In one of my pet projects I have found myself using generics quite a lot and I'm wondering if it is possible to use recursive constraints in a generic class.
Let's imagine I have the following interface
public interface IAmSpecial {}
and the classes implementing it can each be handled by its own class
public class SpecialHandler<T> where T: IAmSpecial {}
I would like to define a class that is generic over any SpecialHandler
public class SpecialWrapperHandler<T> where T : SpecialHandler<SpecialT> where SpecialT: IAmSpecial
{
public SpecialWrapperHandler(T t){}
}
which doesn't compile. I can define the SpecialWrapperHandler class with the following signature
public class SpecialWrapperHandler<T> where T : SpecialHandler<IAmSpecial>
{
public SpecialWrapperHandler(T t){}
}
but then I cannot build it since any SpecialHandler implementation will close the generic value on one implementation of IAmSpecial. Also I cannot declare something like
public class SpecialWrapperHandler<SpecialHandler<T>> where T : IAmSpecial
So is it possible to have a C# construct that recurse generic constraints so I can my SpecialWrapperHandler? If so what construct should I use? And if not, why? A quick read of the generics chapter in the doc doesn't yield many answers...
Pass in a second type parameter for the handler. In that way you can type both parameters.
Like so:
public class SpecialWrapperHandler<H, T> where H : SpecialHandler<T> where T : IAmSpecial
{
public SpecialWrapperHandler(T t){}
}
Proof:
public class X : IAmSpecial { }
public class XHandler : SpecialHandler<X> { }
static void Main(string[] args)
{
X x = new X();
SpecialWrapperHandler<XHandler, X> v = new SpecialWrapperHandler<XHandler, X>(x);
}
Related
I have written some code and I'm curious as to whether or not there is a danger in what I'm doing that I'm unaware of.
I have tried searching and most of the questions I found dealt with how to make things generic which isn't my issue. I also looked in the C# spec for .net 4.5 under section 13.4.3 - Generic Methods and 7.5.2 in regards to Type inference and finally 7.5.2.12 Inferred return type and they don't really cover what I'm trying to do.
Basically I have a hierarchy of classes
public class SomeBaseClass { }
public class SomeClass : SomeBaseClass { }
public class AnotherClass : SomeBaseClass { }
public class BaseData
{
public SomeBaseClass mMember;
public BaseData() { }
public TType GetMember<TType>()
where TType : SomeBaseClass
{
return (TType)mMember;
}
}
public class Data : BaseData
{
public Data()
{
mMember = new SomeClass();
}
//Is this bad
public SomeClass GetMember()
{
return base.GetMember<SomeClass>();
}
}
The compiler doesn't complain because I'm not hiding the base class method. This is shown that intellisense lists them as two separate methods. I've written several tests which all behave the way I would expect them to and when looking at things like List<> there are instances of methods that have both a generic and nongeneric implementation (for example AsParallel from ParallelEnumerable) but the difference is that in this case both methods exist in the same class and take in a generic and nongeneric parameter respectively.
The tests I ran and showed work the way I would expect are listed below.
class Program
{
static void Main(string[] args)
{
BaseData lData = new Data();
Data lData2 = new Data();
//Call base method with type
SomeBaseClass lTest = lData.GetMember<SomeClass>();
//Cast and call derived method
SomeClass lTest2 = ((Data)lData).GetMember();
//Call base method with type and then cast
SomeClass lTest3 = (SomeClass)lData.GetMember<SomeBaseClass>();
//Call derived method directly
SomeClass lTest4 = lData2.GetMember();
//Throw InvalidCastException
SomeBaseClass lTest5 = lData2.GetMember<AnotherClass>();
}
}
The main reason for this is that I would like that any caller code doesn't have to know the generic type when the class itself already has this information. It's to avoid having to write
lData.GetMemberType<...>();
all over the place.
I apologize if this question is too broad or opinionated. Mostly I'm just wondering if there is anything in this scenario that wouldn't work the way I would think or have a hidden bug etc.
Your question is a little too vague to give a very good answer (what are you using this for? what is the purpose of this design?).
I don't think the name overlap is all that problematic, but it does seem like a symptom of a problematic design and a misuse of generics (all that casting should clue you in on that).
Ideally, your class itself should be generic, and you should use the generic type parameter throughout. That will save you from all the casting you are doing:
public class SomeBaseClass { }
public class SomeClass : SomeBaseClass { }
public class AnotherClass : SomeBaseClass { }
public class BaseData<TType> where TType : SomeBaseClass
{
protected TType mMember;
public BaseData() { }
public BaseData(TType member)
: this()
{
mMember = member;
}
public TType GetMember()
{
return mMember;
}
}
public class Data : BaseData<SomeClass>
{
public Data()
: base(new SomeClass())
{
}
// no need to implement GetMember(); base class has it covered
}
I want to force my child classes to pass themselves as as the generic parameter to the parent class.
For example :
class BaseClass<T> where T: BaseClass
{
//FullClassName : Tuple [Save,Update,Delete]
Dictionary<string,Tuple<delegate,delegate,delegate>> dict = new Dictionary...;
static BaseClass()
{
RegisterType();
}
private static void RegisterType()
{
Type t = typeof(T);
var props = t.GetProperties().Where(/* Read all properties with the SomeCustomAttribute */);
/* Create the delegates using expression trees and add the final tuple to the dictionary */
}
public virtual void Save()
{
delegate d = dict[t.GetType().FullName];
d.Item1(this);
}
}
class ChildClass : BaseClass<ChildClass>
{
[SomeCustomAttribute]
public int SomeID {get;set;}
[SomeCustomAttribute]
public string SomeName {get; set;}
}
public class Program
{
public static void Main(string[] args)
{
ChildClass c = new ChildClass();
c.Save();
}
}
Obviously the above code won't compile. I'll restate : I want the child class to pass itself as the generic parameter and not any other child of BaseClass.
(The above code is kind of a psuedo code and will still not compile).
You can do this:
public class BaseClass<T> where T: BaseClass<T> { }
public class ChildClass : BaseClass<ChildClass> { }
But this doesn't force you to use ChildClass as the generic parameter. You could do this public class OtherChildClass : BaseClass<ChildClass> { } which would break the "coontract" that you want to enforce.
The direct answer is that if your accessing a static method then typeof(T) will give you the type for reflection.
However, there is probably better solutions than using reflection. Options:
1) Static constructor on the child class.
2) Abstract method declared in the base class.
I do not know the application, but I get concerned about my design if I feel like using a static constructor, I also get concerned if a base class needs to initialize the child class.
I suggest looking at injection as a solution rather than inheritance. It offers superior unit testing and often a better architecture.
More info (after initial post), this is my preferred solution:
public interface IRegesterable
{
void Register();
}
public class Widget : IRegesterable
{
public void Register()
{
// do stuff
}
}
public class Class1
{
public Class1(IRegesterable widget)
{
widget.Register();
}
}
Hope this helps
The ConcurrentDictionary is being used as a Set<Type>. We can check in the Set<Type> if the type has been initialized. If not we run RegisterType on the type.
public abstract class BaseClass
{
//Concurrent Set does not exist.
private static ConcurrentDictionary<Type, bool> _registeredTypes
= new ConcurrentDictionary<Type, bool>();
protected BaseClass()
{
_registeredTypes.GetOrAdd(GetType(), RegisterType);
}
private static bool RegisterType(Type type)
{
//some code that will perform one time processing using reflections
//dummy return value
return true;
}
}
public class ChildClass : BaseClass
{
}
There are several inefficiencies with this pattern though.
object.GetType() is pretty darn slow, and inefficient.
Even with the HashSet behavior, we are checking for initialization on each instanciation. Its as fast as I can get it, but its still pretty superfluous.
I've written a class that looks like this:
public class MyClass<T>
{
public void doSomething()
{
T.somethingSpecial;
}
}
This code doesn't compile because the compiler has no idea what T is. I would like to constrain T so that it must inherit a certain class that defines somethingSpecial. Bonus points if you can tell me how to do the same thing by contraining T so that it must implement a certain interface.
public class MyClass<T> where T: IAmSomethingSpecial
It's called Constraints on Type Parameters.
Use the following type parameter constraint in the class declaration:
public class MyClass<T> where T : MyBaseClass
You can read more about type parameter contraints for example here at MSDN.
What you want is a generic constraint:
public class MyClass<T> where T : SomeParentClass
You need a Generic Constraint:
public class MyClass<T> where T : ISomeInterface
{
public void doSomething()
{
instanceOfT.somethingSpecial();
}
}
public interface ISomeInterface
{
void DoSomething();
}
public class MyClass<T> where T : ISomeInterface
{
public void doSomething()
{
T.DoSomething();
}
}
The where keyword allows you to specify constraints on the given generic type. You could swap out the interface for a class.
Read the documentation. Generic Constraint.
class MyClass<T> where T : someinterfaceorbaseclassthatTmustinherit
In C#, i have an interface, and there are some class will implement this interface.
i have a generic utility class, which i want to limit so that the utility class can only be declared using types implementing that interface, as show below, how can i do it??
public interface IMyInterface
{}
public class A : IMyInterface {} // can pass into UtilityClass
public interface B : IMyInterface{}
public class C : B {} // can pass into UtilityClass
public class D {} // can Not pass into UtilityClass
public class UtilityClass<T is IMyInterface>
{
// some utility function
}
Thanks.
You're looking for a generic constraint. These are expressed in C# by using the where keyword after the class name.
public class UtilityClass<T> where T:IMyInterface
{
}
public class UtilityClass<T> where T: IMyInterface
{
// some utility function
}
They are called constraints, you can read more here http://msdn.microsoft.com/en-us/library/d5x73970.aspx
use the where clause:
public interface IMyInterface
{}
public class UtilityClass<T> where T : IMyInterface
{
// some utility function
}
You can add restrictions to generic classes like so:
public class UtilityClass<T> where T : IMyInterface
{
// some utility function
}
I am trying to do the following in C# 4.0:
I have a Base Class and 2 derived classes
public class Base {}
public class DerivedClass1 : Base {}
public class DerivedClass2 : Base {}
I want to do something like this, but it doesn't work.
How to I tell a Generic List to accept a Base Class and the derived classes of the base class.
public class Class_1
{
public Class_1()
{
List<DerivedClass2> list = new List<DerivedClass2>();
new Class_2(list);
}
}
public class Class_2
{
public Class_2(List<Base> list)
{
}
}
In Java I can do something like this
public class Class_2
{
public Class_2(List<? extends Base> list)
{
}
}
Does something like that exists in C#
I hope my question is clear, its just about the generics of the List.
Thanks a lot in Advance :)
General case:
function Foo<T>(List<T> list) where T : Base {
...
}
plus for interfaces and delegates, C# allows co/contravariance.
For e.g. IEnumerable<T> and IList<T>, your code will therefore work! Note that if this was allowed for List<T>s, you could insert a Derived1 into a list of Derived2s by using the common base class, which would break type safety. So simply stick to the above, readonly interfaces.
I think you mean either:
// Define other methods and classes here
public class Class_1
{
public Class_1()
{
List<DerivedClass2> list = new List<DerivedClass2>();
new Class_2<DerivedClass2>(list);
}
}
public class Class_2<T> where T : Base
{
public Class_2(List<T> list)
{
}
}
Or, if you want the constructor to be generic, and not the class:
// Define other methods and classes here
public class Class_1
{
public Class_1()
{
List<DerivedClass2> list = new List<DerivedClass2>();
Class_2.Create(list);
}
}
public class Class_2
{
public static Class_2 Create<T>(List<T> list) where T : Base
{
// Stuff
return new Class_2(/*Stuff*/);
}
public Class_2()
{
}
}