requirements satisfy similar motivations for the pattern's use; static classes are a language feature.
As LBushkin's answer discusses, while some of the goals of using singleton can be met using static classes, a particular implementation of singleton may exceed the feature set of static classes alone.
If the class ONLY has private constructors, it cannot be instantiated from outside.
You can also have private constructors and public constructors with different signatures.
If you want to create a factory for a class, you can use a private constructur, and add some static "factory" methods to the class itself to create the class.
An example for this is the Graphics class, with the From* methods.
Private constructors is a special instance constructor. and are used in some cases where we create a class which only have static members, so creating instance of this class is useless, and that is where private constructor comes into play.
If a class has one or more private constructors and no public constructors, other classes (except nested classes) cannot create instances of this class.
example:
class LogClass {
public static double e = Math.E; //2.71828
// Private Constructor:
private LogClass() {
}
}
The declaration of the empty constructor prevents the automatic generation of a parameter less constructor.
If you do not use an access modifier with the constructor it will still be private by default.
Read more: https://qawithexperts.com/tutorial/c-sharp/32/private-constructor-in-c-sharp