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C# static interface or abstract implementation

I have a program that needs to be able to interface with multiple platforms ie read/write files, read/write database or read/write web requests. The platform interface is selected from configuration and does not change while the application is running. I have a single read/write interface class which is inherited by the platform specific classes so that this is abstracted from the rest of the program.
My problem is that I have 10 classes in my framework that will need to use this interface. Instead of making multiple instances of this class, or passing a single reference to every class, I figured it would make sense to make the interface static. Unfortunately I have just learned that Interfaces cannot have static methods, static methods cannot call non-static methods and static methods cannot be abstract.
Can anyone show me another method of approaching this situation?
Edit:
Thanks for everyone's input, here is my solution based on the example given by Patrick Hofman (thank you!)
interface TheInterface
{
void read();
void write();
}
public class X : TheInterface
{
public void read() { //do something }
public void write() { //do something }
}
public class Y : TheInterface
{
public void read() { //do something }
public void write() { //do something }
}
public class FileAccessor
{
public static TheInterface accessor;
public static TheInterface Accessor
{
get
{
if(accessor) return accessor;
}
}
}
This can be called by any class as:
static void Main(string[] args)
{
switch (Config.interface)
{
case "X":
FileAccessor.accessor = new Lazy<X>();
case "Y":
FileAccessor.accessor = new Lazy<Y>();
default:
throw new Lazy<Exception>("Unknown interface: " + Config.interface);
}
FileAccessor.Accessor.read();
}

Indeed, interfaces, or abstract classes can't be static themselves, but the further implementation can. Also, you can use the singleton pattern to make your life easier, and allow inheritance, etc.
public class X : ISomeInterface
{
private X() { }
public static X instance;
public static X Instance
{
get
{
return instance ?? (instance = new X());
}
}
}
Or, using Lazy<T>:
public class X : ISomeInterface
{
private X() { }
public static Lazy<X> instanceLazy = new Lazy<X>(() => new X());
public static X Instance
{
get
{
return instance.Value;
}
}
}

Disclaimer: I am the author of the library described below.
I don't know if this helps you, but I have written a library (very early version yet) that allows you to define static interfaces, by defining normal interfaces and decorating their methods with an attribute named [Static], for example:
public interface IYourInterface
{
[Static]
void DoTheThing();
}
(Note that you don't explicitly add this interface to your implementations.)
Once you have defined the interface, you can instantiate it from within your code with any valid implementation you choose:
return typeof(YourImplementation).ToStaticContract<IYourInterface>();
If the methods can't be found in YourImplementation, this call fails at runtime with an exception.
If the methods are found and this call is successful, then the client code can polymorphically call your static methods like this:
IYourInterface proxy = GetAnImplementation();
proxy.DoTheThing();

You can make a Static Class which has Variable of your Interface.
public static class StaticClass
{
public static ISomeInterface Interface;
}
Now you can access the Instance from everywhere in your Framwork
static void Main(string[] args)
{
StaticClass.Interface = new SomeClass();
}

How to handle a collection of Foo<T>, where T can be different for each item?

Problem description
I am trying to store a collection of generic Foo<T> elements, where T may be different for each item. I also have functions like DoSomething<T>(Foo<T>) that can accept a Foo<T> of any T. It seems like I should be able to call this function on each element of the abovementioned list, because they are all valid parameters for the function, but I can't seem to express this idea to the C# compiler.
The problem, as far as I can tell, is that I can't really express a list like that, because C# does not allow me to write Foo<T> without binding T. What I would want is something like Java's wildcard mechanism (Foo<?>). Here is how it might look in a Pseudo-C#, where this wildcard type existed:
class Foo<T> {
// ...
}
static class Functions {
public static void DoSomething<T>(Foo<T> foo) {
// ...
}
public static void DoSomething(List<Foo<?>> list) {
foreach(Foo<?> item in list)
DoSomething(item);
}
}
This pattern is valid in Java, but how can I do the same in C#? I have experimented a bit to find solutions which I'll post in an answer below, but I feel that there should be a better way.
Note: I have already solved this problem "well enough" for my practical needs, and I know ways to work around it (e.g. using the dynamic type), but I'd really like to see if there is a simpler solution that does not abandon static type safety.
Just using object or a nongeneric supertype, as has been suggested below, does not allow me to call functions that require a Foo<T>. However, this can be sensible even if I don't know anything about T. For example, I could use the Foo<T> to retrieve a List<T> list from somewhere, and a T value from somewhere else, and then call list.Add(value) and the compiler will know that all the types work out right.
Motivation
I was asked why I would ever need something like this, so I'm making up an example that is a bit closer to the everyday experience of most developers. Imagine that you are writing a bunch of UI components which allow the user to manipulate values of a certain type:
public interface IUiComponent<T> {
T Value { get; set; }
}
public class TextBox : IUiComponent<string> {
public string Value { get; set; }
}
public class DatePicker : IUiComponent<DateTime> {
public DateTime Value { get; set; }
}
Apart from the Value property, the components will have have many other members of course (e.g. OnChange events).
Now let's add an undo system. We shouldn't have to modify the UI elements themselves for this, because we have access to all the relevant data already--Just hook up the OnChange events and whenever the user changes a UI component, we store away the value of each IUiComponent<T> (A bit wasteful, but let's keep things simple). To store the values we will use a Stack<T> for each IUiComponent<T> in our form. Those lists are accessed by using the IUiComponent<T> as key. I'll leave out the details of how the lists are stored (If you think this matters I'll provide an implementation).
public class UndoEnabledForm {
public Stack<T> GetUndoStack<T>(IUiComponent<T> component) {
// Implementation left as an exercise to the reader :P
}
// Undo for ONE element. Note that this works and is typesafe,
// even though we don't know anything about T...
private void Undo<T>(IUiComponent<T> component) {
component.Value = GetHistory(component).Pop();
}
// ...but how do we implement undoing ALL components?
// Using Pseudo-C# once more:
public void Undo(List<IUiComponent<?>> components) {
foreach(IUiComponent<?> component in components)
Undo(component);
}
}
We could undo everything by directly calling Undo<T>() on all the IUiComponents (by name):
public void Undo(List<IUiComponent<?>> components) {
Undo(m_TextBox);
Undo(m_DatePicker);
// ...
}
However, I want to avoid this, because it means you will have to touch one more place in the code if you add/remove a component. If you have tens of fields and more functions that you want to perform on all the components (e.g. write all their values to a database and retrieve them again), this can become a lot of duplication.
Sample Code
Here is a small piece of code that you can use to develop/check a solution. The task is to put several Pair<T>-objects into some kind of collection object, and then call a function which accepts this collection object and swaps the First and Second field of each Pair<T> (using Application.Swap()). Ideally, you should not use any casts or reflection. Bonus points if you can manage to do it without modifying the Pair<T>-class in any way :)
class Pair<T> {
public T First, Second;
public override string ToString() {
return String.Format("({0},{1})", First, Second);
}
}
static class Application {
static void Swap<T>(Pair<T> pair) {
T temp = pair.First;
pair.First = pair.Second;
pair.Second = temp;
}
static void Main() {
Pair<int> pair1 = new Pair<int> { First = 1, Second = 2 };
Pair<string> pair2 = new Pair<string> { First = "first", Second = "second" };
// imagine more pairs here
// Silly solution
Swap(pair1);
Swap(pair2);
// Check result
Console.WriteLine(pair1);
Console.WriteLine(pair2);
Console.ReadLine();
}
}

I would suggest you define an interface to invoke the functions you'll want to call as DoSomething<T>(T param). In simplest form:
public interface IDoSomething
{ void DoSomething<T>(T param); }
Next define a base type ElementThatCanDoSomething:
abstract public class ElementThatCanDoSomething
{ abstract public void DoIt(IDoSomething action); }
and a generic concrete type:
public class ElementThatCanDoSomething><T>
{
T data;
ElementThatCanDoSomething(T dat) { data = dat; }
override public void DoIt(IDoSomething action)
{ action.DoIt<T>(data); }
}
Now it's possible to construct an element for any type compile-time T, and pass that element to a generic method, keeping type T (even if the element is null, or if the element is of a derivative of T). The exact implementation above isn't terribly useful, but it can be easily extended in many useful ways. For example, if type T had generic constraints in the interface and concrete type, the elements could be passed to methods which had those constraints on its parameter type (something which is otherwise very difficult, even with Reflection). It may also be useful to add versions of the interface and invoker methods that can accept pass-through parameters:
public interface IDoSomething<TX1>
{ void DoSomething<T>(T param, ref TX1 xparam1); }
... and within the ElementThatCanToSomething
abstract public void DoIt<TX1>(IDoSomething<TX1> action, ref TX1 xparam1);
... and within the ElementThatCanToSomething<T>
override public void DoIt<TX1>(IDoSomething<TX1> action, ref TX1 xparam1)
{ action.DoIt<T>(data, ref xparam1); }
The pattern may easily be extended to any number of pass-through parameters.

EDIT 2: in the case of your overhauled question, the approach is basically the same I've proposed you earlier.
Here I'm adapting it to your scenario and commenting better on what makes it work (plus an unfortunate "gotcha" with value types...)
// note how IPair<T> is covariant with T (the "out" keyword)
public interface IPair<out T> {
T First {get;}
T Second {get;}
}
// I get no bonus points... I've had to touch Pair to add the interface
// note that you can't make classes covariant or contravariant, so I
// could not just declare Pair<out T> but had to do it through the interface
public class Pair<T> : IPair<T> {
public T First {get; set;}
public T Second {get; set;}
// overriding ToString is not strictly needed...
// it's just to "prettify" the output of Console.WriteLine
public override string ToString() {
return String.Format("({0},{1})", First, Second);
}
}
public static class Application {
// Swap now works with IPairs, but is fully generic, type safe
// and contains no casts
public static IPair<T> Swap<T>(IPair<T> pair) {
return new Pair<T>{First=pair.Second, Second=pair.First};
}
// as IPair is immutable, it can only swapped in place by
// creating a new one and assigning it to a ref
public static void SwapInPlace<T>(ref IPair<T> pair) {
pair = new Pair<T>{First=pair.Second, Second=pair.First};
}
// now SwapAll works, but only with Array, not with List
// (my understanding is that while the Array's indexer returns
// a reference to the actual element, List's indexer only returns
// a copy of its value, so it can't be switched in place
public static void SwapAll(IPair<object>[] pairs) {
for(int i=0; i < pairs.Length; i++) {
SwapInPlace(ref pairs[i]);
}
}
}
That's more or less it... Now in your main you can do:
var pairs = new IPair<object>[] {
new Pair<string>{First="a", Second="b"},
new Pair<Uri> {
First=new Uri("http://www.site1.com"),
Second=new Uri("http://www.site2.com")},
new Pair<object>{First=1, Second=2}
};
Application.SwapAll(pairs);
foreach(var p in pairs) Console.WriteLine(p.ToString());
OUTPUT:
(b,a)
(http://www.site2.com/,http://www.site1.com/)
(2,1)
So, your Array is type-safe, because it can only contain Pairs (well, IPairs). The only gotcha is with value types. As you can see I had to declare the last element of the array as a Pair<object> instead of Pair<int> as I would have liked.
This is because covariance/contravariance don't work with value types so I had to box int in an object.
=========
EDIT 1 (old, just leaving there as reference to make sense of the comments below):
you could have both a non-generic marker interface for when you need to act on the container (but don't care about the "wrapped" type) and a covariant generic one for when you need the type information.
Something like:
interface IFoo {}
interface IFoo<out T> : IFoo {
T Value {get;}
}
class Foo<T> : IFoo<T> {
readonly T _value;
public Foo(T value) {this._value=value;}
public T Value {get {return _value;}}
}
Suppose you have this simple hierarchy of classes:
public class Person
{
public virtual string Name {get {return "anonymous";}}
}
public class Paolo : Person
{
public override string Name {get {return "Paolo";}}
}
you could have functions that work either on any IFoo (when you don't care if Foo wraps a Person) or specifically on IFoo<Person> (when you do care):
e.g.
static class Functions
{
// this is where you would do DoSomethingWithContainer(IFoo<?> foo)
// with hypothetical java-like wildcards
public static void DoSomethingWithContainer(IFoo foo)
{
Console.WriteLine(foo.GetType().ToString());
}
public static void DoSomethingWithGenericContainer<T>(IFoo<T> el)
{
Console.WriteLine(el.Value.GetType().ToString());
}
public static void DoSomethingWithContent(IFoo<Person> el)
{
Console.WriteLine(el.Value.Name);
}
}
which you could use like this:
    // note that IFoo can be covariant, but Foo can't,
    // so we need a List<IFoo
var lst = new List<IFoo<Person>>
{
new Foo<Person>(new Person()),
new Foo<Paolo>(new Paolo())
};
foreach(var p in lst) Functions.DoSomethingWithContainer(p);
foreach(var p in lst) Functions.DoSomethingWithGenericContainer<Person>(p);
foreach(var p in lst) Functions.DoSomethingWithContent(p);
// OUTPUT (LinqPad)
// UserQuery+Foo`1[UserQuery+Person]
// UserQuery+Foo`1[UserQuery+Paolo]
// UserQuery+Person
// UserQuery+Paolo
// anonymous
// Paolo
One notable thing in the output is that even the function that only received IFoo still had and printed the full type information which in java would have been lost with type erasure.

It seems that in C#, you have to create a list of Foo, which you use as base type of Foo<T>. However, you can't easily get back to Foo<T> from there.
One solution I found is to add an abstract method to Foo for each function SomeFn<T>(Foo<T>), and implement them in Foo<T> by calling SomeFn(this). However, that would mean that every time you want to define a new (external) function on Foo<T>, you have to add a forwarding function to Foo, even though it really shouldn't have to know about that function:
abstract class Foo {
public abstract void DoSomething();
}
class Foo<T> : Foo {
public override void DoSomething() {
Functions.DoSomething(this);
}
// ...
}
static class Functions {
public static void DoSomething<T>(Foo<T> foo) {
// ...
}
public static void DoSomething(List<Foo> list) {
foreach(Foo item in list)
item.DoSomething();
}
}
A slightly cleaner solution from a design perspective seems to be a Visitor pattern which generalizes the above approach to a degree and severs the coupling between Foo and the specific generic functions, but that makes the whole thing even more verbose and complicated.
interface IFooVisitor {
void Visit<T>(Foo<T> foo);
}
class DoSomethingFooVisitor : IFooVisitor {
public void Visit<T>(Foo<T> foo) {
// ...
}
}
abstract class Foo {
public abstract void Accept(IFooVisitor foo);
}
class Foo<T> : Foo {
public override void Accept(IFooVisitor foo) {
foo.Visit(this);
}
// ...
}
static class Functions {
public static void DoSomething(List<Foo> list) {
IFooVisitor visitor = new DoSomethingFooVisitor();
foreach (Foo item in list)
item.Accept(visitor);
}
}
This would almost be a good solution IMO, if it was easier to create a Visitor. Since C# apparently does not allow generic delegates/lambdas, you cannot specify the visitor inline and take advantage of closures though - As far as I can tell, each Visitor needs to be a new explicitly defined class with possible extra parameters as fields. The Foo type also has to explicitly support this scheme by implementing the Visitor pattern.

For those who may still find this interesting, here is the best solution I could come up with that also meets the "bonus requirement" of not touching the original type in any way. It is basically a Visitor pattern with the twist that we don't store the Foo<T> directly in our container, but rather store a delegate which calls an IFooVisitor on our Foo<T>. Notice how we can easily make a list of those because T is not actually part of the delegates' type.
// The original type, unmodified
class Pair<T> {
public T First, Second;
}
// Interface for any Action on a Pair<T>
interface IPairVisitor {
void Visit<T>(Pair<T> pair);
}
class PairSwapVisitor : IPairVisitor {
public void Visit<T>(Pair<T> pair) {
Application.Swap(pair);
}
}
class PairPrintVisitor : IPairVisitor {
public void Visit<T>(Pair<T> pair) {
Console.WriteLine("Pair<{0}>: ({1},{2})", typeof(T), pair.First, pair.Second);
}
}
// General interface for a container that follows the Visitor pattern
interface IVisitableContainer<T> {
void Accept(T visitor);
}
// The implementation of our Pair-Container
class VisitablePairList : IVisitableContainer<IPairVisitor> {
private List<Action<IPairVisitor>> m_visitables = new List<Action<IPairVisitor>>();
public void Add<T>(Pair<T> pair) {
m_visitables.Add(visitor => visitor.Visit(pair));
}
public void Accept(IPairVisitor visitor) {
foreach (Action<IPairVisitor> visitable in m_visitables)
visitable(visitor);
}
}
static class Application {
public static void Swap<T>(Pair<T> pair) {
T temp = pair.First;
pair.First = pair.Second;
pair.Second = temp;
}
static void Main() {
VisitablePairList list = new VisitablePairList();
list.Add(new Pair<int> { First = 1, Second = 2 });
list.Add(new Pair<string> { First = "first", Second = "second" });
list.Accept(new PairSwapVisitor());
list.Accept(new PairPrintVisitor());
Console.ReadLine();
}
}
Output:
Pair<System.Int32>: (2,1)
Pair<System.String>: (second,first)

static abstract class

I need a way to create a static class where some constants can be case specific, but hard-coded.
What I really want to do is have a class where several constants are provided when the class is extended - I want the 'constants' hard-coded. I figured I will make the some abstract properties and define the get { return constant; } when extending the class.
I know that is not possible, so now I am facing two options and am wondering what would be best and why (if there are options I'm missing please let me know!)
Create a static class with nullable fields and throw an exception if the fields are null when the static method is called.
Give up the static class. Have a non-static class with abstract properties and create an instance of the object wherever I need it even though all the functionality really is static.
I know this might be subjective and case-dependant, however I am going around in circles when thinking about this and could really do with some external input. That plus I hope there might be away of doing what I want and I'm just thinking about this wrong.
Update: Code: I will try to write some code that describes what I'd like to accomplish. I know this code can't work!
Imagine that the abstract class Calculation is in a dll, used by many projects. The functionality is the same for all of them, just the Constant varies from project to project.
public abstract static class Calculation
{
private abstract int Constant { get; } //The constant is unknown at this time
public static int Calculate(int inputValue)
{
return inputValue * Constant;
}
}
The class Calc is defined in a separate project where the functionality is needed and the Constant is known.
public static class Calc : Calculation
{
private override int Constant { get { return 2; }
}
...
static class Program
{
[STAThread]
static void Main()
{
//At some point:
int result = Calc.Calculate(6);
}
}
I suppose the simplest way would be to create a non-static class and create an instance, however I fear having several instances of the class could be expensive and would like to prevent that if possible.
I can't see how I could write this as a singleton pattern without writing it again in each project - having only the Nested class in the dll. That doesn't prevent the implementor to just create an ordinary class and is likely to restart the debate for every project where the code is used.
Update #2 : What I ment with option one is this:
Class in a dll:
public static class Calculation
{
public int? Constant {get; set;}
public static int Calculate(int inputValue)
{
if (Constant == null)
throw new ArgumentNullException();
return inputValue * (int)Constant;
}
}
Usage of the function in a seperate project:
static class Program
{
[STAThread]
static void Main()
{
//At some point:
Calculation.Constant = 2;
int result = Calc.Calculate(6);
}
}
Option one is very simple and elegant, what bothers me about it that nothing forces the implementor to set the Constant. I fear an (admittedly unlikely) scenario where an obscure corner case will cause the property to not be set and for the code to fail (and Constant beeing the last suspect)...

You could make non-static classes that follow singleton, ensuring only one instance of the object ever to exist. I guess that could be the next best thing.

You can't want static and inheritance at the same time ! It simply does not make sense !
If you need to override behavior, you need inheritance !
If you want simplicity of call (one of the advantage of statics), you can use Factory (or singleton if only one instance is needed)
My guess is that you probably have to rethink your model. This set of constants of yours probably represent something that you could extract in a separate class then pass this class to your static method. Would that fit your needs ?

Edit
To your code sample:
public abstract static class Calculation
{
public static int Constant { get; set; }
public static int Calculate(int i) { return i * Constant; }
}
// ...
Calculation.Constant = 6;
Calculation.Calculate(123);
Somewhat more general:
public abstract static class Calculation
{
public struct Context
{
public int Constant, SignificantDigits;
public bool Radians;
}
public static int Calculate(int i, Context ctx) { return i * ctx.Constant; }
}
// ...
Calculation.Calculate(123, new Calculate.Context { Constant = 6 });
First idea:
The closest I can think of is generics:
public interface ISpecifics
{
void DoSomething();
string SomeProp { get; }
}
public static class Static<S>
where S : ISpecifics, new()
{
public static string ExerciseSpecific()
{
var spec = new S();
spec.DoSomething();
return spec.SomeProp;
}
}
Or if you really need a single static type
public static class Static
{
public static string ExerciseSpecific<S>()
where S : ISpecifics, new()
{
var spec = new S();
spec.DoSomething();
return spec.SomeProp;
}
}
Does that help?

I needed pretty much the same thing, so first I made a non-static class with all the functionality.
Then, a static class which instantiates one such non-static class in its static constructor.
Then any of the static methods calls the respective instance methods.
Something like this:
public class CalculationInstance
{
private int constant;
public int Calculate(int inputValue)
{
return inputValue * constant;
}
public void AnyOtherMethod()
{
....
}
public CalculationInstance(int constant)
{
this.constant=constant;
}
}
public static class Calculation
{
const int CONSTANT=2;
private CalculationInstance calc;
static Calculation()
{
calc=new CalculationInstance(CONSTANT);
}
public static int Calculate(int inputValue)
{
return calc.Calculate(inputValue);
}
public static void AnyOtherMethod()
{
calc.AnyOtherMethod();
}
}
static class Program
{
[STAThread]
static void Main()
{
//At some point:
int result = Calculation.Calculate(6);
}
}

I feel this doesn't make sense here, a static class is by default sealed class which means it is sealed for inheritance. So please don't consider having static with abstract.
You can have an abstract class and the child class can inherit and override the methods.

How can I overload a C# method by specific instances of a generic type

Coming from a C++ background, I've run into a snag with overloading based on a specific instance of a generic type. The following doesn't work since only once instance of the code for the Foo<T> class is ever generated, so inside the Method, the type of this is simply Foo<T>, not Foo<A> or Foo<B> as I'd hoped. In C++ I'm used to templates being instantiated as unique types.
using System.Collections.Generic;
class A
{
// Concrete class
}
class B
{
// Concrete class
}
class Bar
{
public void OverloadedMethod(Foo<A> a) {} // do some A related stuff
public void OverloadedMethod(Foo<B> b) {} // do some B related stuff
public void OverloadedMethod(OtherFoo of) {} // do some other stuff
public void VisitFoo(FooBase fb) { fb.Method(this); }
}
abstract class FooBase
{
public abstract void Method(Bar b);
}
class Foo<T> : FooBase
{
// Class that deals with As and Bs in an identical fashion.
public override void Method(Bar b)
{
// Doesn't compile here
b.OverloadedMethod(this);
}
}
class OtherFoo : FooBase
{
public override void Method(Bar b)
{
b.OverloadedMethod(this);
}
}
class Program
{
static void Main(string[] args)
{
List<FooBase> ListOfFoos = new List<FooBase>();
ListOfFoos.Add(new OtherFoo());
ListOfFoos.Add(new Foo<A>());
ListOfFoos.Add(new Foo<B>());
Bar b = new Bar();
foreach (FooBase fb in ListOfFoos)
b.VisitFoo(fb);
// Hopefully call each of the Bar::Overloaded methods
}
}
Is there a way to get something like this to work in C#? I'd rather not have to duplicate the code in Foo as separate classes for every type I want to use it for.
Edit:
Hopefully this is a little clearer.

I now have a genuinely complete piece of code which demonstrates the problem. Note to OP: please try compiling your code before posting it. There were a bunch of things I had to do to get this far. It's good to make it as easy as possible for other people to help you. I've also removed a bunch of extraneous bits. OtherFoo isn't really relevant here, nor is FooBase.
class A {}
class B {}
class Bar
{
public static void OverloadedMethod(Foo<A> a) { }
public static void OverloadedMethod(Foo<B> b) { }
}
class Foo<T>
{
// Class that deals with As and Bs in an identical fashion.
public void Method()
{
// Doesn't compile here
Bar.OverloadedMethod(this);
}
}
Yes, this doesn't compile. What did you expect it to do, exactly? Bear in mind that the overload resolution is performed at compile time, not execution time. As fallen888 says, you could cast and call the appropriate overloaded method - but which of the two overloads would you expect the compiler to pick otherwise? What do you want it to do with Foo<string> instead of Foo<A> or Foo<B>?
This all goes to demonstrate that .NET generics are indeed significantly different from C++ templates, of course...

I haven't tried it but it seems you should be able to achieve what you want by making A & B visitable (e.g. with the acyclic visitor pattern).

This works for the static case. Dealing with instance functions would be a bit more complicated. This post from Jon Skeet might provide a reasonable way to deal with instance methods.
class Program
{
static void Main(string[] args)
{
var testA = new Foo<A>();
testA.Method();
var testB = new Foo<B>();
testB.Method();
Console.ReadLine();
var testString = new Foo<string>(); //Fails
testString.Method();
Console.ReadLine();
}
}
class A { }
class B { }
class Bar
{
public static void OverloadedMethod(Foo<A> a)
{
Console.WriteLine("A");
}
public static void OverloadedMethod(Foo<B> b)
{
Console.WriteLine("B");
}
}
class Foo<T>
{
static Foo()
{
overloaded = (Action<Foo<T>>)Delegate.CreateDelegate(typeof(Action<Foo<T>>), typeof(Bar).GetMethod("OverloadedMethod", new Type[] { typeof(Foo<T>) }));
}
public void Method()
{
overloaded(this);
}
private static readonly Action<Foo<T>> overloaded;
}

Edit: I'm not sure that you can complete this as you're attempting. I've tried all sorts of tricks to attempt to get this to work and can't get it to compile. The best I can do is to pull the method call outside of my Generic class. If your method call is outside, then you can specifically define what T is in the generic. However, inside the method, at compile time, the compiler doesn't know what T will be so it doesn't know which overloaded method to call. The only way I can see around this is to use a switch to determine the type of T and manually specify the overload to call.
The best I can do is this, which isn't quite what you're after, but it could be used to a similar effect:
class Stuff<T>
{
public T value { get; set; }
}
class Program
{
static void DummyFunc(Stuff<int> inst)
{
Console.WriteLine("Stuff<int>: {0}", inst.value.ToString());
}
static void DummyFunc(Stuff<string> inst)
{
Console.WriteLine("Stuff<string>: {0}", inst.value);
}
static void DummyFunc(int value)
{
Console.WriteLine("int: {0}", value.ToString());
}
static void DummyFunc(string value)
{
Console.WriteLine("string: {0}", value);
}
static void Main(string[] args)
{
var a = new Stuff<string>();
a.value = "HelloWorld";
var b = new Stuff<int>();
b.value = 1;
var c = "HelloWorld";
var d = 1;
DummyFunc(a);
DummyFunc(b);
DummyFunc(c);
DummyFunc(d);
}
}
and got output:
Stuff<string>: HelloWorld
Stuff<int>: 1
string: HelloWorld
int: 1
I've got four overloaded functions referencing two referencing generic classes (one for int and one for string) and two referencing regular types (one for int and one for string) and it all works okay... is this what you're after?
Edit: The problem doesn't seem to be with the calling of the overloaded methods, it has to do with your foreach which is trying to convert all items in the list to the same type as the first in order to reference the overloaded method. The first item that doesn't conform to that exact definition will cause your compile to fail.

I was hoping to find an easier way to do this but for now I'm going with this:
Replace Foo<T> class with these classes:
abstract class Foo<T> : FooBase
{
// Class that deals with As and Bs in an identical fashion.
}
class Foo_A : Foo<A>
{
public override void Method(Bar b)
{
b.OverloadedMethod(this);
}
}
class Foo_B : Foo<B>
{
public override void Method(Bar b)
{
// Doesn't compile here
b.OverloadedMethod(this);
}
}
And change the instantiations to
List<FooBase> ListOfFoos = new List<FooBase>();
ListOfFoos.Add(new OtherFoo());
ListOfFoos.Add(new Foo_A());
ListOfFoos.Add(new Foo_B());
This at least doesn't require dublicating the code in Foo<T>, and just requires me to forward the constructors.

Hiding inherited members

I'm looking for some way to effectively hide inherited members. I have a library of classes which inherit from common base classes. Some of the more recent descendant classes inherit dependency properties which have become vestigial and can be a little confusing when using IntelliSense or using the classes in a visual designer.
These classes are all controls that are written to be compiled for either WPF or Silverlight 2.0. I know about ICustomTypeDescriptor and ICustomPropertyProvider, but I'm pretty certain those can't be used in Silverlight.
It's not as much a functional issue as a usability issue. What should I do?
Update
Some of the properties that I would really like to hide come from ancestors that are not my own and because of a specific tool I'm designing for, I can't do member hiding with the new operator. (I know, it's ridiculous)

Override them like Michael Suggests above and to prevent folks from using the overridden (sp?) methods, mark them as obsolete:
[Obsolete("These are not supported in this class.", true)]
public override void dontcallmeanymore()
{
}
If the second parm is set to true, a compiler error will be generated if anyone tries to call that method and the string in the first parm is the message. If parm2 is false only a compiler warning will be generated.

While you cannot prevent usage of those inherited members to my knowledge, you should be able to hide them from IntelliSense using the EditorBrowsableAttribute:
Using System.ComponentModel;
[EditorBrowsable(EditorBrowsableState.Never)]
private string MyHiddenString = "Muahahahahahahahaha";
Edit: Just saw this in the documentation comments, which makes it kinda useless for this purpose:
There is a prominent note that states that this attribute "does not suppress members from a class in the same assembly". That is true but not complete. Actually, the attribute does not suppress members from a class in the same solution.

One potential thing you can do is contain the object rather than extend from the other class. This will give you the most flexibility in terms of exposing what you want to expose, but if you absolutely need the object to be of that type it is not the ideal solution (however you could expose the object from a getter).
Thus:
public class MyClass : BaseClass
{
// Your stuff here
}
Becomes:
public class MyClass
{
private BaseClass baseClass;
public void ExposeThisMethod()
{
baseClass.ExposeThisMethod();
}
}
Or:
public class MyClass
{
private BaseClass baseClass;
public BaseClass BaseClass
{
get
{
return baseClass;
}
}
}

I think you're best least hackish way is to consider composition as opposed to inheritance.
Or, you could create an interface that has the members you want, have your derived class implement that interface, and program against the interface.

I know there's been several answers to this, and it's quite old now, but the simplest method to do this is just declare them as new private.
Consider an example I am currently working on, where I have an API that makes available every method in a 3rd party DLL. I have to take their methods, but I want to use a .Net property, instead of a "getThisValue" and "setThisValue" method. So, I build a second class, inherit the first, make a property that uses the get and set methods, and then override the original get and set methods as private. They're still available to anyone wanting to build something different on them, but if they just want to use the engine I'm building, then they'll be able to use properties instead of methods.
Using the double class method gets rid of any restrictions on being unable to use the new declaration to hide the members. You simply can't use override if the members are marked as virtual.
public class APIClass
{
private static const string DllName = "external.dll";
[DllImport(DllName)]
public extern unsafe uint external_setSomething(int x, uint y);
[DllImport(DllName)]
public extern unsafe uint external_getSomething(int x, uint* y);
public enum valueEnum
{
On = 0x01000000;
Off = 0x00000000;
OnWithOptions = 0x01010000;
OffWithOptions = 0x00010000;
}
}
public class APIUsageClass : APIClass
{
public int Identifier;
private APIClass m_internalInstance = new APIClass();
public valueEnum Something
{
get
{
unsafe
{
valueEnum y;
fixed (valueEnum* yPtr = &y)
{
m_internalInstance.external_getSomething(Identifier, yPtr);
}
return y;
}
}
set
{
m_internalInstance.external_setSomething(Identifier, value);
}
}
new private uint external_setSomething(int x, float y) { return 0; }
new private unsafe uint external_getSomething(int x, float* y) { return 0; }
}
Now valueEnum is available to both classes, but only the property is visible in the APIUsageClass class. The APIClass class is still available for people who want to extend the original API or use it in a different way, and the APIUsageClass is available for those who want something more simple.
Ultimately, what I'll be doing is making the APIClass internal, and only expose my inherited class.

To fully hide and mark not to use, including intellisense which I believe is what most readers expect
[Obsolete("Not applicable in this class.")]
[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
[Browsable(false), EditorBrowsable(EditorBrowsableState.Never)]

I tested all of the proposed solutions and they do not really hide new members.
But this one DOES:
[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
public new string MyHiddenProperty
{
get { return _myHiddenProperty; }
}
But in code-behide it's still accessible, so add as well Obsolete Attribute
[Obsolete("This property is not supported in this class", true)]
[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
public new string MyHiddenProperty
{
get { return _myHiddenProperty; }
}

While clearly stated above that it is not possible in C# to change the access modifiers on inherited methods and properties, I overcame this issue through a sort of "fake inheritance" using implicit casting.
Example:
public class A
{
int var1;
int var2;
public A(int var1, int var2)
{
this.var1 = var1;
this.var2 = var2;
}
public void Method1(int i)
{
var1 = i;
}
public int Method2()
{
return var1+var2;
}
}
Now lets say you want a class B to inherit from class A, but want to change some accessibility or even change Method1 entirely
public class B
{
private A parent;
public B(int var1, int var2)
{
parent = new A(var1, var2);
}
int var1
{
get {return this.parent.var1;}
}
int var2
{
get {return this.parent.var2;}
set {this.parent.var2 = value;}
}
public Method1(int i)
{
this.parent.Method1(i*i);
}
private Method2()
{
this.parent.Method2();
}
public static implicit operator A(B b)
{
return b.parent;
}
}
By including the implicit cast at the end, it allows us to treat B objects as As when we need to. It can also be useful to define an implicit cast from A->B.
The biggest flaw to this approach is that you need to re-write every method/property that you intend to "inherit".
There's probably even more flaws to this approach, but I like to use it as a sort of "fake inheritance".
Note:
While this allows for changing the accessibility of public properties, it doesn't solve the issue of making protected properties public.

You can use an interface
public static void Main()
{
NoRemoveList<string> testList = ListFactory<string>.NewList();
testList.Add(" this is ok ");
// not ok
//testList.RemoveAt(0);
}
public interface NoRemoveList<T>
{
T this[int index] { get; }
int Count { get; }
void Add(T item);
}
public class ListFactory<T>
{
private class HiddenList: List<T>, NoRemoveList<T>
{
// no access outside
}
public static NoRemoveList<T> NewList()
{
return new HiddenList();
}
}