When I run code analysis (VS2013) with the 'Microsoft Managed Recommend Rules' rule set, the only warnings I get for my class library are of type CA1033: 'Interface methods should be callable by child types'. But I don't understand the rule in this situation:
/// An object that has a chemical formula
public interface IChemicalFormula
{
/// The chemical formula of the object
ChemicalFormula ChemicalFormula {get;}
}
public class ChemicalFormula: IChemicalFormula
{
ChemicalFormula IChemicalFormula.ChemicalFormula
{
get { return this; }
}
}
The docs recommends making a protected method with the same name so that deriving types can access it, but you cannot name a method the same as the enclosing type. They also recommend making the class sealed, but I don't want it to be sealed in this case. Is this a time just to ignore this rule, or is there an appropriate way to handle it?
EDIT
To add clarification why the class/interface is designed this way, I have another class, Peptide that contains a IChemicalFormula[] array to store modifications. Not every modification necessarily derives directly from ChemicalFormula, but they need to implement the IChemicalFormula interface. Therefore, if I modify an instance of a peptide withsome molecule (H2O for example), then ChemicalFormula class needs to also implement IChemicalFormula.
This is the description of the rule:
Consider a base type that explicitly implements a public interface
method. A type that derives from the base type can access the
inherited interface method only through a reference to the current
instance (this in C#) that is cast to the interface. If the derived
type re-implements (explicitly) the inherited interface method, the
base implementation can no longer be accessed. The call through the
current instance reference will invoke the derived implementation;
this causes recursion and an eventual stack overflow.
I think you should consider evaluating the usage of this property. A good example where TDD could be used to figure out the interface. There are some possible usages (and some invalid ones) below. I am not yet sure what you intend to achieve by looking at those.
In your example, let's say another class, NewChemicalForumla is derived from ChemicalForumula, and references ChemicalFormula, what does that mean?
public class NewChemicalFormula: ChemicalFormula
{
public void Method()
{
Console.WriteLine("{0}", ChemicalFormula.GetType()); // Compile error
Console.WriteLine("{0}", this.ChemicalFormula.GetType()); // Effectively same as above, compile error
Console.WriteLine("{0}", ((IChemicalFormula)this).ChemicalFormula.GetType()); // Works, is that what you intend?
}
}
Now from outside the class, there are two possibilities:
When you have a handle to a derived class:
new NewChemicalFormula().ChemicalFormula.GetType() // Error
or
// This works, is that what you intend to achieve?
((IChemicalFormula)new NewChemicalFormula()).ChemicalFormula.GetType()
When you have a handle to the IChemicalFormula already. In this case, ChemicalFormula seems redundant:
IChemicalFormula formula = new NewChemicalFormula();
Console.WriteLine("{0}", formula.GetType()); // Works, returns NewChemicalFormula
Console.WriteLine("{0}", formula.ChemicalFormula.GetType()); // Works, returns NewChemicalFormula
Console.WriteLine("{0}", formula.ChemicalFormula.Method()); // Compile error
formula.ChemicalFormula.Method() leads to an error because you must cast it to NewChemicalFormula before you can use Method(). Just because the property returns this doesn't help solve this problem.
So the FXCop warning is worth considering, and evaluating the design.
This question came to mind while I was writing a class that iterates over a list, with methods next() and previous() that will continuously loop (e.g. if at the last object, return it, and then reset index to 0)
In the constructor I was pondering adding a boolean variable, which if true would just act like a regular iterator with only next() methods and no looping. In this case, having the method previous() would make no sense. So I'm curious, is it possible to hide the previous() method in this case. Is it possible to achieve this somehow in Java or C#?.
What about other languages?
C#
It is possible by making the two methods part of two different interfaces, and casting the object to one of the two interfaces. For example:
interface ILoopingIterator
{
void Next();
void Previous();
}
interface INonLoopingIterator
{
void Next();
}
class PlaysItBothWays : ILoopingIterator, INonLoopingIterator
{
void ILoopingIterator.Next()
{
this.NextCore();
}
void ILoopingIterator.Previous()
{
// since this code will never be shared anyway, put it here
}
void INonLoopingIterator.Next()
{
this.NextCore();
}
private void NextCore()
{
// do stuff here; this method only exists so that code can be shared
}
}
Note that I have made the class implement both interfaces explicitly; this way, users of instances are forced to select which "mode" they want to use the class in. You could implement only one interface explicitly instead (providing a "default" mode that can be changed).
and now:
var looping = (ILoopingIterator) new PlaysItBothWays(); // selects mode A
var nonLooping = (INonLoopingIterator) new PlaysItBothWays(); // selects mode B
Of course this does not stop anyone from casting the instance to the "other" interface if they want to, but if the programmer wants to subvert their own code they can also use reflection which allows much more than that.
Java
In Java, the above is not possible. You can come close by having the class expose methods that return instances of one of the two interfaces, and using the returned value. Of course then the object is really a factory and not a service provider, so that's feels like cheating on the problem.
class PlaysItBothWays
{
public ILoopingIterator asLooping() { return /* something */ }
public INonLoopingIterator asNonLooping() { return /* something else */ }
}
Rather than passing a boolean to a constructor, you should simply use inheritance.
Suppose you have a base iterator that supports only next(). If that's the only functionality you need, instantiate it.
To provide more functionality, inherit from this base iterator, make a class called TwoWayIterator or something like that, and provide a previous() method.
Both of these classes will share a common super class, so you can treat them as one, and you can hide the previous() method by treating an instance as its base class.
It is not possible to hide a method like that in a statically typed language. The best you can do is implement the method to throw an exception (or equivalent) if the method is called.
There are tricks that you can do to make it appear like the methods are not there. For instance, having the class implement two interfaces, and using different factory methods to create them. However, they don't work if the constructor is used directly, or if you want the choice to be determined by the value of a constructor or factory method parameter.
Not in Java. You can't "Hide" methods at runtime. I'd suggest you to create Two interfaces
, one with the next method and the other one extending the first one and adding the "previous" method. Then, you can have 2 factories methods to create an instance of one of these classes.
Please take a look to the Java "Iterator" class
interface Iterator<T> {
T next();
}
interface LoopingIterator<T> extends Iterator<T>{
T previous();
}
Then you can cast them. Similar to the previous C# answer
You can't hide class members at run time (well, not in C# anyway - not sure about Java). If you so worried about Previous() method being used in the context where it is not doing anything useful, then simply have it throw InvalidOperationException in that case.
It is also worth noting that .NET already has standard "iterator" interface. It is called IEnumerable (and generic version IEnumerable<T>) and is forward-only.
Ok so I'm currently working with a set of classes that I don't have control over in some pretty generic functions using these objects. Instead of writing literally tens of functions that essentially do the same thing for each class I decided to use a generic function instead.
Now the classes I'm dealing with are a little weird in that the derived classes share many of the same properties but the base class that they are derived from doesn't. One such property example is .Parent which exists on a huge number of derived classes but not on the base class and it is this property that I need to use.
For ease of understanding I've created a small example as follows:
class StandardBaseClass {} // These are simulating the SMO objects
class StandardDerivedClass : StandardBaseClass {
public object Parent { get; set; }
}
static class Extensions
{
public static object GetParent(this StandardDerivedClass sdc) {
return sdc.Parent;
}
public static object GetParent(this StandardBaseClass sbc)
{
throw new NotImplementedException("StandardBaseClass does not contain a property Parent");
}
// This is the Generic function I'm trying to write and need the Parent property.
public static void DoSomething<T>(T foo) where T : StandardBaseClass
{
object Parent = ((T)foo).GetParent();
}
}
In the above example calling DoSomething() will throw the NotImplemented Exception in the base class's implementation of GetParent(), even though I'm forcing the cast to T which is a StandardDerivedClass.
This is contrary to other casting behaviour where by downcasting will force the use of the base class's implementation.
I see this behaviour as a bug. Has anyone else out there encountered this?
I see this behaviour as a bug.
This behavior is correct. Since your method DoSomething is constraining T to StandardBaseClass, you only have access to the specific methods of StandardBaseClass, not any methods or properties of a derived class. Since StandardBaseClass does not have a Parent property, this is invalid, and should be invalid, by design.
There are two potential options here - You can use reflection to pull out the Parent property, or use C# 4's dynamic type, and treat this as a dynamic object. Both bypass the standard type checking in the compiler, however, so will require you to do extra type checking at runtime to verify that the Parent property exists.
Create an interface that contains the Parent property. Have each class that has a Parent property implement that interace. You will then be able to create a generic method that accepts a parameter of type IHaveParent, and it will do the right thing.
For anyone that is interested an succinct answer to this situation is answered by Stephen Cleary on msdn here:
http://social.msdn.microsoft.com/Forums/en-AU/csharpgeneral/thread/95833bb3-fbe1-4ec9-8b04-3e05165e20f8?prof=required
To me this is a divergence in the class hierarchy. By this this I mean that either the base class has parent, or the derived classes with Parent are derived from an abstract child of the base.
Lol as John says, an interface as opposed to an abstract class is sufficient too.
You idea won't work because the compiler can never guarantee that the base class actually would have such a property. And it won't just select the "right" one based on if it has it or not.
The only way you can do this is using reflection and then test at runtime if the requested property exists on the inspected class. You have to judge yourself if that is a viable way to do for your project (reflection is slow and requires maximum rights).
This is correct, as the compiler only knows that it can bind to your type as a StandardBaseClass. The binding is not done at runtime (where it could potentially decide to use the StandardDerivedClass overload.
If you know that it's a StandardDerivedClass, then why not just cast it as such?
object Parent = ((StandardDerivedClass)foo).Parent;
It's a bit ugly, but you can accomplish this using a Registration system, where you register delegates for different possible derived classes that expose the 'shared' property/method and then use something like a Dictionary<Type,Func<SomeT>> to store the delegates. If you know all of the derived types ahead of time and don't have to load plug-ins or the like, you can also use the classic ugly if/else-if structure. Either way you're basically creating your own substitute for what should have been supported by the virtual method table.
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.