Working with C# Generics you can have a class like this:
class Foo<T> where T:new() {}
Which means that the type T should have a constructor without parameters. It would be nice if we could have:
class Foo<T> where T : new(string)
{
private T CreateItem()
{
string s="";
return new T(s);
}
}
Is there any reason that Microsoft haven't added this feature to the language?
Is there any reason that Microsoft haven't added this feature to the language?
The feature you describe is a specific case of the of the more general feature "allow a constraint that requires a particular method to exist". For example, you might say:
void M<T>(T t) where T has an accessible method with signature double Foo(int)
{
double x = t.Foo(123);
}
We don't have that feature in C# because features have to be justified by a cost-benefit analysis. That would be a pretty expensive feature from both a design and implementation point of view -- a feature that would drive requirements onto not just C# but every .NET language. What's the compelling benefit that justifies the feature?
Moreover: suppose we did design that feature. How would it be implemented efficiently? The constraints in the generic type system have been carefully designed so that the jitter can generate efficient code once that can then be shared for every reference type. How would we generate efficient code for arbitrary method pattern matching? That sort of efficient dispatch is pretty straightforward when the method's slot can be known at compile time; with this feature we would no longer have that advantage.
The feature you want is the same feature, just with the kind of method restricted to a constructor.
Remember, the purpose of generics is to let you write generically typed code. If you're requiring constraints that are more specific than things that can be captured in the type system then you might be trying to abuse generics.
Rather than try to guess why Microsoft decided on a particular implementation, here's a workaround for you, using the factory pattern
public interface IFactory<T>
{
T CreateItem(string s);
}
class Foo<TFactory,T> where TFactory : IFactory<T>, new()
{
private T CreateItem()
{
var factory = new TFactory();
string s="";
return factory.CreateItem(s);
}
}
Using this pattern, say you have a class Bar which has a constructor taking a single string:
public class Bar
{
public Bar(string laa)
{}
}
You just need a BarFactory which implements IFactory<Bar>
public class BarFactory : IFactory<Bar>
{
public BarFactory () {}
public Bar CreateItem(string s)
{
return new Bar(s);
}
}
Now you can use that factory with Foo
var foo = new Foo<BarFactory,Bar>(); // calls to CreateItem() will construct a Bar
The solution I adopted when I wanted a generic function that invoked a constructor with arguments was to use reflection to find and invoke it.
Given that I had control of both the generic, and all the classes it was implemented over (and this function was the only place those classes were constructed), I think I might have been better to give the classes a default constructor and added an Initialize method in the interface they all implemented.
(A solution that allowed adding static methods in general to an interface would be ideal.)
Related
Basically, I have the following scenario:
public abstract class FooBase<T> where T : FooBase<T>
{
public bool IsSpecial { get; private set; }
public static T GetSpecialInstance()
{
return new T() { IsSpecial = true };
}
}
public sealed class ConcreteFooA : FooBase<ConcreteFooA> { ... }
public sealed class ConcreteFooB : FooBase<ConcreteFooB> { ... }
But, the problem I see here is that I could have done ConcreteFooB : FooBase<ConcreteFooA> { ... }, which would completely mess up the class at runtime (it wouldn't meet the logic I'm trying to achieve), but still compile correctly.
Is there some way I haven't thought of to enforce the generic, T, to be whatever the derived class is?
Update: I do end up using the generic parameter, T, in the FooBase<T> class, I just didn't list every method that has it as an out and in parameter, but I do have a use for T.
To answer your question:
No, there is no compile time solution to enforce this.
There are a couple of ways to enforce this rule:
Unit Testing - You could write up a unit test (or unit tests) to ensure that the compiled types are passing themselves in as the generic parameter.
Code Analysis - You could create a custom code analysis rule that enforces this, and then set that rule as an error (vs warning). This would be checked at compile-time.
FxCop Rule - Similar to the Code Analysis rule, except if you don't have a version of Visual Studio that has built-in support for Code Analysis, then you can use FxCop instead.
Of course, none of these rules are enforced on a standard compilation, but instead require additional tools (Unit Testing, Code Analysis, FxCop). If someone took your code and compiled it without using these tools you'd run into the same issue... of course, at that point why is someone else compiling your code without running your unit tests or Code Analysis/FxCop rules?
Alternatively, and I don't recommend this, you could throw a run-time error. Why not? According to Microsoft:
If a static constructor throws an exception, the runtime will not
invoke it a second time, and the type will remain uninitialized for
the lifetime of the application domain in which your program is
running.
That really doesn't solve your issue. On top of that, throwing an exception during static initialization is a violation of Code Analysis CA1065:DoNotRaiseExceptionsInUnexpectedLocations. So, you're going in the wrong direction if you do this.
There is no compile-time way to enforce this, as far as I know. It can, however, be enforced using a run-time check. No unusual user actions would typically be able to cause this, (just incorrect coding) so it's similar to having Debug.Assert in places (and, in fact, you could implement it using that, if you like). E.g.
public abstract class FooBase<T> where T : FooBase<T>
{
protected FooBase()
{
Debug.Assert(this.GetType() == typeof(T));
}
}
I don't know why you have this as a requirement. I would first suggest that you go back and look at 'your object model and determine why you feel you need this requirement and determine if there's a better way to accomplish whatever it is you're trying to achieve.
I think I see one problem with what you have above: no generic parameters in your definitions/declarations of classes ConcreteFooA and ConcreteFooB.
It looks as though it may be better for you to create an interface IFooBase and have your concrete implementations implement the interface. In every instance where you want to work with an IFooBase, you'd use a variable of type IFooBase.
So:
public interface IFooBase { /* Interface contract... */ }
public class ConcreteFooA : IFooBase { /* Implement interface contract */ }
public class ConcreteFooB : IFooBase { /* Implement interface contract */ }
// Some class that acts on IFooBases
public class ActionClass
{
public ActionClass(IFooBase fooBase) { this._fooBase = foobase };
public DoSomething() { /* Do something useful with the FooBase */ }
// Or, you could use method injection on static methods...
public static void DoSomething(IFooBase fooBase) { /* Do some stuff... */ }
}
Just some ideas. But I don't think you can accomplish what you want to do with Generics alone.
It's not possible and it should not be, because according to L in SOLID:
Liskov substitution principle: “objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program”.
So actually the compiler is doing what it was meant to do.
Maybe you need to change the design and implementation of your classes for example by employing a Behavioral Pattern. For instance if an object should present different algorithms for a specific calculation you could use Strategy Pattern.
But I can not advise on that since I am not aware what exactly you want to achieve.
I suspect the answer is no, but I want to know if it is possible to do something like this:
public class MyGenericClass<TSomeClass> {
public void MyGenericMethod<TSomeInterface>()
// This doesn't compile.
where TSomeClass : TSomeInterface
{
//...
}
}
What I mean to indicate in the above (non-working) example is to constrain TSomeInterface such that it can be any base class, implemented interface, or (if you really want to get fancy) implicit conversion of MyGenericClass.
NOTE:
I suspect that the reason why this was never implemented in C# is that generic constraints are not really meant to be code contracts, which is how I am trying to use them here. I really don't care what type TSomeInterface is, so long as it is implemented by TSomeClass.
So far, I have hacked this together:
public class MyGenericClass<TSomeClass> {
public void MyGenericMethod<TIntermediateType, TSomeInterface>()
where TIntermediateType : TSomeClass, TSomeInterface
{
//...
}
}
This more or less enforces the constraint that I want (that TSomeClass must inherit from, or in the case of an interface, implement, TSomeInterface), but calling it is very clumsy, because I have to specify TIntermediateType (even though I really want it to evaluate against TSomeClass):
var myGenericInstance = new MyGenericClass<TSomeClass>();
myGenericInstance.MyGenericMethod(TSomeClass, TSomeInterface);
Additionally, the above hack is broken because a caller could in theory specify a subclass of TSomeClass as the first type parameter, where only the subclass implements TSomeInterface.
The reason that I want to do this is that I am writing a fluent factory pattern for a WCF service, and I would like to prevent the caller (at compile time) from trying to create an endpoint with a contract that the service class doesn't implement. I can obviously check this at runtime (WCF in fact does this for me), but I am a big fan of compile-time checking.
Is there a better/more elegant way to achieve what I am after here?
The way I was able to wrap my head around the reason why this doesn't compile is the following:
Consider this program compiles:
class Program {
class Class1 { }
class Class2 { }
public class MyGenericClass<TSomeClass> {
public void MyGenericMethod<TSomeInterface>() where TSomeClass : TSomeInterface {
}
}
static void Main(string[] args) {
var inst = new MyGenericClass<Class1>();
}
}
Everything is good. The compiler is happy. Now consider I change the Main method:
static void Main(string[] args) {
var inst = new MyGenericClass<Class1>();
inst.MyGenericMethod<Class2>();
}
The compiler will complain that Class1 does not implement Class2. But which line is wrong? The constraint is on the call to MyGenericMethod, but the offending line of code is the creation of MyGenericClass.
In other words, which one gets the red squiggly line?
As discussed in this linked question, you can't use a type parameter that isn't from the current declaration, on the left side of a where clause.
So as suggested by w0lf in that other question, what you can do is provide both types in your interface (rather than method) declaration:
public class MyGenericClass<TSomeClass, TSomeInterface> {
where TSomeClass : TSomeInterface
public void MyGenericMethod() // not so generic anymore :(
{
//...
}
}
That, however, greatly limits your MyGenericMethod and forces your class to declare before-hand what base interface you with to allow.
So another option is to use a static method with more type parameters:
public class MyGenericClass<TSomeClass> {
public static void MyGenericMethod<TSomeClass, TSomeInterface>
(MyGenericClass<TSomeClass> that)
where TSomeClass : TSomeInterface
{
// use "that" instead of this
}
}
Possibly you could make it an extension method to make it appear to the user like an actual method.
Neither of these is exactly what you wanted, but probably better than the intermediate type solution.
As for the reason for why not?, my guess is that it would complicate the compiler without adding enough value. Here's a discussion by Angelika Langer of the same subject but about Java. Although there are significant differences between C# and Java, I think her conclusion might apply here as well:
The bottom line is that the usefulness of lower bounds on type
parameters is somewhat debatable. They would be confusing and perhaps
even misleading when used as type parameters of a generic class. On
the other hand, generic methods would occasionally profit from a type
parameter with a lower bound. For methods, a work-around for the
lack of a lower bound type parameter can often be found. Such a
work-around typically involves a static generic method or a lower
bound wildcard.
She also gives a nice use case, see the link above.
An extension method provides the best solution, though it doesn't totally solve all your concerns.
public class MyGenericClass<TSomeClass>
{
}
public static class MyGenericClassExtensions
{
public static void MyGenericMethod<TSomeClass, TSomeInterface>(this MyGenericClass<TSomeClass> self)
where TSomeClass : TSomeInterface
{
//...
}
}
It is still necessary to specify both types when calling MyGenericMethod, but it prevents the caller from specifying an incorrect type for TSomeClass as is possible with the approach you came up with. With this approach, the method can be called like so:
var myGenericInstance = new MyGenericClass<TSomeClass>();
myGenericInstance.MyGenericMethod<TSomeClass, TSomeInterface>();
It will be a compile error if the type parameter MyGenericClass is declared with does not match the first type parameter to MyGenericMethod.
Since the first type parameter can be inferred by the this argument, it is often possible for the compiler to infer both type parameters if their additional parameters to the method.
I have a conceptual / theoretical question about loose coupling and interfaces.
So one way to use an interface might be to encapsulate the parameters required by a certain constructor:
class Foo
{
public Foo(IFooInterface foo)
{
// do stuff that depends on the members of IFooInterface
}
}
So as long as the object passed in implements the contract, everything will work. From my understanding the main benefit here is that it enables polymorphism, but I'm not sure whether this really has anything to do with loose coupling.
Lets say for the sake of argument that an IFooInterface is as follows:
interface IFooInterface
{
string FooString { get; set; }
int FooInt { get; set; }
void DoFoo(string _str);
}
From a loose coupling standpoint, wouldnt it much better to NOT to use an IFooInterface in the above constructor, and instead set up the Foo like so:
class Foo
{
public Foo(string _fooString, int _fooInt, Action<string> _doFoo)
{
// do the same operations
}
}
Because say I want to drop the functionality of Foo into another project. That means that other project also has to reference IFooInterface, adding another dependency. But this way I can drop Foo into another project and it expresses exactly what it requires in order to work. Obviously I can just use overloaded constructors, but lets say for the sake of argument I dont want to and/or cannot modify Foo's constructors.
The most salient downside (to me atleast) is that if you have a method with a bunch of primitive parameters it gets ugly and hard to read. So I had the idea to create a sort of wrapping function that allows you to still pass in an interface rather than all the primitive types:
public static Func<T, R> Wrap<T, R>(Func<T, object[]> conversion)
{
return new Func<T, R>(t =>
{
object[] parameters = conversion(t);
Type[] args = new Type[parameters.Length];
for (int i = 0; i < parameters.Length; i++)
{
args[i] = parameters[i].GetType();
}
ConstructorInfo info = typeof(R).GetConstructor(args);
return (R)info.Invoke(parameters);
});
}
The idea here is that I can get back a function that takes an instance of some interface which conforms to the requirements of Foo, but Foo literally doesnt know anything about that interface. It could be used like so:
public Foo MakeFoo(IFooInterface foo)
{
return Wrap<IFooInterface, Foo>(f =>
new object[] { f.FooString, f.FooInt, f.DoFoo })(foo);
}
I've heard discussion about how interfaces are supposed to enable loose-coupling, but was wondering about this.
Wondering what some experienced programmers think.
In your initial example you're pretty close to the Parameter Object pattern, though it's more common to use a simple class (often with auto-properties) here without the extra abstraction of an interface.
Typically when you hear about passing an interface into a constructor, it's not to replace primitives but as a form of dependency injection. Instead of depending on MyFooRepository directly, one would take a dependency on IFooRepository which would remove the coupling to a specific implementation.
My first thought is that you did not provide Action<string> and Action<int> for the setters of FooString and FooInt, respectively. The implementation of IFooInterface may have rules concerning those setters, and may require access to other implementation details not exposed on the interface.
In the same vein, you should accept a Func<string> and Func<int> as well: the implementation of IFooInterface may have rules about what FooString and FooInt are as time progresses. For example, DoFoo may recalculate those values; you can't assume that they are just pass-throughs to fields that never change.
Taking this even further, if the getters, setters, or DoFoo require access to common state, the functions and actions will need to close over the same set of variables when you create them. At that point, you will be doing some mental gymnastics to comprehend the variable lifetimes and the relationships between the delegates.
This pairing of state and behavior is exactly what a class expresses, and the hiding of implementation details is exactly what an interface provides. Breaking those concepts into their component elements is certainly achievable, but it also breaks the coherence gained by grouping the members with a type.
To put it another way, you can give me noodles, sauce, vegetables, and hamburger, but that's not spaghetti and meatballs :-)
Is it possible to define an Interface with optional implementation methods? For example I have the following interface definition as IDataReader in my core library:
public interface IDataReader<T> {
void StartRead(T data);
void Stop();
}
However, in my current implementations, the Stop() method has never been used or implemented. In all my implementation classes, this method has to be implemented with throw NotImplementedExcetion() as default:
class MyDataReader : IDataReader<MyData> {
...
public void Stop()
{
// this none implementaion looks like uncompleted codes
throw NotImplementedException();
}
Of course, I can remove the throw exception code and leave it empty.
When I designed this data reader interface, I thought it should provide a way to stop the reading process. Maybe we will use Stop() sometime in the future.
Anyway, not sure if it is possible to make this Stop() method as an optional implementation method? The only way I can think is to either to define two interfaces one with stop and another without such as IDataReader and IDataReader2. Another option is to break this one into to interfaces like this:
interface IDataReader<T> {
void StartRead(T data);
}
interface IStop {
void Stop();
}
In my implementation cases, I have to cast or use as IStop to check if my implementation supports Stop() method:
reader.StartRead(myData);
....
// some where when I need to stop reader
IStop stoppable = reader as IStop;
if (stoppable != null ) stoppable.Stop();
...
Still I have to write those codes. Any suggestions? Not sure if there is any way to define optional implementation methods in an interface in .Net or C#?
Interesting. I'll have to quote you here:
However, in my current
implementations, the Stop() method has
never been used or implemented. In all
my implementation classes, this method
has to be implemented with throw
NotImplementedExcetion() as default:
If this is the case, then you have two options:
Remove the Stop() method from the interface. If it isn't used by every implementor of the interface, it clearly does not belong there.
Instead of an interface, convert your interface to an abstract base class. This way there is no need to override an empty Stop() method until you need to.
Update The only way I think methods can be made optional is to assign a method to a variable (of a delegate type similar to the method's signature) and then evaluating if the method is null before attempting to call it anywhere.
This is usually done for event handlers, wherein the handler may or may not be present, and can be considered optional.
For info, another approach fairly common in the BCL is Supports* on the same interface, i.e.
bool SupportsStop {get;}
void Stop();
(examples of this, for example, in IBindingList).
I'm not pretending that it is "pure" or anything, but it works - but it means you now have two methods to implement per feature, not one. Separate interfaces (IStoppableReader, for example) may be preferable.
For info, if the implementation is common between all implementations, then you can use extension methods; for a trivial example:
public static void AddRange<T>(this IList<T> list, IEnumerable<T> items) {
foreach(T item in items) list.Add(item);
}
(or the equivalent for your interface). If you provide a more specialized version against the concrete type, then it will take precedence (but only if the caller knows about the variable as the concrete type, not the interface). So with the above, anyone knowingly using a List<T> still uses List<T>'s version of AddRange; but if the have a List<T> but only know about it as IList<T>, it'll use the extension method.
If the method is inappropriate for your implementation, throw InvalidOperationException just like most iterators do when you call Reset on them. An alternative is NotSupportedException which tends to be used by System.IO. The latter is more logical (as it has nothing to do with the current state of the object, just its concrete type) but the former is more commonly used in my experience.
However, it's best to only put things into an interface when you actually need them - if you're still in a position where you can remove Stop, I would do so if I were you.
There's no unified support for optional interface members in the language or the CLR.
If no classes in your code actually implement Stop(), and you don't have definite plans to do so in the future, then you don't need it in your interface. Otherwise, if some but not all of your objects are "stoppable", then the correct approach is indeed to make it a separate interface such as IStoppable, and the clients should then query for it as needed.
If your implementation does not implement the interface method Stop, then it breaks obviousily the contract that comes with your interface. Either you implement the Stop method appropriately (not by throwing an Exception and not by leaving it empty) or you need to redesign your interface (so to change the contract).
Best Regards
C# version 4 (or vNext) is considering default implementation for interfaces - I heard that on channel9 a few months ago ;).
Interfaces with default implementation would behave somewhat like abstract base classes. Now that you can inherit multiple interfaces this could mean that C# might get multiple inheritance in form of interfaces with default implementations.
Until then you might get away with extension methods...
Or your type could make use of the delegates.
interface IOptionalStop
{
Action Stop { get; }
}
public class WithStop : IOptionalStop
{
#region IOptionalStop Members
public Action Stop
{
get;
private set;
}
#endregion
public WithStop()
{
this.Stop =
delegate
{
// we are going to stop, honest!
};
}
}
public class WithoutStop : IOptionalStop
{
#region IOptionalStop Members
public Action Stop
{
get;
private set;
}
#endregion
}
public class Program
{
public static string Text { get; set; }
public static void Main(string[] args)
{
var a = new WithStop();
a.Stop();
var o = new WithoutStop();
// Stop is null and we cannot actually call it
a.Stop();
}
}
I'd like to override a class method without inheriting the base class because it'd take a lot of time and modifications and, therefore, more and more tests. It's like this:
class TestClass{
public void initialMethod(){
...
}
}
And somewhere on the code, I'd like to do something like this:
public testMethod()
{
return;
}
test(){
changeMethod(TestClass.initialMethod, testMethod);
}
And this changeMethod function would override the TestClass initialMethod so that it'd call testMethod instead.
Inheriting and overriding the method using normal practices is not an option, as this class A is a graphic component and, inhereting it (and changing it) would break lots of code.
Edit: We don't have the base code for the TestClass, so it's not an option to modify the code there defining the initialMethod as a delegate.
Edit 2: Since this is a graphical component, the designer added a lot of code automatically. If I were to inherit this code, I would have to replace all code added by the designer. That's why I wouldn't like to replace this component.
You need the Strategy pattern.
Main steps:
Create an interface with ie. Do() signature
Your initialMethod() should call a strategy.Do(), where strategy is type of your interface
Create a class that implements this interface. Do() is your testmethod now.
Inject into your main class an instance of this class
If the job it's not so big (let's say just a color replacement or something) then I agree with Jhonny D. Cano's solution with C# (anonymous)delegates.
Edit (after edit 2)
May - just as proof-of-concept - you should inherit the class and replace all references from base class to this new. Do this, and nothing else. If it works, you can think about the next steps (new methods or delegates etc.)
You need only a new checkout from your version control system, and if it maybe fails you can abandon it. It's worth trying.
Perhaps you can do it as a delegate.
class TestClass {
public Action myAction;
public void initialMethod(){
...
}
initialMethod
public TestClass() {
myAction = initialMethod;
}
}
and then on TestMethod
public testMethod()
{
return;
}
test() {
testClassInstance.myAction = testMethod;
}
I think your best bet might be to use a AOP framework like LinFu. There's a codeproject article explaining it:
Introducing LinFu, Part VI: LinFu.AOP – Pervasive Method Interception and Replacement for Sealed Types in Any .NET Language
If 'TestClass' is something you defined, you could replace the 'initialMethod' definition with a property and delegate which can then be set to any method with a given signature. (Even anonymous ones.)
class TestClass {
Action _myMethod;
Action MyMethod {
get { return _myMethod; }
set { _myMethod = value; }
}
var tc = new TestClass()
tc.MyMethod = () -> Console.WriteLine("Hello World!");
tc.MyMethod()
The above code is untested.
The short and simple answer is: if you can't adjust the base TestClass code, no, there's no way you can modify the class to replace a method by another. Once we started doing stuff like that, we'd be in a completely different kind of language, like JavaScript.
The longer answer is: it depends on who is calling the replaced method.
If it's other classes, see if you can't implement a Proxy in between them and the unmodifiable concrete class. Whether this is doable depends on whether that class implements interfaces, or is its own interface.
If it's the class itself, then your only option is to decompile and modify the class, at design time using Reflector (or equivalent tools), or at runtime using Reflection.Emit. However, you'd have to be hurting pretty badly to go this route, as it's sure to be painful and brittle.
Unfortunately you still haven't explained what you are trying do and why. Replacing methods on the go is powerful stuff in the languages that permit it directly... There might be mocking libraries that can be twisted sufficiently far to do the reflection stuff, but then you'd be skating on thin ice.
If you don't have the code use Extension Methods.
public void doSmth(this objectYOUWANT arg)
{
//Do Something
}
Here you use the principle Closed for Modification Open for Extension.
This will add functionality to the library you don't have the source code. It's very clean to do it this way.
Edition:
In FrameWork 3.5 there is something new called Extension Methods. These kind of methods adds functionality to a closed Assembly letting you Extend in functionality a closed dll/assembly.
To use this for example you have a dll that you import, that is called Graphics.dll (you have the reference on your project)
First of all you shoud create a new static class called for example Extension:
public static class Extensions
{
}
Second, you want to add extra functionality to a class contained in Graphics.dll named ChartGraph. You will do this:
public static class Extensions
{
public static void draw(this ChartGraph g)
{
// DO SOMETHING
}
}
Third, when you instantiate a new object from the graphics.dll you now will have the new method you have created:
ChartGraph myG = new ChartGraph();
myG.draw();
As you can see there you have added new functionality without much effort without recompiling the dll, this is good if you don't have the source code.