I have a Generic Type Interface and want a constructor of an object to take in the Generic Interface.
Like:
public Constructor(int blah, IGenericType<T> instance)
{}
I want the code that creates this object to specify the IGenericType (use Inversion of Control). I have not seen a way for this to happen. Any suggestions to accomplish this?
I want someone to create the object like:
Constructor varname = new Constructor(1, new GenericType<int>());
You can't make constructors generic, but you can use a generic static method instead:
public static Constructor CreateInstance<T>(int blah, IGenericType<T> instance)
and then do whatever you need to after the constructor, if required. Another alternative in some cases might be to introduce a non-generic interface which the generic interface extends.
EDIT: As per the comments...
If you want to save the argument into the newly created object, and you want to do so in a strongly typed way, then the type must be generic as well.
At that point the constructor problem goes away, but you may want to keep a static generic method anyway in a non-generic type: so you can take advantage of type inference:
public static class Foo
{
public static Foo<T> CreateInstance<T>(IGenericType<T> instance)
{
return new Foo<T>(instance);
}
}
public class Foo<T>
{
public Foo(IGenericType<T> instance)
{
// Whatever
}
}
...
IGenericType<string> x = new GenericType<string>();
Foo<string> noInference = new Foo<string>(x);
Foo<string> withInference = Foo.CreateInstance(x);
Related
Is it possible in C# to have a method of a generic driven class require new, but not require new on the whole class?
public class Response<T> //not where T:new()
{
public void AddTotal<T>() where T : new()
{
var totaledObject = new T();
// do all the useful stuff I need with the new generic
}
}
I use this response for many different scenarios, the T does not always have a new(), and for those scenarios I will not use the AddTotal function, but for a few I would like to have it. Is this possible?
Note:
I know I can do a different generic variable name and just pass that to the function, but this breaks the indication that it has to be of the same type as T.
You can get around this by pushing the problem out to the calling code. Add an argument to either have the calling code provide the object instance you need or have the calling code provide the method you need to create the object:
public void AddTotal<T>(T totaledObject)
{
// do all the useful stuff I need with totaledObject
}
//call it: AddTotal(new Object())
or
public void AddTotal<T>(Func<T> createObject)
{
var totaledObject = createObject();
// do all the useful stuff I need with the new generic
}
The trick with this option is that you can't just pass a constructor to that object. You have to wrap calls in another delegate, which can be done with a quick lambda expression:
AddTotal(() => new Object());
Is it possible in C# to have a method of a generic driven class
require new, but not require new on the whole class?
Only if you specify different type parameter, which you don't want to do. A generic type parameter constraint exists for any generic type argument you specify. As you want T for the class and method declaration to be the same T, the constraint will exist for both.
If you want to specify another generic type parameter, you could do something hacky like this:
public class X<T>
{
public void Y<OtherT>() where OtherT : T, new()
{
}
}
Thus constraining OtherT to be of type T declared in the class or a derivative. There's no guarantee there will be an exact match on the type.
You don't have to constrain T to new (). Just allow it. Use 'where T : class, new()' as your class constraint. You don't need anything on the method as new T() can now be used.
I would like to implement my generic IQueue<T> interface in an efficient way by doing one implementation if T is struct and another if T is a class.
interface IQueue<T> { ... }
class StructQueue<T> : IQueue<T> where T : struct { ... }
class RefQueue<T> : IQueue<T> where T : class { ... }
The, I'd like to have a factory method which based on T's kind returns an instance of one or the other:
static IQueue<T> CreateQueue<T>() {
if (typeof(T).IsValueType) {
return new StructQueue<T>();
}
return new RefQueue<T>();
}
Of course, the compiler indicates that T should be non-nullable/nullable type argument respectively.
Is there a way to cast T into a struct kind (and into a class kind) to make the method compile? Is this kind of runtime dispatching even possible with C#?
You can use Reflection to do it like this:
static IQueue<T> CreateQueue<T>()
{
if (typeof(T).IsValueType)
{
return (IQueue<T>)Activator
.CreateInstance(typeof(StructQueue<>).MakeGenericType(typeof(T)));
}
return (IQueue<T>)Activator
.CreateInstance(typeof(RefQueue<>).MakeGenericType(typeof(T)));
}
This code uses the Activator.CreateInstance method to create queues at runtime. This method takes in the type of the object you want to create.
To create a Type that represents the generic class, this code uses the MakeGenericType method to create a closed generic Type object from the open generic types like StructQueue<>.
Yacoub Massad's answer is correct, but with a little modification, you don't need to run MakeGenericType for each call to CreateQueue.
The code below runs MakeGenericType once per type, since a separate static variable exists for each type of QueueFactory<T>, i.e QueueFactory<int>.queueType will get StructQueue<int>, while QueueFactory<string>.queueType will get RefQueue<int>
public class QueueFactory<T>
{
static Type queueType = typeof(T).IsValueType ?
typeof(StructQueue<>).MakeGenericType(typeof(T)) : typeof(RefQueue<>).MakeGenericType(typeof(T));
public static IQueue<T> CreateQueue()
{
return (IQueue<T>)Activator.CreateInstance(queueType);
}
}
In my semi-scientific test, it created 1 million instances in about a tenth of the time.
say I have a class
public abstract class A<T>
{
public T GetT{get;}
public ISomeInterface AMethod{get;}
}
Then I have some other method in another class where I take an object and I want to check it is of type A<> then if it is get the type of T and call the method AMethod. So I'm doing this:
if (theObject.GetType().GetGenericTypeDefinition() == typeof (A<>))
{
Type TType = theObject.GetType().GetGenericArguments()[0];
dynamic dynamicObject= theObject;
ISomeInterface filter = dynamicObject.AMethod;
//...some other stuff using TType
}
Is there a way to do this without using the dynamic object, since I can't declare the type of the variable using the TType or using the generic type defintion A<> at runtime...
If you're able to, put all the non-generic stuff in an abstract non-generic base class:
public abstract class A
{
public ISomeInterface AMethod{get;}
}
public abstract class A<T> : A
{
public T GetT{get;}
}
Then you can just use:
A foo = theObject as A;
if (foo != null)
{
ISomeInterface filter = foo.AMethod;
}
As mentioned by Mr. Skeet, pulling items which don't depend on the class type into a non-generic class or interface is generally the way to go when practical. Otherwise, I would suggest writing a generic method with type parameter T which will return a generic singleton (generated via Reflection) that can pass an object to a generic method with parameter types U such that T:A<U>. Such a method would only have to use Reflection once for any particular type T, no matter how many times it was used to handle instances of that type.
Because delegates are only usable with closed generics, one would probably have to define some generic interfaces:
// User code will supply an implementation of one of these interfaces to the dispatching
// object, which will call its "Act" method with a proper type
interface IActUponGenericA
{
void Act<T>(A<T> param);
}
interface IActUponGenericA<PT1>
{
void Act<T>(A<T> param, PT1 extraParam1);
}
interface IActUponGenericA<PT1,PT2>
{
void Act<T>(A<T> param, PT1 extraParam1, PT2 extraParam2);
}
// The dispatching object itself will implement this interface:
interface IWrapActUponGenericA<T>
{
void CallAction(IActUponGenericA act, T param);
void CallAction<PT1>(IActUponGenericA<PT1> act, T param, PT1 extraParam1);
void CallAction<PT1,PT2>(IActUponGenericA<PT1,PT2> act, T param,
PT1 extraParam1, PT2 extraParam2);
}
Then given a parameter type T (which implements A<Q> for some Q), the Reflection-based method code will generate a singleton which implements interface IWrapActUponGenericA<T>. That implementation can take an object of type T, and an implementation of IActUponGenericA, and call that implementation's Act<Q> method.
This approach will be more complicated than using dynamic, and may or may not perform better; it will, however, be able to deal with some situations that dynamic can't.
I have a generic class Proxy<T>, and I want to write another generic class with its type parameter being a Proxy.
I want to write:
public class MyClass<U> where U : Proxy<T>
but the compiler reports The type or namespace name T could not be found.
A solution I've found is to declare it like this:
public class MyClass<U, T> where U : Proxy<T>
but this seems clumsy as the client will have to declare two type parameters, like this:
public class SomeClass { ... }
public class SomeProxy : Proxy<SomeClass> { ... }
and then in a client somewhere:
var proxyWrapper = new MyClass<SomeProxy, SomeClass>();
How can I do this without having to have two generic types on MyClass. After all, if we know the first is SomeProxy, it should follow that the second is SomeClass.
Maybe something like this would do the job, too?
class Test<T> {
public Test(Proxy<T> proxy) { this.MyProxy = proxy; }
public Proxy<T> MyProxy { get; private set; }
}
Sorry, you just can't do this in C# without having MyClass generic on both types (unless you want to use reflection to create instances of it.)
You can have an interface IMyClass<SomeProxy> and a factory method that creates and returns an instance of MyClass<SomeProxy, SomeClass>. You may need to create the instance using Reflection.
I have a code example here of a similar situation: the end user only cares about a single type parameter, but the implementation needs to have two. In my example, I don't have to use Reflection to create the instance, but it sounds like you may need to.
What you're trying to do is possible using compile-time constructs such as C++ templates, but not run-time constructs such as C# generics.
If you want T to remain generic in Myclass, then the MyClass instance still needs to resolve all internally used generic types and you HAVE TO declare it somewhere. The way to go is the verbose way you mentioned:
public class MyClass<U, T> where U : Proxy<T>
If you don't care about the generic type T in MyClass then create interface and use it instead:
public interface IProxy { ... }
public class SomeClass { ... }
public class SomeProxy : Proxy<SomeClass>, IProxy { ... }
public class MyClass<U> where U : IProxy
and then in a client somewhere:
var proxyWrapper = new MyClass<SomeProxy>();
But do note that you cannot use type T in your interface declaration and Type U is now more general then before.
It turns out that all of the SomeProxy classes I want to deal with actually just override one method of Proxy<T> which has the signature:
T LoadInternal(Identifier id)
So, what I've done is created an internal class inside MyClass which takes a Func<Identifier, T> in its constructor. I can then pass a Func<Identifier, T> as a parameter to the constructor of MyClass and use my subclass in place of SomeProxy.
Seems a bit convoluted, but it works for me. To summarise, I now have:
public class MyClass<T>{
private SomeProxy theProxy;
public MyClass(Func<Identifier, T> loadDelegate){
theProxy = new SomeProxy(loadDelegate);
}
/* Other methods here */
class SomeProxy : Proxy<T>{
private Func<Identifier, T> m_loadInternal;
public SomeProxy(Func<Identifier, T> loadInternal){
m_loadInternal = loadInternal;
}
protected override T LoadInternal(Identifier id){
return m_loadInternal(id);
}
}
}
So, from client code, instead of writing a class which extends Proxy and then overriding LoadInternal in that class, I just create MyClass using:
var myClass = new MyClass<T>(x => CodeWhichReturnsT());
How can I do this without having to have two generic types on MyClass. After all, if we know the first is SomeProxy, it should follow that the second is SomeClass.
Although you seem to have found an answer to the main part of the question, I figured I'd offer my understanding about this part. It sounds like you wish you could do something like this:
class Proxy<T>
{
T Value { get; set; }
}
class MyClass<U> where U : Proxy<> { }
and have the compiler fill in the Proxy type parameter when you provide U. Since you have declared U as inheriting from Proxy, you must intend to use one of the methods on Proxy, that probably use the T parameter, like so:
class MyClass<U> where U : Proxy<>
{
void SomeMethod(U parameter)
{
var local = parameter.Value;
//more code here...
}
}
Now, what is the compiler supposed to infer for local here? This is the main problem I see that makes such a feature, if possible, hard to implement. If you don't want to use any methods that use the generic type of Proxy, you could instead make a non-generic base class and use that for U and sidestep the entire problem.
I am not a compiler writer, but a couple possibilities of how this could be dealt with come to mind. It could just say object (or whatever other restriction you put on the type parameter in Proxy), but that doesn't seem quite right or quite what normal generics seem to do. This would also require the CLR to allow open generic types as a constraint on the generic parameter, which I doubt it does. The other option I could see is for the type to actually have the second type parameter, and the compiler to give you syntactic sugar to make it easier.
Any way you go, this feature seems like a lot of work for a little benefit in what is probably a rare scenario, thus not likely to make the cut to get implemented.
public class BinarySearchTree<T>
where T : IComparable<T>
{
public static BinarySearchTree<char> InitializeSampleCharacterBST()
{
var bst = new BinarySearchTree<char>();
bst.Insert('F');
bst.Insert('B');
bst.Insert('A');
bst.Insert('D');
bst.Insert('C');
bst.Insert('G');
bst.Insert('I');
bst.Insert('H');
return bst;
}
class Program
{
static void Main(string[] args)
{
var bst = BinarySearchTree.InitializeSampleCharacterBST();
}
}
Why is this illegal? It's expecting me to provide a type parameter to the method call for the class which makes no sense. A generic class or method has no use for a type parameter in a static context.
It wants me to write the call like this:
var bst = BinarySearchTree<foo>.InitializeSampleCharacterBST();
Where foo can be any type I want regardless of the fact that the static method call returns a specifically typed generic object.
the class BinarySearchTree and BinarySeachTree<Foo> are completely separate; the language allows generic type overloading. Perhaps declare this method on a non-generic twin class:
public static class BinarySearchTree {
public static BinarySearchTree<char> InitializeSampleCharacterBST() {...}
}
public class BinarySearchTree<T> {...} // rest of the code
Otherwise... what T would it use? And what if the static method talked to static fields? Let alone which T to use, each T gets different static fields (i.e. SomeType<Foo> has separate fields to SomeType<Bar>).
As Marc said, it's sometimes useful to overload the type to have a non-generic class - and it would be in this case.
As for why it's necessary, suppose that the static method were actually implemented as:
public static BinarySearchTree<char> InitializeSampleCharacterBST()
{
Console.WriteLine(typeof(T));
return null;
}
That would be perfectly valid code - it's in a generic type, so it should have access to the type parameter... but you're trying to call the method without providing a generic type parameter, so it couldn't possibly work. In your case you happen to not use T anywhere within the method, but that's a coincidence. It's a bit like having an instance method which doesn't use this: you're not using the instance, but you still can't call it as if it were a static method.
As well as having separate static classes, another design technique which can be useful is to split your type into non-generic and generic pieces. That way, in cases where it can be awkward to work out which exact type you have, you don't actually need to know it in order to call some of the members. For example, a collection interface hierarchy might have:
public interface ISomeCollection
{
int Count { get; }
void Clear();
}
public interface ISomeCollection<T> : ISomeCollection
{
void Add(T item);
}
I've used this technique myself for my Protocol Buffers port to C#, and it's proved very useful (if somewhat complicated).
You're forgetting that type parameters don't only appear in the parameter/return type of a method. They can also appear in the implementation:
public static BinarySearchTree<char> InitializeSampleCharacterBST()
{
var forSomeReason = new T();
By placing your method inside a static class with a type parameter, you are saying that the implementation of the method may (now or in some future revision) depend upon that type parameter.
If this isn't the case, you've put the method in the wrong place.
Because the type itself is Generic, you have to provide a type argument, even if the static method you are interested in does not make use of that type argument. Its just the nature of generics in C#...they don't exist in a non-generic form at any time. If they did, that would cause conflicts with a non-generic version of the same type.