I have an interface based class that I want to have a few static classes as properties. However, I can't seem to find a way to use a static class as an instance property on a class based on an interface.
public interface IHttpHelp
{
ItemsManager {get;set;}
}
public static class ItemsManager
{
//static methods
}
public class HttpHelper
{
public ItemsManager { get { return ItemsManager;}
}
The above code won't work because of the "ItemsManager is used like a variable but it's a type error." Is there anyway to use a class this way?
For some insight into what I'm doing - I have a few static helper classes that access the httpruntime and current context. I currently use them directly, but wanted to move into a container class that will be used IoC. I could make them instance classes and forget about it, but I'm wondering f there's a way to this.
You can't use a static class like that, because by definition you can't create an instance of it, so you can't return it from a property. Make it a singleton instead:
public class ItemsManager
{
#region Singleton implementation
// Make constructor private to avoid instantiation from the outside
private ItemsManager()
{
}
// Create unique instance
private static readonly ItemsManager _instance = new ItemsManager();
// Expose unique instance
public static ItemsManager Instance
{
get { return _instance; }
}
#endregion
// instance methods
// ...
}
public class HttpHelper
{
public ItemsManager ItemsManager { get { return ItemsManager.Instance; } }
}
This is not supported by the language directly. You can either write a proxy class manually or use a library like the Duck Typing Project to emit a proxy class at runtime.
Both will have the same result: you will have a class that implements the interface, and proxies all calls to the static methods of the static class. Whether you want to write this yourself or use the duck typing library is up to you.
EDIT: Thomas' answer of using a singleton would be the way to go, if you have that option.
Static classes can't implement interfaces--it really wouldn't make much sense. An interface provides a standard API that all instances will support and you can swap instances and polymorphically access the methods through the standard interface. With a static class, all references to it are through the class anyways.
Typically in this situation you want a factory to support DI of an instance class that implements your helper.
public interface IHttpHelper
{ }
public class RealHttpHelper
{ ... }
public class FakeHttpHelper
{ ... }
public static class HttpHelper
{
public static IHttpHelper Instance
{
get
{
return whatever ? new RealHttpHelper() : new FakeHttpHelper();
}
}
}
...
HttpHelper.Instance.Context...
...
Related
I have multiple classes in the service layer of my app.
Let's say I need to access some method from AbcService.cs in my controller. Then, I need to access some method from XyzService.cs in the same controller. Then another.......For this, I would need to create an object of each Service class separately in the constructor. Also, if I needed to access these methods in another cntroller I would again have to create objects of AbcService, XyzService, etc. I want to have one instance that can give me access to methods of all service classes.
Something like:
generalService.AbcService.MethodName();
generalService.AbcService.MethodName();
How do I do this in the best possible way?
You can use inheritance and create a class that (eventually) inherits from all of them, thus inheriting their methods. Or you can make them inherit from each other (this way you'll have to use the service class that inherits from both, as it would have all the methods that can be inherited).
To put it very simply, it can go like this using inheritance (I assume the service classes were not inheriting from anything until now):
public class ServiceClassA
{
//Certain Methods
}
public class ServiceClassB : ServiceClassA
{
//Other methods, this class also has ServiceClassA methods
}
public class ServiceClassC : ServiceClassB
{
//Even more methods, this class also has ServiceClassA and ServiceClassB methods
}
//... and so on
Assuming the lowest class in the inheritance tree is ServiceClassC for example, you only need a ServiceClassC object and you'll be able to access the needed (inherited) methods.
Create service layer so that other componentns can access easily.
public class Program
{
static void Main(string[] args)
{
Global.ServiceABC.MethodA();
Global.ServiceXYZ.MethodB();
}
}
public class Global
{
private static ABC serviceABC;
public static ABC ServiceABC { get
{
if (serviceABC == null)
{
serviceABC = new ABC();
}
return serviceABC;
}
}
private static XYZ serviceXYZ;
public static XYZ ServiceXYZ
{
get
{
if (serviceXYZ == null)
{
serviceXYZ = new XYZ();
}
return serviceXYZ;
}
}
}
public class ABC
{
public void MethodA() { }
}
public class XYZ
{
public void MethodB() { }
}
Unfortunately in C# you can NOT inherit from two classes at once so something like
public class CombinedService : AbcService, XyzService {
}
is not possible.
You can however use static methods to have only one instance per service like this:
public static class ServiceManager {
// The variable holding the instance
private static AbcService _abcService = null;
// Access to the instance and single instance creator
public static AbcService AbcServiceInstance {
get {
if (_abcService == null) {
// Create your Instance here
_abcService = new AbcService();
}
return _abcService;
}
}
// The variable holding the instance
private static XyzService _xyzService = null;
// Access to the instance and single instance creator
public static XyzService XyzServiceInstance {
get {
if (_xyzService == null) {
// Create your Instance here
_xyzService = new XyzService();
}
return _xyzService;
}
}
}
Because they are static you can access them from everywhere in the code just by calling the static class properties like this:
ServiceManager.AbcServiceInstance.SomeMethod();
ServiceManager.XyzServiceInstance.SomeMethod();
You can also shorten the instantiation and accessor like this:
private static AbcService _abcService = new AbcService();
public static XyzService XyzServiceInstance {
get { return _abcService; }
}
If the instances can just be created like this and don't need any more parameters or configuration.
You should consider using a dependency injection container like Autofac. Register your service classes as Single Instance scope. You will get only one instance of a service class whenever you request it in every individual controller and even you do not need to create instance of it on your own.All is done by Ioc Container.
var builder = new ContainerBuilder();
builder.RegisterType<SomeService>().SingleInstance();
It somewhat depends how you choose to implement your services, but generally speaking you want some layer that encapsulate your services which is a singleton or that you inject\produce a single instance of it.
Encapsulation:
First try to look at a facade design pattern.
http://www.dofactory.com/net/facade-design-pattern
And btw if you don't need something complex and don't mind have an extra level of indirection then you can have some version of the facade like
KindOfSimpleFacade
{
public IServiceA ServiceA { get; }
public IServiceB ServiceB { get; }
}
Regarding the singleton there are a few ways to get it:
inject the facade object to the (just provide the same instance to each one of the controllers as an input).
(Facade) Factory -http://tutorialspoint.com/design_pattern/factory_pattern.htm
The factory will produce a single instance of the facade.
use static members inside the facade for example:
public class KindOfSimpleFacade
{
private static readonly serviceA = new ServiceA();
private static readonly serviceB = new ServiceB();
public IServiceA ServiceA { get { return serviceA; } }
public IServiceB ServiceB { get { return serviceB; } }
}
I have a scenario that (simplified) is as follows:
public static class Example
{
public const int CONSTANT_VALUE = 1;
public static Example<T> Create<T>(IEnumerable<T> argument)
where T : class
{
return new Example<T>(argument);
}
//More overloads of Create<T>, possibly other static methods, etc..
}
public class Example<T>
where T : class
{
public Example(IEnumerable<T> argument)
{
//Do Stuff
//Nothing like this in the real code, just example
//that constants are used from the other class.
if (something == Example.CONSTANT_VALUE)
{
//Do A Thing
}
}
//Lots more code
}
The basic idea is that I can have static methods, constants, etc. available through the name of the class through the static class, while the actual implementation is in the type-argumented non-static class.
My question is whether or not this is a good way to set this up. Is there a way to put some static methods and constants that don't care what the type argument is on Example<T>? Is there otherwise a more recommended pattern? What I have works fine, but I wanted to know if there are other ways since this is the first time I've ended up doing something like this (not that it's conceptually new to me, just never had need).
This would only make sense if the constants are public. If they are only for internal use inside Example<T> then this is pointless, becuase you can reference them without a fully qualified name.
If the constants are of public use, I wouldn't use this pattern anayways; Example and Example<T> are two different classes, it is potentially confusing to any user, and not immeadiately obvious, that constants defined in the non generic class are aplicable to the generic one.
You are only avoding the user a few keystrokes, I'm not really sure it is worth it.
Update: other options
In this scenario, I'd use the following factory pattern (assuming the users are outside your assembly)
public class Example<T>
{
internal Example() { } //disallow users from instantiating this class
...
}
public static class Example
{
public const int Constant = ...
public static Example<T> Create<T>() { return new ... }
}
And now all users will interact only with Example and avoid using Example<T>. You could even enforce this with users of your own assembly, you'd just need to make Example<T> a private nested class implementing a public interface:
public interface IExample<T>
{
...
}
public static class Example
{
private class Example<T>: IExample<T> { ... }
public static IExample<T> Create<T>() { ... }
....
}
Unless there is a reason this wouldn't work in your case, I would prefer to use a non-static base class Example, and then let Example<T> inherit from this class. That way you get direct access to all the methods in Example, without having to qualify with the name. Of course, this assumes that the Example class is exclusively to be used in connection with the various typed classes Example<T>.
How can i create a class with zero constructor, just like MessageBox class which has no constructor.
I can not make this class static, beacause a public static method is declared in it, and that method makes object of this class.
in C# 3.5
i want to make this class just like System.Windows.Forms.MessageBox class,
in which there is no constructor and
when we create object of this class error occurres :
this class has no constructor
where as a class with a private constructor when object creates error occurrs -
the constructor is not accessible due to its protection level.
The only way to create a class without a constructor is to use static class.
However, it seem you want to be able to create instances of this class from inside the class itself, which is not possible with a static class. For that, you should give the class a private constructor:
class Foo
{
private Foo() { }
public static Foo Create()
{
return new Foo(); // Only members of Foo can directly invoke the constructor.
}
}
If a method outside of Foo in the same assembly tries to instantiate Foo, the message given will be that the constructor is not accessible due to its protection level. If you try to access it from another assembly, it will give the message that Foo has no constructors.
The methods on MessageBox are static; you can do that with the static modifier:
public static class Foo {
public static void Bar() {...}
}
then:
Foo.Bar();
In earlier versions of c# (before static was allowed on classes) you had to cheat:
public class Foo {
private Foo() {} // hide the constructor
public static void Bar() {...}
}
Make it static class with no constructor or make it an Abstract class.
Make a static class, or make a class with a private constructor.
You can add public STATIC methods into your class and you would acheve the same as in messagebox.
Remember that static methods cannot access non static properties or methods in the same class.
Hope it helps.
Consider usage of Creational patterns, described in GOF ("Gang Of Four")
There are the following ways:
1) If you want to have only one instance of object to be created, use Singleton
There is a good example of thread-safe singleton on MSDN
In this strategy, the instance is created the first time any member of
the class is referenced
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
private Singleton(){}
public static Singleton Instance
{
get
{
return instance;
}
}
}
2) If you don't want to specify the exact class to create, use Factory method
Here is an extract from an article on C#-Corner Factory method Design pattern using C#
abstract class Factory
{
public abstract Product GetProduct(); //Factory Method Declaration
}
class concreteFactoryforProcuct1 : Factory
{
public override Product GetProduct() //Factory Method Implementation
{
return new Product1();
}
}
3) If there is a group of objects to be created this way, use Abstract factory
Here are extracts from an article on codeproject: Understanding and implementing abstract factory pattern in C#
Creating the Abstract Factory
interface IPhoneFactory //'I' stands for interface no relation with Iphone
{
ISmart GetSmart();
IDumb GetDumb();
}
Creating the Concrete Factories
class SamsungFactory : IPhoneFactory
{
public ISmart GetSmart()
{
return new GalaxyS2();
}
public IDumb GetDumb()
{
return new Primo();
}
}
...
Creating the Client
enum MANUFACTURERS
{
SAMSUNG,
HTC,
NOKIA
}
class PhoneTypeChecker
{
IPhoneFactory factory;
...
public PhoneTypeChecker(MANUFACTURERS m)
{
m_manufacturer= m;
}
public void CheckProducts()
{
switch (m_manufacturer)
{
case MANUFACTURERS.SAMSUNG:
factory = new SamsungFactory();
break;
case MANUFACTURERS.HTC:
factory = new HTCFactory();
break;
case MANUFACTURERS.NOKIA:
factory = new NokiaFactory();
break;
}
...
factory.GetSmart();
factory.GetDumb();
...
}
}
static void Main(string[] args)
{
PhoneTypeChecker checker = new PhoneTypeChecker(MANUFACTURERS.SAMSUNG);
checker.CheckProducts();
...
}
4) Use you common sense to develop your own design that would satisfy your needs.
If the purpose of it is not allowing user it instantiate new instances of the class you could make
all constructors less visible then public .
For example - protected.
Is there a way to limit the instantiation of the nested class in C#? I want to prevent nested class being instantiated from any other class except the nesting class, but to allow full access to the nested class from other code.
Usually I create an interface for the functionality you want to expose to other classes, then make the nested class private and implement that interface. This way the nested class definition can stay hidden:
public class Outer
{
private class Nested : IFace
{
public Nested(...)
{
}
//interface member implementations...
}
public IFace GetNested()
{
return new Nested();
}
}
If you need to meet one of the following requirements:
You want the nested class to be sealed,
You don't want to copy all the nested class's method signatures to an interface like in Lee's answer,
I found a solution similar to the one posted by ak99372, but without using a static initializer:
public class Outer
{
private interface IPrivateFactory<T>
{
T CreateInstance();
}
public sealed class Nested
{
private Nested() {
// private constructor, accessible only to the class Factory.
}
public class Factory : IPrivateFactory<Nested>
{
Nested IPrivateFactory<Nested>.CreateInstance() { return new Nested(); }
}
}
public Nested GetNested() {
// We couldn't write these lines outside of the `Outer` class.
IPrivateFactory<Nested> factory = new Nested.Factory();
return factory.CreateInstance();
}
}
The idea is that the Nested class's constructor is accessible only to the Factory class, which is nested one level deeper. The Factory class explicitly implements the method CreateInstance from the private interface IPrivateFactory, so that only those who can see IPrivateFactory can call CreateInstance and get a new instance of Nested.
Code outside the Outer class can't freely create instances of Nested without asking Outer.GetNested(), because
Outer.Nested's constructor is privated, so they can't call it directly
Outer.Nested.Factory can be instantiated, but can't be cast to IPrivateFactory, so its CreateInstance() method can't be called.
Note that I wouldn't recommend using that pattern heavily in production code, but it's a trick I find useful to have up my sleeve on rare occasions.
In short, no, you cannot do that. There is an accessibity modifier "public" which means "accessible by anything inside me or outside me" and there is an accessibility modifier "private" which means "accessible by anything inside me". There is no modifier which means "accessible to the thing immediately outside me but not to anything outside it", which is what you would need to mark the constructor as. That's simply not a concept that the designers of the type system thought would be useful.
Can you describe why you want this crazy kind of accessibility? Perhaps there is a better way to get what you want.
Since there is nothing in C# syntax you'll have to implement something like "a contract" between them. You can take advantage of the fact that nested class can access private fields of its parent:
public class ParentClass
{
private static Func<FriendClass> _friendContract;
public class FriendClass
{
static FriendClass()
{
_friendContract= () => new FriendClass();
}
private FriendClass() { }
}
///Usage
public FriendClass MethodUse()
{
var fInstance = _friendContract();
//fInstance.DoSomething();
return fInstance;
}
}
Of course you can adjust the contract to handle different parameters
private static Func<Arg1,Arg2,FriendClass> _friendContract;
For the answer proposed by Joshua Smith I found it necessary to force the static constructor of FriendClass to run, achieved by calling an empty static Initalize() method on FriendClass from the static constructor of ParentClass.
With the new static abstract interface members of C# 11 you can limit the instantiation of nested classes quite neatly:
public class Outer
{
protected interface INestedFactory<T> where T : INestedFactory<T>
{
public static abstract T CreateInstance();
}
public class SomeNested : INestedFactory<SomeNested>
{
private SomeNested() { }
static SomeNested INestedFactory<SomeNested>.CreateInstance()
{
return new SomeNested();
}
}
protected void CreateNested<T>() where T : INestedFactory<T>
{
T.CreateInstance();
}
}
public class Outer
{
public class Nested
{
readonly Outer Outer;
public Nested(Outer outer /* , parameters */)
{
Outer = outer;
// implementation
}
// implementation
}
public Nested GetNested(/* parameters */) => new Nested(this /* , parameters */);
}
Note that you can access private members of Outer from Nested.
What is the best way to implement polymorphic behavior in classes that I can't modify? I currently have some code like:
if(obj is ClassA) {
// ...
} else if(obj is ClassB) {
// ...
} else if ...
The obvious answer is to add a virtual method to the base class, but unfortunately the code is in a different assembly and I can't modify it. Is there a better way to handle this than the ugly and slow code above?
Hmmm... seems more suited to Adapter.
public interface ITheInterfaceYouNeed
{
void DoWhatYouWant();
}
public class MyA : ITheInterfaceYouNeed
{
protected ClassA _actualA;
public MyA( ClassA actualA )
{
_actualA = actualA;
}
public void DoWhatYouWant()
{
_actualA.DoWhatADoes();
}
}
public class MyB : ITheInterfaceYouNeed
{
protected ClassB _actualB;
public MyB( ClassB actualB )
{
_actualB = actualB;
}
public void DoWhatYouWant()
{
_actualB.DoWhatBDoes();
}
}
Seems like a lot of code, but it will make the client code a lot closer to what you want. Plus it'll give you a chance to think about what interface you're actually using.
Check out the Visitor pattern. This lets you come close to adding virtual methods to a class without changing the class. You need to use an extension method with a dynamic cast if the base class you're working with doesn't have a Visit method. Here's some sample code:
public class Main
{
public static void Example()
{
Base a = new GirlChild();
var v = new Visitor();
a.Visit(v);
}
}
static class Ext
{
public static void Visit(this object b, Visitor v)
{
((dynamic)v).Visit((dynamic)b);
}
}
public class Visitor
{
public void Visit(Base b)
{
throw new NotImplementedException();
}
public void Visit(BoyChild b)
{
Console.WriteLine("It's a boy!");
}
public void Visit(GirlChild g)
{
Console.WriteLine("It's a girl!");
}
}
//Below this line are the classes you don't have to change.
public class Base
{
}
public class BoyChild : Base
{
}
public class GirlChild : Base
{
}
I would say that the standard approach here is to wrap the class you want to "inherit" as a protected instance variable and then emulate all the non-private members (method/properties/events/etc.) of the wrapped class in your container class. You can then mark this class and its appropiate members as virtual so that you can use standard polymorphism features with it.
Here's an example of what I mean. ClosedClass is the class contained in the assembly whose code to which you have no access.
public virtual class WrapperClass : IClosedClassInterface1, IClosedClassInterface2
{
protected ClosedClass object;
public ClosedClass()
{
object = new ClosedClass();
}
public void Method1()
{
object.Method1();
}
public void Method2()
{
object.Method2();
}
}
If whatever assembly you are referencing were designed well, then all the types/members that you might ever want to access would be marked appropiately (abstract, virtual, sealed), but indeed this is unfortunately not the case (sometimes you can even experienced this issue with the Base Class Library). In my opinion, the wrapper class is the way to go here. It does have its benefits (even when the class from which you want to derive is inheritable), namely removing/changing the modifier of methods you don't want the user of your class to have access to. The ReadOnlyCollection<T> in the BCL is a pretty good example of this.
Take a look at the Decorator pattern. Noldorin actually explained it without giving the name of the pattern.
Decorator is the way of extending behavior without inheriting. The only thing I would change in Noldorin's code is the fact that the constructor should receive an instance of the object you are decorating.
Extension methods provide an easy way to add additional method signatures to existing classes. This requires the 3.5 framework.
Create a static utility class and add something like this:
public static void DoSomething(this ClassA obj, int param1, string param2)
{
//do something
}
Add a reference to the utility class on the page, and this method will appear as a member of ClassA. You can overload existing methods or create new ones this way.