Related
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
When to Use Static Classes in C#
I set my classes as static a lot, but I am not sure when use static or not, or what's the difference it makes to use it or not.
can anybody explain please?
Making a class static just prevents people from trying to make an instance of it. If all your class has are static members it is a good practice to make the class itself static.
If a class is declared as static then the variables and methods need to be declared as static.
A class can be declared static, indicating that it contains only static members. It is not possible to create instances of a static class using the new keyword. Static classes are loaded automatically by the .NET Framework common language runtime (CLR) when the program or namespace containing the class is loaded.
Use a static class to contain methods that are not associated with a particular object. For example, it is a common requirement to create a set of methods that do not act on instance data and are not associated to a specific object in your code. You could use a static class to hold those methods.
->The main features of a static class are:
They only contain static members.
They cannot be instantiated.
They are sealed.
They cannot contain Instance Constructors or simply constructors as we know that they are associated with objects and operates on data when an object is created.
Example
static class CollegeRegistration
{
//All static member variables
static int nCollegeId; //College Id will be same for all the students studying
static string sCollegeName; //Name will be same
static string sColegeAddress; //Address of the college will also same
//Member functions
public static int GetCollegeId()
{
nCollegeId = 100;
return (nCollegeID);
}
//similarly implementation of others also.
} //class end
public class student
{
int nRollNo;
string sName;
public GetRollNo()
{
nRollNo += 1;
return (nRollNo);
}
//similarly ....
public static void Main()
{
//Not required.
//CollegeRegistration objCollReg= new CollegeRegistration();
//<ClassName>.<MethodName>
int cid= CollegeRegistration.GetCollegeId();
string sname= CollegeRegistration.GetCollegeName();
} //Main end
}
Static classes can be useful in certain situations, but there is a potential to abuse and/or overuse them, like most language features.
As Dylan Smith already mentioned, the most obvious case for using a static class is if you have a class with only static methods. There is no point in allowing developers to instantiate such a class.
The caveat is that an overabundance of static methods may itself indicate a flaw in your design strategy. I find that when you are creating a static function, its a good to ask yourself -- would it be better suited as either a) an instance method, or b) an extension method to an interface. The idea here is that object behaviors are usually associated with object state, meaning the behavior should belong to the object. By using a static function you are implying that the behavior shouldn't belong to any particular object.
Polymorphic and interface driven design are hindered by overusing static functions -- they cannot be overriden in derived classes nor can they be attached to an interface. Its usually better to have your 'helper' functions tied to an interface via an extension method such that all instances of the interface have access to that shared 'helper' functionality.
One situation where static functions are definitely useful, in my opinion, is in creating a .Create() or .New() method to implement logic for object creation, for instance when you want to proxy the object being created,
public class Foo
{
public static Foo New(string fooString)
{
ProxyGenerator generator = new ProxyGenerator();
return (Foo)generator.CreateClassProxy
(typeof(Foo), new object[] { fooString }, new Interceptor());
}
This can be used with a proxying framework (like Castle Dynamic Proxy) where you want to intercept / inject functionality into an object, based on say, certain attributes assigned to its methods. The overall idea is that you need a special constructor because technically you are creating a copy of the original instance with special added functionality.
I have several classes that do not really need any state. From the organizational point of view, I would like to put them into hierarchy.
But it seems I can't declare inheritance for static classes.
Something like that:
public static class Base
{
}
public static class Inherited : Base
{
}
will not work.
Why have the designers of the language closed that possibility?
Citation from here:
This is actually by design. There seems to be no good reason to inherit a static class. It has public static members that you can always access via the class name itself. The only reasons I have seen for inheriting static stuff have been bad ones, such as saving a couple of characters of typing.
There may be reason to consider mechanisms to bring static members directly into scope (and we will in fact consider this after the Orcas product cycle), but static class inheritance is not the way to go: It is the wrong mechanism to use, and works only for static members that happen to reside in a static class.
(Mads Torgersen, C# Language PM)
Other opinions from channel9
Inheritance in .NET works only on instance base. Static methods are defined on the type level not on the instance level. That is why overriding doesn't work with static methods/properties/events...
Static methods are only held once in memory. There is no virtual table etc. that is created for them.
If you invoke an instance method in .NET, you always give it the current instance. This is hidden by the .NET runtime, but it happens. Each instance method has as first argument a pointer (reference) to the object that the method is run on. This doesn't happen with static methods (as they are defined on type level). How should the compiler decide to select the method to invoke?
(littleguru)
And as a valuable idea, littleguru has a partial "workaround" for this issue: the Singleton pattern.
The main reason that you cannot inherit a static class is that they are abstract and sealed (this also prevents any instance of them from being created).
So this:
static class Foo { }
compiles to this IL:
.class private abstract auto ansi sealed beforefieldinit Foo
extends [mscorlib]System.Object
{
}
Think about it this way: you access static members via type name, like this:
MyStaticType.MyStaticMember();
Were you to inherit from that class, you would have to access it via the new type name:
MyNewType.MyStaticMember();
Thus, the new item bears no relationships to the original when used in code. There would be no way to take advantage of any inheritance relationship for things like polymorphism.
Perhaps you're thinking you just want to extend some of the items in the original class. In that case, there's nothing preventing you from just using a member of the original in an entirely new type.
Perhaps you want to add methods to an existing static type. You can do that already via extension methods.
Perhaps you want to be able to pass a static Type to a function at runtime and call a method on that type, without knowing exactly what the method does. In that case, you can use an Interface.
So, in the end you don't really gain anything from inheriting static classes.
Hmmm... would it be much different if you just had non-static classes filled with static methods..?
What you want to achieve by using class hierarchy can be achieved merely through namespacing. So languages that support namespapces ( like C#) will have no use of implementing class hierarchy of static classes. Since you can not instantiate any of the classes, all you need is a hierarchical organization of class definitions which you can obtain through the use of namespaces
You can use composition instead... this will allow you to access class objects from the static type. But still cant implements interfaces or abstract classes
Although you can access "inherited" static members through the inherited classes name, static members are not really inherited. This is in part why they can't be virtual or abstract and can't be overridden. In your example, if you declared a Base.Method(), the compiler will map a call to Inherited.Method() back to Base.Method() anyway. You might as well call Base.Method() explicitly. You can write a small test and see the result with Reflector.
So... if you can't inherit static members, and if static classes can contain only static members, what good would inheriting a static class do?
A workaround you can do is not use static classes but hide the constructor so the classes static members are the only thing accessible outside the class. The result is an inheritable "static" class essentially:
public class TestClass<T>
{
protected TestClass()
{ }
public static T Add(T x, T y)
{
return (dynamic)x + (dynamic)y;
}
}
public class TestClass : TestClass<double>
{
// Inherited classes will also need to have protected constructors to prevent people from creating instances of them.
protected TestClass()
{ }
}
TestClass.Add(3.0, 4.0)
TestClass<int>.Add(3, 4)
// Creating a class instance is not allowed because the constructors are inaccessible.
// new TestClass();
// new TestClass<int>();
Unfortunately because of the "by-design" language limitation we can't do:
public static class TestClass<T>
{
public static T Add(T x, T y)
{
return (dynamic)x + (dynamic)y;
}
}
public static class TestClass : TestClass<double>
{
}
You can do something that will look like static inheritance.
Here is the trick:
public abstract class StaticBase<TSuccessor>
where TSuccessor : StaticBase<TSuccessor>, new()
{
protected static readonly TSuccessor Instance = new TSuccessor();
}
Then you can do this:
public class Base : StaticBase<Base>
{
public Base()
{
}
public void MethodA()
{
}
}
public class Inherited : Base
{
private Inherited()
{
}
public new static void MethodA()
{
Instance.MethodA();
}
}
The Inherited class is not static itself, but we don't allow to create it. It actually has inherited static constructor which builds Base, and all properties and methods of Base available as static. Now the only thing left to do make static wrappers for each method and property you need to expose to your static context.
There are downsides like the need for manual creation of static wrapper methods and new keyword. But this approach helps support something that is really similar to static inheritance.
P.S.
We used this for creating compiled queries, and this actually can be replaced with ConcurrentDictionary, but a static read-only field with its thread safety was good enough.
My answer: poor design choice. ;-)
This is an interesting debate focused on syntax impact. The core of the argument, in my view, is that a design decision led to sealed static classes. A focus on transparency of the static class's names appearing at the top level instead of hiding ('confusing') behind child names? One can image a language implementation that could access the base or the child directly, confusing.
A pseudo example, assuming static inheritance was defined in some way.
public static class MyStaticBase
{
SomeType AttributeBase;
}
public static class MyStaticChild : MyStaticBase
{
SomeType AttributeChild;
}
would lead to:
// ...
DoSomethingTo(MyStaticBase.AttributeBase);
// ...
which could (would?) impact the same storage as
// ...
DoSomethingTo(MyStaticChild.AttributeBase);
// ...
Very confusing!
But wait! How would the compiler deal with MyStaticBase and MyStaticChild having the same signature defined in both? If the child overrides than my above example would NOT change the same storage, maybe? This leads to even more confusion.
I believe there is a strong informational space justification for limited static inheritance. More on the limits shortly. This pseudocode shows the value:
public static class MyStaticBase<T>
{
public static T Payload;
public static void Load(StorageSpecs);
public static void Save(StorageSpecs);
public static SomeType AttributeBase
public static SomeType MethodBase(){/*...*/};
}
Then you get:
public static class MyStaticChild : MyStaticBase<MyChildPlayloadType>
{
public static SomeType AttributeChild;
public static SomeType SomeChildMethod(){/*...*/};
// No need to create the PlayLoad, Load(), and Save().
// You, 'should' be prevented from creating them, more on this in a sec...
}
Usage looks like:
// ...
MyStaticChild.Load(FileNamePath);
MyStaticChild.Save(FileNamePath);
doSomeThing(MyStaticChild.Payload.Attribute);
doSomething(MyStaticChild.AttributeBase);
doSomeThing(MyStaticChild.AttributeChild);
// ...
The person creating the static child does not need to think about the serialization process as long as they understand any limitations that might be placed on the platform's or environment's serialization engine.
Statics (singletons and other forms of 'globals') often come up around configuration storage. Static inheritance would allow this sort of responsibility allocation to be cleanly represented in the syntax to match a hierarchy of configurations. Though, as I showed, there is plenty of potential for massive ambiguity if basic static inheritance concepts are implemented.
I believe the right design choice would be to allow static inheritance with specific limitations:
No override of anything. The child cannot replace the base
attributes, fields, or methods,... Overloading should be ok, as
long as there is a difference in signature allowing the compiler to
sort out child vs base.
Only allow generic static bases, you cannot inherit from a
non-generic static base.
You could still change the same store via a generic reference MyStaticBase<ChildPayload>.SomeBaseField. But you would be discouraged since the generic type would have to be specified. While the child reference would be cleaner: MyStaticChild.SomeBaseField.
I am not a compiler writer so I am not sure if I am missing something about the difficulties of implementing these limitations in a compiler. That said, I am a strong believer that there is an informational space need for limited static inheritance and the basic answer is that you can't because of a poor (or over simple) design choice.
Static classes and class members are used to create data and functions that can be accessed without creating an instance of the class. Static class members can be used to separate data and behavior that is independent of any object identity: the data and functions do not change regardless of what happens to the object. Static classes can be used when there is no data or behavior in the class that depends on object identity.
A class can be declared static, which indicates that it contains only static members. It is not possible to use the new keyword to create instances of a static class. Static classes are loaded automatically by the .NET Framework common language runtime (CLR) when the program or namespace that contains the class is loaded.
Use a static class to contain methods that are not associated with a particular object. For example, it is a common requirement to create a set of methods that do not act on instance data and are not associated to a specific object in your code. You could use a static class to hold those methods.
Following are the main features of a static class:
They only contain static members.
They cannot be instantiated.
They are sealed.
They cannot contain Instance Constructors (C# Programming Guide).
Creating a static class is therefore basically the same as creating a class that contains only static members and a private constructor. A private constructor prevents the class from being instantiated.
The advantage of using a static class is that the compiler can check to make sure that no instance members are accidentally added. The compiler will guarantee that instances of this class cannot be created.
Static classes are sealed and therefore cannot be inherited. They cannot inherit from any class except Object. Static classes cannot contain an instance constructor; however, they can have a static constructor. For more information, see Static Constructors (C# Programming Guide).
When we create a static class that contains only the static members and a private constructor.The only reason is that the static constructor prevent the class from being instantiated for that we can not inherit a static class .The only way to access the member of the static class by using the class name itself.Try to inherit a static class is not a good idea.
I run into the problem when trying to code an IComparer<T> implementation against a third-party library where T is an enum embedded in a class as in the following:
public class TheClass
{
public enum EnumOfInterest
{
}
}
But because the enum is defined within a third-party library class, I can't write the comparer because the following gives a "cannot extends list" error:
public class MyComparer : IComparer<TheClass.EnumOfInterest>
{
}
I'm not even extending a static class -- I'm just implementing a comparer of a enum defined in a class.
I would like to access a class everywhere in my application, how can I do this?
To make it more clear, I have a class somewhere that use some code. I have an other class that use the same code. I do not want to duplicate so I would like to call the same code in both place by using something. In php I would just include("abc.php") in both... I do not want to create the object everytime I want to use the code.
Do you want to access the class or access an instance of the class from everywhere?
You can either make it a static class - public static class MyClass { } - or you can use the Singleton Pattern.
For the singleton pattern in its simplest form you can simply add a static property to the class (or some other class) that returns the same instance of the class like this:
public class MyClass
{
private static MyClass myClass;
public static MyClass MyClass
{
get { return myClass ?? (myClass = new MyClass()); }
}
private MyClass()
{
//private constructor makes it where this class can only be created by itself
}
}
The concept of global classes in C# is really just a simple matter of referencing the appropriate assembly containing the class. Once you have reference the needed assembly, you can refer to the class of choice either by it's fully qualified Type name, or by importing the namespace that contains the class. (Concrete instance or Static access to that class)
Or
You can have a Singleton class to use it everywhere but some people won't recommend you this way to proceed.
The other answers that you've been given about using a static class or a singleton pattern are correct.
Please consider, however, the fact that doing so does compromise your ability to test. In general, if you can, prefer dependency injection over globally accessed classes. I know this isn't always possible (or practical).
Just on that, you should also look up the abstract factory pattern. It allows you to have a well known factory class that produces the actual instance of a class that you're using. To have a globally accessed logging class, for example, don't directly create a logging class. Instead, use a logging factory to create it for you and return an interface to a logging class. That way it's easier to swap in and out different logging classes.
Since you do not want to create the object every time and it sounds like you are talking about some sort of utility methods...
I suggest you use static methods in an assembly which you can reference where needed
I'm new to mock objects, but I understand that I need to have my classes implement interfaces in order to mock them.
The problem I'm having is that in my data access layer, I want to have static methods, but I can't put a static method in an interface.
What's the best way around this? Should I just use instance methods (which seems wrong) or is there another solution?
Yes, you use instance methods. Static methods basically say, "There is one way to accomplish this functionality - it's not polymorphic." Mocking relies on polymorphism.
Now, if your static methods logically don't care about what implementation you're using, they might be able to take the interfaces as parameters, or perhaps work without interacting with state at all - but otherwise you should be using instances (and probably dependency injection to wire everything together).
I found a blog via google with some great examples on how to do this:
Refactor class to be an instance class and implement an interface.
You have already stated that you don't want to do this.
Use a wrapper instance class with delegates for static classes members
Doing this you can simulate a static interface via delegates.
Use a wrapper instance class with protected members which call the static class
This is probably the easiest to mock/manage without refactoring as it can just be inherited from and extended.
I would use a method object pattern. Have a static instance of this, and call it in the static method. It should be possible to subclass for testing, depending on your mocking framework.
i.e. in your class with the static method have:
private static final MethodObject methodObject = new MethodObject();
public static void doSomething(){
methodObject.doSomething();
}
and your method object can be a very simple, easily-tested:
public class MethodObject {
public void doSomething() {
// do your thang
}
}
You might be trying to test at too deep a starting point. A test does not need to be created to test each and every method individually; private and static methods should be tested by calling the public methods that then call the private and static ones in turn.
So lets say your code is like this:
public object GetData()
{
object obj1 = GetDataFromWherever();
object obj2 = TransformData(obj1);
return obj2;
}
private static object TransformData(object obj)
{
//Do whatever
}
You do not need to write a test against the TransformData method (and you can't). Instead write a test for the GetData method that tests the work done in TransformData.
Use instance methods where possible.
Use public static Func[T, U] (static function references that can be substituted for mock functions) where instance methods are not possible.
A simple solution is to allow to change the static class's implementation via a setter:
class ClassWithStatics {
private IClassWithStaticsImpl implementation = new DefaultClassWithStaticsImpl();
// Should only be invoked for testing purposes
public static void overrideImplementation(IClassWithStaticsImpl implementation) {
ClassWithStatics.implementation = implementation;
}
public static Foo someMethod() {
return implementation.someMethod();
}
}
So in the setup of your tests, you call overrideImplementation with some mocked interface. The benefit is that you don't need to change clients of your static class. The downside is that you probably will have a little duplicated code, because you'll have to repeat the methods of the static class and it's implementation. But some times the static methods can use a ligther interface which provide base funcionality.
The problem you have is when you're using 3rd party code and it's called from one of your methods. What we ended up doing is wrapping it in an object, and calling passing it in with dep inj, and then your unit test can mock 3rd party static method call the setter with it.
In C#, I have base class Product and derived class Widget.
Product contains a static method MyMethod().
I want to call static method Product.MyMethod() from static method Widget.MyMethod().
I can't use the base keyword, because that only works with instance methods.
I can call Product.MyMethod() explicitly, but if I later change Widget to derive from another class, I have to revise the method.
Is there some syntax in C# similar to base that allows me to call a static method from a base class from a static method of a derived class?
static methods are basically a method to fallback from object oriented concepts. As a consequence, they are not very flexible in inheritance hierarchies and it's not possible to do such a thing directly.
The closest thing I can think of is a using directive.
using mybaseclass = Namespace.BaseClass;
class MyClass : mybaseclass {
static void MyMethod() { mybaseclass.BaseStaticMethod(); }
}
It can be done, but I don't recommend it.
public class Parent1
{
public static void Foo()
{
Console.WriteLine("Parent1");
}
}
public class Child : Parent1
{
public new static void Foo()
{
Type parent = typeof(Child).BaseType;
MethodInfo[] methods = parent.GetMethods();
MethodInfo foo = methods.First(m => m.Name == "Foo");
foo.Invoke(null, null);
}
}
Calling a static method using reflection is exactly the same as calling an instance method except that you pass null for the instance. You need FlattenHierarchy because it's defined in an ancestor.
var type = assy.GetType("MyNamespace.MyType");
MethodInfo mi = type.GetMethod("MyStaticMethod",
BindingFlags.Static | BindingFlags.Public | BindingFlags.FlattenHierarchy);
mi.Invoke(null, null);
Further reading and thinking leaves me asking the same questions as others who have responded: why use static methods like this? Are you trying to do functional programming, and if so why not use lambda expressions instead? If you want polymophic behaviours with shared state, instance methods would be better.
It can be done:
public class Parent1
{
protected static void Foo()
{
Console.WriteLine("Parent1");
}
}
public class Child : Parent1
{
public static void Foo()
{
return Parent1.Foo();
}
}
Can be useful for unit testing protected static methods (for example).
First and foremost, if you're worried about re-parenting a class, then you're probably doing inheritance wrong. Inheritance should be used to establish "is-a" relationships, not simply foster code reuse. If you need code re-use alone, consider using delegation, rather than inheritance. I suppose you could introduce an intermediate type between a sub-type and its parent, but I would let that possibility drive my design.
Second, if you need to use functionality from the base class but extend it AND the use case calls for a static method, then you might want to consider using some external class to hold the functionality. The classic case for this in my mind is the Factory pattern. One way to implement the Factory pattern is through Factory Methods, a static method on a class that constructs an instance of that class. Usually the constructor is protected so that the factory method is the only way to build the class from outside.
One way to approach re-use with Factory Methods in an inheritance hierarchy would be to put the common code in a protected method and call that method from the Factory Method rather than directly call the base class Factory Method from a sub-types Factory Method. A better implementation might use the same technique but move the Factory Methods to a Factory class and use the constructor logic (internal now, not private), perhaps in conjunction with an initialization method(s), to create the object. If the behavior you are inheriting is external from the class (decryption/validation/etc), you can use shared methods (or composition) within the Factory to allow re-use between the Factory methods.
Without knowing the goal of your use of static methods it's difficult to give you an exact direction, but hopefully this will help.
Static methods are not polymorphic, so what you want to do is impossible.
Trying to find a way to treat static methods as polymorphic is possible but dangerous, because the language itself doesn't support it.
Some suggestions:
Reflection
Aliasing the base class (such as Mehrdad's example)
Having into account that a Static method shoudn't relay in instance data... you should have a static "MyMethod" that behaves diferent based on a parameter or something like that.
Remember that the static method defined in one class is just a way to order your code... but it is the same if that method is placed elsewhere... because it don't relay on the object where it is placed.
EDIT: I think your best choise is to use Product.MyMethod explicitly... If you think it... it should't be probable that your Widget change its base clase... and also in that case it is a minor change in code.
Static methods are "Class-Level" methods. They are intended to be methods that apply to all instances of a specific class. Thus, inheritance of this method does not make sense as it would change the meaning to apply to instances of the class. For instance, if you were looping through a collection of Products (some of Widget, some not) and called MyMethod on each Product then the method that was called would be determined by the instance of the class. This violates the purpose of static methods.
You can probably do something like you want more cleanly by simply not using static methods at all, because in your example it does not seem like MyMethod applies to all instances of Product. However, you can achieve the same affect you are describing by using an interface class like ‘IMyMethod’. This approach is still not using a static method. I guess I’m not seeing a need for a static method. Why do you want to use a static method to begin with?
It's very simple. Much simpler than using aliasing, reflections, etc. Maybe it's been made easier in newer additions of .NET, IDK, but this works perfectly fine. Just like with instance methods, accessing base methods doesn't require base be used, it is optional, usually necessary when the inheriting and base class have a method of the same name. Even without the base keyword, you can access the static methods of a base class as if they were in the class you are calling from. I only tested this when calling a base static method from a derived class's static method. Might not work if you are calling from an instance method to a static method.
public class BaseThings
{
protected static void AssertPermissions()
{
//something
}
}
public class Person:BaseThings
{
public static void ValidatePerson(Person person)
{
//just call the base static method as if it were in this class.
AssertPermissions();
}
}