This is more of a documentation than a real question. This does not seem to have been addressed on SO yet (unless I missed it), so here goes:
Imagine a generic class that contains a static member:
class Foo<T> {
public static int member;
}
Is there a new instance of the member for each specific class, or is there only a single instance for all Foo-type classes?
It can easily be verified by code like this:
Foo<int>.member = 1;
Foo<string>.member = 2;
Console.WriteLine (Foo<int>.member);
What is the result, and where is this behavior documented?
A static field is shared across all instances of the same type. Foo<int> and Foo<string> are two different types. This can be proven by the following line of code:
// this prints "False"
Console.WriteLine(typeof(Foo<int>) == typeof(Foo<string>));
As for where this is documented, the following is found in section 1.6.5 Fields of the C# Language Specification (for C# 3):
A static field identifies exactly one
storage location. No matter how many
instances of a class are created,
there is only ever one copy of a
static field.
As stated before; Foo<int> and Foo<string> are not the same class; they are two different classes constructed from the same generic class. How this happens is outlined in section 4.4 of the above mentioned document:
A generic type declaration, by itself,
denotes an unbound generic type that
is used as a “blueprint” to form many
different types, by way of applying
type arguments.
The problem here is actually the fact that "generic classes" are not classes at all.
Generic class definitions are just templates for classes, and until their type parameters are specified, they are just a piece of text (or a handful of bytes).
At runtime, one can specify a type parameter for the template, thus bringing it to life, and creating a class of the, now, fully specified type. That's why static properties are not template-wide, and that's why you cannot cast between List<string> and List<int>.
That relationship kinda mirrors the class-object relationship. Just like classes do not exist* until you instantiate an object from them, generic classes do not exist, until you make a class based on the template.
P.S. It's quite possible to declare
class Foo<T> {
public static T Member;
}
From this is kinda obvious that the static members cannot be shared, as T is different for different specializations.
They are not shared. Not sure where it's documented but analysis warning CA1000 (Do not declare static members on generic types) warns against just this due to the risk of making the code more complicated.
C# implementation of generics is more closer to C++. In both of these languages MyClass<Foo> and MyClass<Bar> don't share static members but in Java they do. In C# and C++ MyClass<Foo> internally creates entirely new type at compile time as if generics are kind of macros. You can usually see their generated names in stack trace, like MyClass'1 and MyClass'2. This is why they don't share static variables. In Java, generics are implemented by more simpler method of compiler generating code using non-generic types and adding type casts all over. So MyClass<Foo> and MyClass<Bar> don't generate two entirely new class in Java, instead they both are same class MyClass underneath and that's why they share static variables.
They are not really shared.
Because the member doesn't belong to the instance at all.
A static class member belongs to the class itself.
So, if you have MyClass.Number it is the same for all MyClass.Number objects because it not even depends on the object.
You can even call or modify MyClass.Number without any object.
But since Foo< int > is not the same class as Foo< string > these two numbers are not shared.
An example to show this:
TestClass<string>.Number = 5;
TestClass<int>.Number = 3;
Console.WriteLine(TestClass<string>.Number); //prints 5
Console.WriteLine(TestClass<int>.Number); //prints 3
IMO, you need to test it, but I think that
Foo<int>.member = 1;
Foo<string>.member = 2;
Console.WriteLine (Foo<int>.member);
will output 1 because I think that, during compilation, the compilator create 1 class for every generic class you use (in you example : Foo<int> and Foo<string>).
But I'm not 100% sure =).
Remark : I think it's not a good design nor a good practice to use such kind of static attributes.
I have a situation where I would like to have objects of a certain type be able to be used as two different types. If one of the "base" types was an interface this wouldn't be an issue, but in my case it is preferable that they both be concrete types.
I am considering adding copies of the methods and properties of one of the base types to the derived type, and adding an implicit conversion from the derived type to that base type. Then users will be able treat the derived type as the base type by using the duplicated methods directly, by assigning it to a variable of the base type, or by passing it to a method that takes the base type.
It seems like this solution will fit my needs well, but am I missing anything? Is there a situation where this won't work, or where it is likely to add confusion instead of simplicity when using the API?
EDIT: More details about my specific scenario:
This is for a potential future redesign of the way indicators are written in RightEdge, which is an automated trading system development environment. Price data is represented as a series of bars, which have values for the open, low, high, and close prices for a given period (1 minute, 1 day, etc). Indicators perform calculations on series of data. An example of a simple indicator is the moving average indicator, which gives the moving average of the most recent n values of its input, where n is user-specified. The moving average might be applied to the bar close, or it could be applied to the output of another indicator to smooth it out.
Each time a new bar comes in, the indicators compute the new value for their output for that bar.
Most indicators have only one output series, but sometimes it is convenient to have more than one output (see MACD), and I want to support this.
So, indicators need to derive from a "Component" class which has the methods that are called when new data comes in. However, for indicators which have only one output series (and this is most of them), it would be good for them to act as a series themselves. That way, users can use SMA.Current for the current value of an SMA, instead of having to use SMA.Output.Current. Likewise, Indicator2.Input = Indicator1; is preferable to Indicator2.Input = Indicator1.Output;. This may not seem like much of a difference, but a lot of our target customers are not professional .NET developers so I want to make this as easy as possible.
My idea is to have an implicit conversion from the indicator to its output series for indicators that have only one output series.
You don't provide too many details, so here is an attempt to answering from what you provide.
Take a look at the basic differences:
When you have a base type B and a derived type D, an assignment like this:
B my_B_object = my_D_object;
assigns a reference to the same object. On the other hand, when B and D are independent types with an implicit conversion between them, the above assignment would create a copy of my_D_object and store it (or a reference to it if B is a class) on my_B_object.
In summary, with "real" inheritance works by reference (changes to a reference affect the object shared by many references), while custom type conversions generally work by value (that depends on how you implement it, but implementing something close to "by reference" behavior for converters would be nearly insane): each reference will point to its own object.
You say you don't want to use interfaces, but why? Using the combo interface + helper class + extension methods (C# 3.0 and .Net 3.5 or newer required) can get quite close to real multiple inheritance. Look at this:
interface MyType { ... }
static class MyTypeHelper {
public static void MyMethod(this MyType value) {...}
}
Doing that for each "base" type would allow you to provide default implementations for the methods you want to.
These won't behave as virtual methods out-of-the-box; but you may use reflection to achieve that; you would need to do the following from within the implementation on the Helper class:
retrieve a System.Type with value.GetType()
find if that type has a method matching the signature
if you find a matching method, invoke it and return (so the rest of the Helper's method is not run).
Finally, if you found no specific implementation, let the rest of the method run and work as a "base class implementation".
There you go: multiple inheritance in C#, with the only caveat of requiring some ugly code in the base classes that will support this, and some overhead due to reflection; but unless your application is working under heavy pressure this should do the trick.
So, once again, why you don't want to use interfaces? If the only reason is their inability to provide method implementations, the trick above solves it. If you have any other issue with interfaces, I might try to sort them out, but I'd have to know about them first ;)
Hope this helps.
[EDIT: Addition based on the comments]
I've added a bunch of details to the original question. I don't want to use interfaces because I want to prevent users from shooting themselves in the foot by implementing them incorrectly, or accidentally calling a method (ie NewBar) which they need to override if they want to implement an indicator, but which they should never need to call directly.
I've looked at your updated question, but the comment quite summarizes it. Maybe I'm missing something, but interfaces + extensions + reflection can solve everything multiple inheritance could, and fares far better than implicit conversions at the task:
Virtual method behavior (an implementation is provided, inheritors can override): include method on the helper (wrapped in the reflection "virtualization" described above), don't declare on the interface.
Abstract method behavior (no implementation provided, inheritors must implement): declare method on the interface, don't include it on the helper.
Non-virtual method behavior (an implementation is provided, inheritors may hide but can't override): Just implement it as normal on the helper.
Bonus: weird method (an implementation is provided, but inheritors must implement anyway; they may explicitly invoke the base implementation): that's not doable with normal or multiple inheritance, but I'm including it for completeness: that's what you'd get if you provide an implementation on the helper and also declare it on the interface. I'm not sure of how would that work (on the aspect of virtual vs. non-virtual) or what use it'd have, but hey, my solution has already beaten multiple inheritance :P
Note: On the case of the non-virtual method, you'd need to have the interface type as the "declared" type to ensure that the base implementation is used. That's exactly the same as when an inheritor hides a method.
I want to prevent users from shooting themselves in the foot by implementing them incorrectly
Seems that non-virtual (implemented only on the helper) will work best here.
or accidentally calling a method (ie NewBar) which they need to override if they want to implement an indicator
That's where abstract methods (or interfaces, which are a kind of super-abstract thing) shine most. The inheritor must implement the method, or the code won't even compile. On some cases virtual methods may do (if you have a generic base implementation but more specific implementations are reasonable).
but which they should never need to call directly
If a method (or any other member) is exposed to client code but shouldn't be called from client code, there is no programmatic solution to enforce that (actually, there is, bear with me). The right place to address that is on the documentation. Because you are documenting you API, aren't you? ;) Neither conversions nor multiple inheritance could help you here. However, reflection may help:
if(System.Reflection.Assembly.GetCallingAssembly()!=System.Reflection.Assembly.GetExecutingAssembly())
throw new Exception("Don't call me. Don't call me!. DON'T CALL ME!!!");
Of course, you may shorten that if you have a using System.Reflection; statement on your file. And, BTW, feel free to change the Exception's type and message to something more descriptive ;).
I see two issues:
User-defined type conversion operators are generally not very discoverable -- they don't show up in IntelliSense.
With an implicit user-defined type conversion operator, it's often not obvious when the operator is applied.
This doesn't been you shouldn't be defining type conversion operators at all, but you have to keep this in mind when designing your solution.
An easily discoverable, easily recognizable solution would be to define explicit conversion methods:
class Person { }
abstract class Student : Person
{
public abstract decimal Wage { get; }
}
abstract class Musician : Person
{
public abstract decimal Wage { get; }
}
class StudentMusician : Person
{
public decimal MusicianWage { get { return 10; } }
public decimal StudentWage { get { return 8; } }
public Musician AsMusician() { return new MusicianFacade(this); }
public Student AsStudent() { return new StudentFacade(this); }
}
Usage:
void PayMusician(Musician musician) { GiveMoney(musician, musician.Wage); }
void PayStudent(Student student) { GiveMoney(student, student.Wage); }
StudentMusician alice;
PayStudent(alice.AsStudent());
It doesn't sound as if your method would support a cross-cast. True multiple inheritance would.
An example from C++, which has multiple inheritance:
class A {};
class B {};
class C : public A, public B {};
C o;
B* pB = &o;
A* pA = dynamic_cast<A*>(pB); // with true MI, this succeeds
Then users will be able treat the derived type as the base type by using the duplicated methods directly, by assigning it to a variable of the base type, or by passing it to a method that takes the base type.
This will behave differently, however. In the case of inheritance, you're just passing your object. However, by implementing an implicit converter, you'll always be constructing a new object when the conversion takes place. This could be very unexpected, since it will behave quite differently in the two cases.
Personally, I'd make this a method that returns the new type, since it would make the actual implementation obvious to the end user.
Maybe I'm going too far off with this, but your use case sounds suspiciously as if it could heavily benefit from building on Rx (Rx in 15 Minutes).
Rx is a framework for working with objects that produce values. It allows such objects to be composed in a very expressive way and to transform, filter and aggregate such streams of produced values.
You say you have a bar:
class Bar
{
double Open { get; }
double Low { get; }
double High { get; }
double Close { get; }
}
A series is an object that produces bars:
class Series : IObservable<Bar>
{
// ...
}
A moving average indicator is an object that produces the average of the last count bars whenever a new bar is produced:
static class IndicatorExtensions
{
public static IObservable<double> MovingAverage(
this IObservable<Bar> source,
int count)
{
// ...
}
}
The usage would be as follows:
Series series = GetSeries();
series.MovingAverage(20).Subscribe(average =>
{
txtCurrentAverage.Text = average.ToString();
});
An indicator with multiple outputs is similar to GroupBy.
This might be a stupid idea, but: if your design requires multiple inheritance, then why don't you simply use a language with MI? There are several .NET languages which support multiple inheritance. Off the top of my head: Eiffel, Python, Ioke. There's probable more.
Why was C# designed this way?
As I understand it, an interface only describes behaviour, and serves the purpose of describing a contractual obligation for classes implementing the interface that certain behaviour is implemented.
If classes wish to implement that behavour in a shared method, why shouldn't they?
Here is an example of what I have in mind:
// These items will be displayed in a list on the screen.
public interface IListItem {
string ScreenName();
...
}
public class Animal: IListItem {
// All animals will be called "Animal".
public static string ScreenName() {
return "Animal";
}
....
}
public class Person: IListItem {
private string name;
// All persons will be called by their individual names.
public string ScreenName() {
return name;
}
....
}
Assuming you are asking why you can't do this:
public interface IFoo {
void Bar();
}
public class Foo: IFoo {
public static void Bar() {}
}
This doesn't make sense to me, semantically. Methods specified on an interface should be there to specify the contract for interacting with an object. Static methods do not allow you to interact with an object - if you find yourself in the position where your implementation could be made static, you may need to ask yourself if that method really belongs in the interface.
To implement your example, I would give Animal a const property, which would still allow it to be accessed from a static context, and return that value in the implementation.
public class Animal: IListItem {
/* Can be tough to come up with a different, yet meaningful name!
* A different casing convention, like Java has, would help here.
*/
public const string AnimalScreenName = "Animal";
public string ScreenName(){ return AnimalScreenName; }
}
For a more complicated situation, you could always declare another static method and delegate to that. In trying come up with an example, I couldn't think of any reason you would do something non-trivial in both a static and instance context, so I'll spare you a FooBar blob, and take it as an indication that it might not be a good idea.
My (simplified) technical reason is that static methods are not in the vtable, and the call site is chosen at compile time. It's the same reason you can't have override or virtual static members. For more details, you'd need a CS grad or compiler wonk - of which I'm neither.
For the political reason, I'll quote Eric Lippert (who is a compiler wonk, and holds a Bachelor of Mathematics, Computer science and Applied Mathematics from University of Waterloo (source: LinkedIn):
...the core design principle of static methods, the principle that gives them their name...[is]...it can always be determined exactly, at compile time, what method will be called. That is, the method can be resolved solely by static analysis of the code.
Note that Lippert does leave room for a so-called type method:
That is, a method associated with a type (like a static), which does not take a non-nullable “this” argument (unlike an instance or virtual), but one where the method called would depend on the constructed type of T (unlike a static, which must be determinable at compile time).
but is yet to be convinced of its usefulness.
Most answers here seem to miss the whole point. Polymorphism can be used not only between instances, but also between types. This is often needed, when we use generics.
Suppose we have type parameter in generic method and we need to do some operation with it. We dont want to instantinate, because we are unaware of the constructors.
For example:
Repository GetRepository<T>()
{
//need to call T.IsQueryable, but can't!!!
//need to call T.RowCount
//need to call T.DoSomeStaticMath(int param)
}
...
var r = GetRepository<Customer>()
Unfortunately, I can come up only with "ugly" alternatives:
Use reflection
Ugly and beats the idea of interfaces and polymorphism.
Create completely separate factory class
This might greatly increase the complexity of the code. For example, if we are trying to model domain objects, each object would need another repository class.
Instantiate and then call the desired interface method
This can be hard to implement even if we control the source for the classes, used as generic parameters. The reason is that, for example we might need the instances to be only in well-known, "connected to DB" state.
Example:
public class Customer
{
//create new customer
public Customer(Transaction t) { ... }
//open existing customer
public Customer(Transaction t, int id) { ... }
void SomeOtherMethod()
{
//do work...
}
}
in order to use instantination for solving the static interface problem we need to do the following thing:
public class Customer: IDoSomeStaticMath
{
//create new customer
public Customer(Transaction t) { ... }
//open existing customer
public Customer(Transaction t, int id) { ... }
//dummy instance
public Customer() { IsDummy = true; }
int DoSomeStaticMath(int a) { }
void SomeOtherMethod()
{
if(!IsDummy)
{
//do work...
}
}
}
This is obviously ugly and also unnecessary complicates the code for all other methods. Obviously, not an elegant solution either!
I know it's an old question, but it's interesting. The example isn't the best. I think it would be much clearer if you showed a usage case:
string DoSomething<T>() where T:ISomeFunction
{
if (T.someFunction())
...
}
Merely being able to have static methods implement an interface would not achieve what you want; what would be needed would be to have static members as part of an interface. I can certainly imagine many usage cases for that, especially when it comes to being able to create things. Two approaches I could offer which might be helpful:
Create a static generic class whose type parameter will be the type you'd be passing to DoSomething above. Each variation of this class will have one or more static members holding stuff related to that type. This information could supplied either by having each class of interest call a "register information" routine, or by using Reflection to get the information when the class variation's static constructor is run. I believe the latter approach is used by things like Comparer<T>.Default().
For each class T of interest, define a class or struct which implements IGetWhateverClassInfo<T> and satisfies a "new" constraint. The class won't actually contain any fields, but will have a static property which returns a static field with the type information. Pass the type of that class or struct to the generic routine in question, which will be able to create an instance and use it to get information about the other class. If you use a class for this purpose, you should probably define a static generic class as indicated above, to avoid having to construct a new descriptor-object instance each time. If you use a struct, instantiation cost should be nil, but every different struct type would require a different expansion of the DoSomething routine.
None of these approaches is really appealing. On the other hand, I would expect that if the mechanisms existed in CLR to provide this sort of functionality cleanly, .net would allow one to specify parameterized "new" constraints (since knowing if a class has a constructor with a particular signature would seem to be comparable in difficulty to knowing if it has a static method with a particular signature).
Short-sightedness, I'd guess.
When originally designed, interfaces were intended only to be used with instances of class
IMyInterface val = GetObjectImplementingIMyInterface();
val.SomeThingDefinedinInterface();
It was only with the introduction of interfaces as constraints for generics did adding a static method to an interface have a practical use.
(responding to comment:) I believe changing it now would require a change to the CLR, which would lead to incompatibilities with existing assemblies.
To the extent that interfaces represent "contracts", it seems quiet reasonable for static classes to implement interfaces.
The above arguments all seem to miss this point about contracts.
Interfaces specify behavior of an object.
Static methods do not specify a behavior of an object, but behavior that affects an object in some way.
Because the purpose of an interface is to allow polymorphism, being able to pass an instance of any number of defined classes that have all been defined to implement the defined interface... guaranteeing that within your polymorphic call, the code will be able to find the method you are calling. it makes no sense to allow a static method to implement the interface,
How would you call it??
public interface MyInterface { void MyMethod(); }
public class MyClass: MyInterface
{
public static void MyMethod() { //Do Something; }
}
// inside of some other class ...
// How would you call the method on the interface ???
MyClass.MyMethod(); // this calls the method normally
// not through the interface...
// This next fails you can't cast a classname to a different type...
// Only instances can be Cast to a different type...
MyInterface myItf = MyClass as MyInterface;
Actually, it does.
As of Mid-2022, the current version of C# has full support for so-called static abstract members:
interface INumber<T>
{
static abstract T Zero { get; }
}
struct Fraction : INumber<Fraction>
{
public static Fraction Zero { get; } = new Fraction();
public long Numerator;
public ulong Denominator;
....
}
Please note that depending on your version of Visual Studio and your installed .NET SDK, you'll either have to update at least one of them (or maybe both), or that you'll have to enable preview features (see Use preview features & preview language in Visual Studio).
See more:
https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/tutorials/static-virtual-interface-members
https://blog.ndepend.com/c-11-static-abstract-members/
https://khalidabuhakmeh.com/static-abstract-members-in-csharp-10-interfaces#:~:text=Static%20abstract%20members%20allow%20each,like%20any%20other%20interface%20definition.
Regarding static methods used in non-generic contexts I agree that it doesn't make much sense to allow them in interfaces, since you wouldn't be able to call them if you had a reference to the interface anyway. However there is a fundamental hole in the language design created by using interfaces NOT in a polymorphic context, but in a generic one. In this case the interface is not an interface at all but rather a constraint. Because C# has no concept of a constraint outside of an interface it is missing substantial functionality. Case in point:
T SumElements<T>(T initVal, T[] values)
{
foreach (var v in values)
{
initVal += v;
}
}
Here there is no polymorphism, the generic uses the actual type of the object and calls the += operator, but this fails since it can't say for sure that that operator exists. The simple solution is to specify it in the constraint; the simple solution is impossible because operators are static and static methods can't be in an interface and (here is the problem) constraints are represented as interfaces.
What C# needs is a real constraint type, all interfaces would also be constraints, but not all constraints would be interfaces then you could do this:
constraint CHasPlusEquals
{
static CHasPlusEquals operator + (CHasPlusEquals a, CHasPlusEquals b);
}
T SumElements<T>(T initVal, T[] values) where T : CHasPlusEquals
{
foreach (var v in values)
{
initVal += v;
}
}
There has been lots of talk already about making an IArithmetic for all numeric types to implement, but there is concern about efficiency, since a constraint is not a polymorphic construct, making a CArithmetic constraint would solve that problem.
Because interfaces are in inheritance structure, and static methods don't inherit well.
What you seem to want would allow for a static method to be called via both the Type or any instance of that type. This would at very least result in ambiguity which is not a desirable trait.
There would be endless debates about whether it mattered, which is best practice and whether there are performance issues doing it one way or another. By simply not supporting it C# saves us having to worry about it.
Its also likely that a compilier that conformed to this desire would lose some optimisations that may come with a more strict separation between instance and static methods.
You can think of the static methods and non-static methods of a class as being different interfaces. When called, static methods resolve to the singleton static class object, and non-static methods resolve to the instance of the class you deal with. So, if you use static and non-static methods in an interface, you'd effectively be declaring two interfaces when really we want interfaces to be used to access one cohesive thing.
To give an example where I am missing either static implementation of interface methods or what Mark Brackett introduced as the "so-called type method":
When reading from a database storage, we have a generic DataTable class that handles reading from a table of any structure. All table specific information is put in one class per table that also holds data for one row from the DB and which must implement an IDataRow interface. Included in the IDataRow is a description of the structure of the table to read from the database. The DataTable must ask for the datastructure from the IDataRow before reading from the DB. Currently this looks like:
interface IDataRow {
string GetDataSTructre(); // How to read data from the DB
void Read(IDBDataRow); // How to populate this datarow from DB data
}
public class DataTable<T> : List<T> where T : IDataRow {
public string GetDataStructure()
// Desired: Static or Type method:
// return (T.GetDataStructure());
// Required: Instantiate a new class:
return (new T().GetDataStructure());
}
}
The GetDataStructure is only required once for each table to read, the overhead for instantiating one more instance is minimal. However, it would be nice in this case here.
FYI: You could get a similar behavior to what you want by creating extension methods for the interface. The extension method would be a shared, non overridable static behavior. However, unfortunately, this static method would not be part of the contract.
Interfaces are abstract sets of defined available functionality.
Whether or not a method in that interface behaves as static or not is an implementation detail that should be hidden behind the interface. It would be wrong to define an interface method as static because you would be unnecessarily forcing the method to be implemented in a certain way.
If methods were defined as static, the class implementing the interface wouldn't be as encapsulated as it could be. Encapsulation is a good thing to strive for in object oriented design (I won't go into why, you can read that here: http://en.wikipedia.org/wiki/Object-oriented). For this reason, static methods aren't permitted in interfaces.
Static classes should be able to do this so they can be used generically. I had to instead implement a Singleton to achieve the desired results.
I had a bunch of Static Business Layer classes that implemented CRUD methods like "Create", "Read", "Update", "Delete" for each entity type like "User", "Team", ect.. Then I created a base control that had an abstract property for the Business Layer class that implemented the CRUD methods. This allowed me to automate the "Create", "Read", "Update", "Delete" operations from the base class. I had to use a Singleton because of the Static limitation.
Most people seem to forget that in OOP Classes are objects too, and so they have messages, which for some reason c# calls "static method".
The fact that differences exist between instance objects and class objects only shows flaws or shortcomings in the language.
Optimist about c# though...
OK here is an example of needing a 'type method'. I am creating one of a set of classes based on some source XML. So I have a
static public bool IsHandled(XElement xml)
function which is called in turn on each class.
The function should be static as otherwise we waste time creating inappropriate objects.
As #Ian Boyde points out it could be done in a factory class, but this just adds complexity.
It would be nice to add it to the interface to force class implementors to implement it. This would not cause significant overhead - it is only a compile/link time check and does not affect the vtable.
However, it would also be a fairly minor improvement. As the method is static, I as the caller, must call it explicitly and so get an immediate compile error if it is not implemented. Allowing it to be specified on the interface would mean this error comes marginally earlier in the development cycle, but this is trivial compared to other broken-interface issues.
So it is a minor potential feature which on balance is probably best left out.
The fact that a static class is implemented in C# by Microsoft creating a special instance of a class with the static elements is just an oddity of how static functionality is achieved. It is isn't a theoretical point.
An interface SHOULD be a descriptor of the class interface - or how it is interacted with, and that should include interactions that are static. The general definition of interface (from Meriam-Webster): the place or area at which different things meet and communicate with or affect each other. When you omit static components of a class or static classes entirely, we are ignoring large sections of how these bad boys interact.
Here is a very clear example of where being able to use interfaces with static classes would be quite useful:
public interface ICrudModel<T, Tk>
{
Boolean Create(T obj);
T Retrieve(Tk key);
Boolean Update(T obj);
Boolean Delete(T obj);
}
Currently, I write the static classes that contain these methods without any kind of checking to make sure that I haven't forgotten anything. Is like the bad old days of programming before OOP.
C# and the CLR should support static methods in interfaces as Java does. The static modifier is part of a contract definition and does have meaning, specifically that the behavior and return value do not vary base on instance although it may still vary from call to call.
That said, I recommend that when you want to use a static method in an interface and cannot, use an annotation instead. You will get the functionality you are looking for.
Static Methods within an Interface are allowed as of c# 9 (see https://www.dotnetcurry.com/csharp/simpler-code-with-csharp-9).
I think the short answer is "because it is of zero usefulness".
To call an interface method, you need an instance of the type. From instance methods you can call any static methods you want to.
I think the question is getting at the fact that C# needs another keyword, for precisely this sort of situation. You want a method whose return value depends only on the type on which it is called. You can't call it "static" if said type is unknown. But once the type becomes known, it will become static. "Unresolved static" is the idea -- it's not static yet, but once we know the receiving type, it will be. This is a perfectly good concept, which is why programmers keep asking for it. But it didn't quite fit into the way the designers thought about the language.
Since it's not available, I have taken to using non-static methods in the way shown below. Not exactly ideal, but I can't see any approach that makes more sense, at least not for me.
public interface IZeroWrapper<TNumber> {
TNumber Zero {get;}
}
public class DoubleWrapper: IZeroWrapper<double> {
public double Zero { get { return 0; } }
}
As per Object oriented concept Interface implemented by classes and
have contract to access these implemented function(or methods) using
object.
So if you want to access Interface Contract methods you have to create object. It is always must that is not allowed in case of Static methods. Static classes ,method and variables never require objects and load in memory without creating object of that area(or class) or you can say do not require Object Creation.
Conceptually there is no reason why an interface could not define a contract that includes static methods.
For the current C# language implementation, the restriction is due to the allowance of inheritance of a base class and interfaces. If "class SomeBaseClass" implements "interface ISomeInterface" and "class SomeDerivedClass : SomeBaseClass, ISomeInterface" also implements the interface, a static method to implement an interface method would fail compile because a static method cannot have same signature as an instance method (which would be present in base class to implement the interface).
A static class is functionally identical to a singleton and serves the same purpose as a singleton with cleaner syntax. Since a singleton can implement an interface, interface implementations by statics are conceptually valid.
So it simply boils down to the limitation of C# name conflict for instance and static methods of the same name across inheritance. There is no reason why C# could not be "upgraded" to support static method contracts (interfaces).
An interface is an OOPS concept, which means every member of the interface should get used through an object or instance. Hence, an interface can not have static methods.
When a class implements an interface,it is creating instance for the interface members. While a static type doesnt have an instance,there is no point in having static signatures in an interface.
I imagine everyone has seen code like:
public void Server2ClientEnumConvert( ServerEnum server)
{
switch(server)
{
case ServerEnum.One:
return ClientEnum.ABC
//And so on.
Instead of this badness we could do somthing like:
public enum ServerEnum
{
[Enum2Enum(ClientEnum.ABC)]
One,
}
Now we can use reflection to rip through ServerEnum and get the conversion mappings from the enum declaration itself.
The problem I am having here is in the declaration of the Enum2Enum attribute.
This works but replacing object o with Enum e does not. I do not want to be able to pass in objects to the constructor, only other enums.
public class EnumToEnumAttribute : Attribute
{
public EnumToEnumAttribute(object o){}
}
This fails to compile.
public class EnumToEnumAttribute : Attribute
{
public EnumToEnumAttribute(Enum e){}
}
Is there a reason for the compile error? How else could I pass in the information needed to map besides:
EnumtoEnumAttribute(Type dest, string enumString)
This seems too verbose but if it is the only way then I guess I will use it.
Using almost the same example, you can achieve this directly in the enum:
public enum ServerEnum
{
One = ClientEnum.ABC,
}
This has the benefit of not requiring Reflection, is easier to read (in my opinion), and overall requires less overhead.
I would probably use struct as the type, and then throw an exception if it isn't an Enum type. I don't see how your (Type, string) option is any safer than using object or struct.
#Danial Jennings I read through the rules there and found: "An enum type, provided it has public accessibility and the types in which it is nested (if any) also have public accessibility (Section 17.2).".
How does trying for Enum e in the constructor fail based on the quoted rule? Is it because being of type enum does not guarantee that the enums passed in are publicly visibly? This seems right. Is there a way for force this rule at compile time?
# bdukes You are exactly correct. I should have thought about that more.
It looks like run time type checking is my only option to make sure I am only mapping enums to other enums.
Why not use a Dictionary? This could be a static property of your class, initialized with those fancy schmancy object initializers we got in 3.0? You would not be typing more code (the mapping has to be done even with the Attribute sollution).
Here are the rules for the types that can be included as Attribute parameters: