Note: Even though there are a lot of similar questions, non that i have found answered my question.
Problem:
I would like to implement a class in C# which uses the singleton pattern in the following way.
namespace DAL
{
public class CDAL : IDAL
{
/* Singleton Pattern */
private CDAL instance = null;
private CDAL()
{
}
public IDAL getInstance()
{
if (instance != null)
{
return instance;
}
else
{
CDAL.instance = new CDAL();
return CDAL.instance;
}
}
}
}
the problem is that instance and the method getInstance should be static, as i want to 'ask' the class for that instance and not an object.
but using c# i can't seem to do anything static in an interface.
how can i solve this?
You're right, you cannot do anything static on an interface, since it does not make any sense.
Use an abstract class instead of the interface to implement static logic.
It does not make any sense creating an interface with a static member.
Interfaces are a contract while the static member is always accessed by the class name, not the instance name. so briefly your interface does not know which the correct instance implementing the right logic.
in your case you don't need your method getInstance() to be defined in the interface.
Interface when used with Singleton is often just for unit testing purposes
Related
I am trying to implement a Singleton base class in my Unity3d project. But attempting to access the instance of a Singleton raises compiler errors:
Type 'Singleton' does not contain a definition for 'getPlaneTextureType' and no extension method 'getPlaneTextureType' of type 'Singleton' could be found (are you missing a using directive or an assembly reference?)
Do I need to override the method/property instance for each class that derives from Singleton? Ie, change the return type to the child class type? Or is there 1 simple change I can make to the base class Singleton that will fix this? Like for example; using c++ like templates?
public class Singleton : MonoBehaviour {
#region Static Variables
private static Singleton _instance = null;
#endregion
#region Singleton Class Generation
public static Singleton instance {
get { return _instance; }
}
protected Singleton() { }
void Awake() {
if (_instance != null) {
GameObject.Destroy( this );
return;
}
_instance = this;
}
#endregion
}
public class TerrainManager : Singleton {
public PlaneTexture getPlaneTextureType() { }
}
// Usage that throws compiler error: Type 'Singleton' does not contain a definition for 'getPlaneTextureType' and no extension method 'getPlaneTextureType' of type 'Singleton' could be found (are you missing a using directive or an assembly reference?)
TerrainManager.instance.getPlaneTextureType();
Sometimes you want a Singleton in Unity to be just a plain Singleton Design pattern without it being a MonoBehaviour.
Othertimes you need your Singleton to be a MonoBehaviour. When it must be a MonoBehaviour you can do this to avoid calling new which you can't do on MonoBehaviours.
Solution
Makes a Singleton out of a MonoBehaviour
public class Singleton : MonoBehaviour
{
private static Singleton _instance;
// Will be called before anything, don't even worry about it, instance be initialized.
void Awake()
{
_instance = this;
}
public static Singleton getInstance()
{
return _instance;
}
}
This should also allow you to inherit from this Singleton like this:
public class Singleton2 : Singleton
{
}
Also the Destroy() code you are using won't be a good idea unless you are protecting the Singleton from being destroyed on level change with DontDestroyOnLoad()
Other Solution
You can find it here: Unity Wiki Singleton It is the same code Imran posted but he forgot to include the reference link where he got it from.
You can either move getPlaneTextureType() method into the Singleton class for TerrainManager.instance.getPlaneTextureType() to work you can change getPlaneTextureType() into a static method and called it as such... TerrainManager.getPlaneTextureType().
Inheritance won't help you implement a singleton in C# very well. The reason is that if you put the static field in the base class, it is the same static field for across all inherited classes. This exactly what you don't want. You want a static field and/or property in each inherited class that is different.
It's not really that much code to implement one in the first place so inheritance isn't really letting you reuse a lot of code. This is your basic singleton template:
public class Singleton
{
private static readonly Singleton instance = new Singleton();
private Singleton() { }
public static Singleton Instance { get { return instance; } }
}
I would just copy and paste that every time I needed one, but I find myself using the singleton pattern only sparingly.
generally you applying singleton for
"It is a design pattern that restricts the Instantiation of a class to one object. And, if you're here, you probably want to use that basically for implementing Global Variables. For any other usage"
Now try for implementation
Other help plz link
As it is not possible to override a static class in c#, if i want to override a method I generally define a delegate matching the signature of the static method, then modify the method along the lines of:
public static void foo(int bar)
{
if (delegatename!=null)
{
delegatename.Invoke(bar);
}
else
{
//execute previous code as normal
}
}
I feel a twinge of guilt, knowing this is a bit messy.
Can anyone suggest a neater solution to this problem (other than rewriting the original structure)
It seems that you are using the static class as a way to provide a single access point to some resource in your application. If it is the case, you should consider to use an implementation of Singleton design pattern. Doing it, you could take advantage of use inheritance on your non-static classes.
public abstract class Base { ... }
public class Impl : Base { ... }
public class Singleton : Impl
{
#region Static Members
static readonly Singleton _instance = new Singleton();
static Singleton() { }
static public Singleton Instance
{
get { return _instance; }
}
#endregion Static Members
#region Instance Members
private Singleton() { }
// Method overrides goes here...
#endregion Instance Members
}
A deeper discussion on how to implement the singleton design pattern on C# can be found on Implementing the Singleton Pattern in C# article.
Consider using dependency injection, as static singletons make test isolation essentially impossible and re-usability extremely painful.
Singletons are cool, just don't do it using statics. There are many resources on "dependency injection," which Google can easily find for you.
Although a static class has only one instance and can't be instantiated, a class with a private constructor can't be instantiated (as the constructor can't be seen), so every time you call this class, this is the same one instance?
Factory classes always follow the last convention (instance class with private constructor). Why is this?
Thanks
There's nothing stopping the class with the private constructor from having a public static method which returns instances of the class:
public class NoPublicConstructor
{
private NoPublicConstructor()
{
}
public static NoPublicConstructor NewInstance()
{
return new NoPublicConstructor();
}
}
As you can see, the static method does not return the same one instance.
edit: One of the reasons factory classes do this is to be able to separate responsibility in future versions: while your code always calls the factory creation method, the author may move all the "guts" out of that class into a different one and your code won't need to know the difference. Calling that class' (public) constructor ties it to an extent to the original class implementation.
You can't* get an instance from outside the class, but you can from inside. A static method or an inner class can create and return an instance of the class with a private constructor. The static class cannot be instanced by anything.
class Foo
{
private Foo()
{
}
public class Bar
{
public Bar()
{
}
public Foo GetFoo()
{
return new Foo();
}
}
}
..
Foo.Bar fooBar = new Foo.Bar();
Foo foo = fooBar.GetFoo();
Edit: *I use the term "can't" loosely. Brian Rasmussen pointed out in the comments to the OP that another method to obtain an instance is through a call through System.Runtime.Serialization.FormatterServices, and this is external to the class itself.
Foo foo = (Foo)System.Runtime.Serialization.FormatterServices.GetSafeUninitializedObject(typeof(Foo));
Creating a class with private constructor is the common pattern for implementing a "Singleton" object.
The Singleton usually will instantiate an instance of itself, and only allow access to it through a static "Instance" property, which means there's only ever one instance of the class.
The advantage of using a Singleton over a purely static class is that you can utilize interfaces and different implementation classes within the singleton. Your "Singleton" might expose an interface for a set of methods, and you can choose which exact implementation class to instantiate under the covers. If you were using a purely static class, it would be hard to swap out a completely different implementation, without impacting other code.
The main downside of Singleton is that it's difficult to swap out the implementation class for testing, because it's controlled within the Singleton private methods, but there are ways to get around that.
I am currently designing a class library that will provide data to a web application graph rendering engine in C#. I am currently defining the interfaces of this library.
I have a IGraphData interface which I would like to cache using a service that accesses the cache, this is called IGraphDataCacheService and has set and get methods to add and retrieve IGraphData objects to and from the cache. the cache service will be a singleton.
I am confused about the correct way to implement this, so that there is only one cache service that can get and set generic IgraphData objects.
I came up with this:
interface IGraphDataCacheService {
IGraphData<object> Get(string identifier);
void Set(IGraphData<object> graphData);}
or this:
T Get<T, P>(string identifier) where T : IGraphData<P>;
void Set<T,P>(T graphData) where T : IGraphData<P>;
Can any one offer any advice help?
Thanks
Why don't you just make the interface generic instead?
interface ICacheService<T> {
T Get(string identifier);
void Set(T graphData);
}
if you wanted, you could type-constrain T to be of type IGraphData, or you could write it as:
interface IGraphDataCacheService<T> {
IGraphData<T> Get(string identifier);
void Set(IGraphData<T> graphData);
}
A few points:
I'd probably rename the interface methods to be more emblematic of a caching service. For example, Fetch and Store instead of Get and Set, which makes it sound like you're getting or setting the provider rather than the data to be cached.
Ensuring that there is only one cache is an implementation detail, not an interface one.
To implement a singleton, try something like:
public class SingletonCacheService : IGraphDataCacheService {
private static Singleton instance;
private Singleton() {}
// snip implementation of IGraphDataCacheService methods ...
public static Singleton Instance {
get {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
}
Note that this simple version isn't threadsafe.
Both alternatives seem plausible at a glance; my hunch is that you need to write some 'typical' client code to decide. e.g. Does the typical client 'know' the type of data associated with the identifier it's looking up? Good API design requires identifying the main use scenarios and using that to inform the design.
If I understand your question correctly you are wanting to treat the generic types like they are the same, but in current .NET implementation you can't do this.
IGraphData<string> can't be passed as a IGraphData<object> they are actually different types even though string is an object, the generic types are not related and can't be cast or passed like they are the same.
If you control the IGraphData interface you can create a IGraphData interface and derive IGraphData from it and use IGraphData to access the cache. It just depends on how you are using it and what you have the control over.
You can do what you want in C# 4.0. There is an article about it here
You can't ensure there's only a single instance implementing an interface. However, you can make a class (e.g. GraphDataCacheServiceImpl) implementing the interface a singleton by sealing it and providing only a getter property, with the object created as a static variable from a private constructor. See the below. As far as generics, it's not exactly clear what you're seeking to accomplish. But I would guess the below is close to what you want.
interface IGraphDataCacheService<T> {
IGraphData<T> Get(string identifier);
void Set(IGraphData<T> graphData);
}
public sealed class GraphDataCacheServiceImpl<T> : IGraphDataCacheService<T>
{
private GraphDataCacheServiceImpl()
{
// ..
}
static GraphDataCacheServiceImpl()
{
Instance = new GraphDataCacheServiceImpl<T>();
}
public IGraphData<T> Get(string id)
{
return new GraphDataImpl<T>();
}
public void Set(IGraphData<T> graphData)
{
}
public static GraphDataCacheServiceImpl<T> Instance {get; private set;}
}
I'm still trying to get a better understanding of Interfaces. I know about what they are and how to implement them in classes.
What I don't understand is when you create a variable that is of one of your Interface types:
IMyInterface somevariable;
Why would you do this? I don't understand how IMyInterface can be used like a class...for example to call methods, so:
somevariable.CallSomeMethod();
Why would you use an IMyInterface variable to do this?
You are not creating an instance of the interface - you are creating an instance of something that implements the interface.
The point of the interface is that it guarantees that what ever implements it will provide the methods declared within it.
So now, using your example, you could have:
MyNiftyClass : IMyInterface
{
public void CallSomeMethod()
{
//Do something nifty
}
}
MyOddClass : IMyInterface
{
public void CallSomeMethod()
{
//Do something odd
}
}
And now you have:
IMyInterface nifty = new MyNiftyClass()
IMyInterface odd = new MyOddClass()
Calling the CallSomeMethod method will now do either something nifty or something odd, and this becomes particulary useful when you are passing in using IMyInterface as the type.
public void ThisMethodShowsHowItWorks(IMyInterface someObject)
{
someObject.CallSomeMethod();
}
Now, depending on whether you call the above method with a nifty or an odd class, you get different behaviour.
public void AnotherClass()
{
IMyInterface nifty = new MyNiftyClass()
IMyInterface odd = new MyOddClass()
// Pass in the nifty class to do something nifty
this.ThisMethodShowsHowItWorks(nifty);
// Pass in the odd class to do something odd
this.ThisMethodShowsHowItWorks(odd);
}
EDIT
This addresses what I think your intended question is - Why would you declare a variable to be of an interface type?
That is, why use:
IMyInterface foo = new MyConcreteClass();
in preference to:
MyConcreteClass foo = new MyConcreteClass();
Hopefully it is clear why you would use the interface when declaring a method signature, but that leaves the question about locally scoped variables:
public void AMethod()
{
// Why use this?
IMyInterface foo = new MyConcreteClass();
// Why not use this?
MyConcreteClass bar = new MyConcreteClass();
}
Usually there is no technical reason why the interface is preferred. I usually use the interface because:
I typically inject dependencies so the polymorphism is needed
Using the interface clearly states my intent to only use members of the interface
The one place where you would technically need the interface is where you are utilising the polymorphism, such as creating your variable using a factory or (as I say above) using dependency injection.
Borrowing an example from itowlson, using concrete declaration you could not do this:
public void AMethod(string input)
{
IMyInterface foo;
if (input == "nifty")
{
foo = new MyNiftyClass();
}
else
{
foo = new MyOddClass();
}
foo.CallSomeMethod();
}
Because this:
public void ReadItemsList(List<string> items);
public void ReadItemsArray(string[] items);
can become this:
public void ReadItems(IEnumerable<string> items);
Edit
Think of it like this:
You have to be able to do this.
rather than:
You have to be this.
Essentially this is a contract between the method and it's callers.
Using interface variables is the ONLY way to allow handler methods to be written which can accept data from objects that have different base classes.
This is about as clear as anyone is going to get.
An interface is used so you do not need to worry about what class implements the interface. An example of this being useful is when you have a factory method that returns a concrete implementation that may be different depending on the environment you are running in. It also allows an API designer to define the API while allowing 3rd parties to implement the API in any way they see fit. Sun does this with it's cryptographic API's for Java.
public interface Foo {
}
public class FooFactory {
public static Foo getInstance() {
if(os == 'Windows') return new WinFoo();
else if(os == 'OS X') return new MacFoo();
else return new GenricFoo();
}
}
Your code that uses the factory only needs to know about Foo, not any of the specific implementations.
I was in same position and took me few days to figure out why do we have to use interface variable.
IDepartments rep = new DepartmentsImpl();
why not
DepartmentsImpl rep = new DepartmentsImpl();
Imagine If a class implements two interfaces that contain a member with the same signature, then implementing that member on the class will cause both interfaces to use that member as their implementation.
class Test
{
static void Main()
{
SampleClass sc = new SampleClass();
IControl ctrl = (IControl)sc;
ISurface srfc = (ISurface)sc;
// The following lines all call the same method.
sc.Paint();
ctrl.Paint();
srfc.Paint();
}
}
interface IControl
{
void Paint();
}
interface ISurface
{
void Paint();
}
class SampleClass : IControl, ISurface
{
// Both ISurface.Paint and IControl.Paint call this method.
public void Paint()
{
Console.WriteLine("Paint method in SampleClass");
}
}
// Output:
// Paint method in SampleClass
// Paint method in SampleClass
// Paint method in SampleClass
If the two interface members do not perform the same function, however, this can lead to an incorrect implementation of one or both of the interfaces.
public class SampleClass : IControl, ISurface
{
void IControl.Paint()
{
System.Console.WriteLine("IControl.Paint");
}
void ISurface.Paint()
{
System.Console.WriteLine("ISurface.Paint");
}
}
The class member IControl.Paint is only available through the IControl interface, and ISurface.Paint is only available through ISurface. Both method implementations are separate, and neither is available directly on the class. For example:
IControl c = new SampleClass();
ISurface s = new SampleClass();
s.Paint();
Please do correct me if i am wrong as i am still learning this Interface concept.
Lets say you have class Boat, Car, Truck, Plane.
These all share a common method TakeMeThere(string destination)
You would have an interface:
public interface ITransportation
{
public void TakeMeThere(string destination);
}
then your class:
public class Boat : ITransportation
{
public void TakeMeThere(string destination) // From ITransportation
{
Console.WriteLine("Going to " + destination);
}
}
What you're saying here, is that my class Boat will do everything ITransportation has told me too.
And then when you want to make software for a transport company. You could have a method
Void ProvideServiceForClient(ITransportation transportationMethod, string whereTheyWantToGo)
{
transportationMethod.TakeMeThere(whereTheyWantToGo); // Cause ITransportation has this method
}
So it doesn't matter which type of transportation they want, because we know it can TakeMeThere
This is not specific to C#,so i recommend to move to some othere flag.
for your question,
the main reason why we opt for interface is to provide a protocol between two components(can be a dll,jar or any othere component).
Please refer below
public class TestClass
{
static void Main()
{
IMyInterface ob1, obj2;
ob1 = getIMyInterfaceObj();
obj2 = getIMyInterfaceObj();
Console.WriteLine(ob1.CallSomeMethod());
Console.WriteLine(obj2.CallSomeMethod());
Console.ReadLine();
}
private static bool isfirstTime = true;
private static IMyInterface getIMyInterfaceObj()
{
if (isfirstTime)
{
isfirstTime = false;
return new ImplementingClass1();
}
else
{
return new ImplementingClass2();
}
}
}
public class ImplementingClass1 : IMyInterface
{
public ImplementingClass1()
{
}
#region IMyInterface Members
public bool CallSomeMethod()
{
return true;
}
#endregion
}
public class ImplementingClass2 : IMyInterface
{
public ImplementingClass2()
{
}
#region IMyInterface Members
public bool CallSomeMethod()
{
return false;
}
#endregion
}
public interface IMyInterface
{
bool CallSomeMethod();
}
Here the main method does not know about the classes still it is able to get different behaviour using the interface.
The purpose of the Interface is to define a contract between several objects, independent of specific implementation.
So you would usually use it when you have an Intrace ISomething, and a specific implementation
class Something : ISomething
So the Interface varialbe would come to use when you instantiate a contract:
ISomething myObj = new Something();
myObj.SomeFunc();
You should also read interface C#
Update:
I will explaing the logic of using an Interface for the variable and not the class itself by a (real life) example:
I have a generic repositor interace:
Interface IRepository {
void Create();
void Update();
}
And i have 2 seperate implementations:
class RepositoryFile : interface IRepository {}
class RepositoryDB : interface IRepository {}
Each class has an entirely different internal implementation.
Now i have another object, a Logger, that uses an already instansiated repository to do his writing. This object, doesn't care how the Repository is implemented, so he just implements:
void WriteLog(string Log, IRepository oRep);
BTW, this can also be implemented by using standard classes inheritance. But the difference between using interfaces and classes inheritance is another discussion.
For a slightly more details discussion on the difference between abstract classes and interfaces see here.
Say, for example, you have two classes: Book and Newspaper. You can read each of these, but it wouldn't really make sense for these two to inherit from a common superclass. So they will both implement the IReadable interface:
public interface IReadable
{
public void Read();
}
Now say you're writing an application that will read books and newspapers for the user. The user can select a book or newspaper from a list, and that item will be read to the user.
The method in your application that reads to the user will take this Book or Newspaper as a parameter. This might look like this in code:
public static void ReadItem(IReadable item)
{
item.Read();
}
Since the parameter is an IReadable, we know that the object has the method Read(), thus we call it to read it to the user. It doesn't matter whether this is a Book, Newspaper, or anything else that implements IReadable. The individual classes implement exactly how each item will be read by implementing the Read() method, since it will most likely be different for the different classes.
Book's Read() might look like this:
public void Read()
{
this.Open();
this.TurnToPage(1);
while(!this.AtLastPage)
{
ReadText(this.CurrentPage.Text);
this.TurnPage();
}
this.Close();
}
Newspaper's Read() would likely be a little different:
public void Read()
{
while(!this.OnBackPage)
{
foreach(Article article in this.CurrentPage.Articles)
{
ReadText(article.Text);
}
}
}
The point is that the object contained by a variable that is an interface type is guaranteed to have a specific set of methods on it, even if the possible classes of the object are not related in any other way. This allows you to write code that will apply to a variety of classes that have common operations that can be performed on them.
No, it is not possible. Designers did not provide a way. Of course, it is of common sense also. Because interface contains only abstract methods and as abstract methods do not have a body (of implementation code), we cannot create an object..
Suppose even if it is permitted, what is the use. Calling the abstract method with object does not yield any purpose as no output. No functionality to abstract methods.
Then, what is the use of interfaces in Java design and coding. They can be used as prototypes from which you can develop new classes easily. They work like templates for other classes that implement interface just like a blue print to construct a building.
I believe everyone is answering the polymorphic reason for using an interface and David Hall touches on partially why you would reference it as an interface instead of the actual object name. Of course, being limited to the interface members etc is helpful but the another answer is dependency injection / instantiation.
When you engineer your application it is typically cleaner, easier to manage, and more flexible if you do so utilizing dependency injection. It feels backwards at first if you've never done it but when you start backtracking you'll wish you had.
Dependency injection normally works by allowing a class to instantiate and control the dependencies and you just rely on the interface of the object you need.
Example:
Layer the application first. Tier 1 logic, tier 2 interface, tier 3 dependency injection. (Everyone has their own way, this is just for show).
In the logic layer you reference the interfaces and dependency layer and then finally you create logic based on only the interfaces of foreign objects.
Here we go:
public IEmployee GetEmployee(string id)
{
IEmployee emp = di.GetInstance<List<IEmployee>>().Where(e => e.Id == id).FirstOrDefault();
emp?.LastAccessTimeStamp = DateTime.Now;
return emp;
}
Notice above how we use di.GetInstance to get an object from our dependency. Our code in that tier will never know or care about the Employee object. In fact if it changes in other code it will never affect us here. If the interface of IEmployee changes then we may need to make code changes.
The point is, IEmployee emp = never really knows what the actual object is but does know the interface and how to work with it. With that in mind, this is when you want to use an interface as opposed to an object becase we never know or have access to the object.
This is summarized.. Hopefully it helps.
This is a fundamental concept in object-oriented programming -- polymorphism. (wikipedia)
The short answer is that by using the interface in Class A, you can give Class A any implementation of IMyInterface.
This is also a form of loose coupling (wikipedia) -- where you have many classes, but they do not rely explicitly on one another -- only on an abstract notion of the set of properties and methods that they provide (the interface).