Develop Reference

Working with Generic classes with different constraints

I have two legacy C# user controls I need to work together.
I have an existing dialog onto which I need to add an existing generic usercontrol.
I have tried to sample the hierarchy below
interface Foo<T> {}
interface Bar<T>
{
T DataObject { get; set; }
}
public class ClassA<T> where T : Foo<T>
{
public ClassA(T dataObject)
{
//Do stuff if T implements Bar<T> - Pseudocode ahead
if(var T is Bar<T>)
{
var x = new ClassB<T>();
//x is typesafe, and I can set DataObject
x.DataObject = dataObject;
}
}
}
public class ClassB<T> where T : Bar<T>
{
T DataObject { get; set; }
}
The existing dialog, ClassA currently don't have any generic contraints, but could easily be changed to require T implementing Foo<T>.
The userControl, ClassB is based on another interface, Bar<T>. In practice, Bar<T> objects always implements Foo<T> - in theory of course not.
Are there any construction I can use to declare an object of type ClassB, and get compile time validation?
The construction above will give me a compile error stating:
The type 'T' cannot be used as type parameter 'T' in the generic type
of method ClassB<T>. There is no implicit reference conversion from
'T' for Bar<T>
I can make the ClassB object with Reflection, setting the properties using Reflection as well - but I would prefer a compile time solution.
But in my current situation with two existing dialogs - i'm not sure I am able to.
Any help is appreciated - also just if it is stating what I expect, that it can't be done.
--EDIT
Trying to elaborate a bit.
The problem rises when I have a ClassC that implements both Foo<T> and Bar<T>
public class ClassC<T> : Foo<T>, Bar<T>
{
T DataProperty
}
If I make an instance of ClassA<ClassC>, that is T in the specific instance is ClassC - then is there a way in code I can use T in creating an instance of ClassB- in this case T in ClassA does live up to the ClassB constraints, since T is ClassC.
I havent figured out how or if possible - tend to believe I can't.
As I wrote above, I have a solution based on reflection, i'm just not fan of using reflection and getting run-time validation only. But in this case with two legacy objects that need to work together I might be running out of options.

First of all, your types are a bit weird. They are kind-of recursive, with ClassB<T> requiring a T that implements Bar<T> which has the same structure as ClassB<T>. Maybe you meant to ClassB<T> to implement Bar<T> instead of requiring it as a type parameter?
Anyway you cannot do this. In order to be able to write ClassB<T>, the compiler needs to ensure that T is a valid type parameter for ClassB<> at runtime. This can only be the case when the type parameters on ClassA<T> for T are at least as restrictive as the ones from ClassB<T>.
Unfortunately, even a hard type check which would ensure that this is the case will not allow you to write ClassB<T>.
So without being able to write ClassB<T>, you will not be able to get static type safety at compile-time. So even when you create an instance of ClassB<T> (which you can), you won’t be able to access DataProperty on it since you will not be able to cast it as a ClassB<T>.
So in order to solve this, you would either have to access DataProperty using reflection only, or call a method inside ClassA<T> that does have the type constraint. I’ll show you both solutions:
public class ClassA<T>
where T : Foo<T>
{
public ClassA(T dataObject)
{
if (typeof(Bar<T>).IsAssignableFrom(typeof(T)))
{
// method 1, calling a generic function
MethodInfo mi = typeof(ClassA<T>).GetMethod("SetBDataObject").MakeGenericMethod(typeof(Bar<T>));
mi.Invoke(this, new object[] { dataObject });
// method 2, doing it all with reflection
Type type = typeof(ClassB<>).MakeGenericType(typeof(T));
object x = Activator.CreateInstance(type);
type.GetProperty("DataObject").SetValue(x, dataObject);
}
}
public object SetBDataObject<TB> (TB obj)
where TB : Bar<TB>
{
var x = new ClassB<TB>();
x.DataObject = obj;
return x;
}
}

The first thing that probably is confusing in your code, is that you have used the same letter T as the Type parameter in both classes ClassA<T> and ClassB<T>.
I'll start by stating the obvious:
when you call var x = new ClassB<T>(); the constraint for T here is in the context of ClassA<T> (i.e. T : Foo<T>), while new ClassB<T>() expects T to match the constraint of T : Bar<T>.
It seems to me that underlying reason for the issue you are having is a design problem. It looks like you a little mix up between types and classes.
Lets walk it through:
from the Gang of Four Design Patterns book:
An objects's class defines how the object is implemented .The class
defines object's internal state and the implementation of its
operations.
In contrast, an objects's type only refers to its interface -the set
of requests to which it can respond.
An object can have many type, and object of different classes can have
the same type.
The usage of interfaces in your code implies coding against types (that's good!).
Checking for if (dataObject is Bar<T>) and upon the result constructing a ClassB<U> where !typeof(U).Equals(typeof(T) implies heavily relying on implementation (e.g. class).
If you ask me, I think you should try one of the following:
Use the factory pattern for constructing ClassB. In the dedicated factory you can add some more logics and verifications in order to decide how to construct it (from your code, it is not clear since the types do not match...).
If possible, resolve the relation between Foo<T> and Foo<T> and declare the constraints in the interfaces. In that case, both interfaces should have same constraints fto T

How to initialize an interface that has type aguments of the type of another Interface

ive run into a problem - these is my class structure
public interface IModel{}
public interface IGenericMapper<T> where T : IModel {...}
public class ActualModel:IModel {...}
public class ActualMapper: IGenericMapper<ActualModel> {...}
My actual code to initialse the mapper is:
IGenericMapper<IModel> mapper;
mapper= new ActualMapper();
It does not compile. I get the error
Cannot implicitly convert type 'ActualMapper' to 'IGenericMapper'.
An explicit conversion exists (are you missing a cast?)
When I do cast it using
mapper= new ActualMapper() as IGenericMapper<IModel>;
the mapper does not get initialized properly (it comes back as NULL)
What am I missing - since ActualMapper() implements IGeneric Mapper and its type impliments `IModel' why can it not initialize mapper.
Is there another way to structure this so achieve what I need?
Thank you so much
Note the solution people have proposed gives me other compilation errors as the mapping Interface has the following members
T GetModel(busO bBusinessObject);
busO SetBusObject(T source, busO target);
apparently you cant have the generic type as an input parameter when its declared at "out"

Pretty sure you're moving into the realm of covarience here with the Generic change;
try this:
public interface IModel{}
public interface IGenericMapper< out T> where T : IModel{}
public class ActualModel : IModel{}
public class ActualMapper : IGenericMapper<ActualModel> {}
and then:
IGenericMapper<IModel> blah = new ActualMapper();
with out the 'out T' the best you can do is:
IGenericMapper<ActualModel> blah = new ActualMapper();
This is a rabbit hole, so be careful especially if you ever try to mix the two :)
http://msdn.microsoft.com/en-us/library/ee207183.aspx
[Edit]
If you want to be able to downcast the generic T, then it has to be out and you cannot use it as an input. You can, however, move some of it to real time in your implementation; i.e. to a check to see if you can cast it to a model type.
interface IGenericMapper<out TModel, in TKeyOrIdent>
TModel GetModel(TKeyOrIdent bBusinessObject);
void SetModel(object model, TKeyOrIdent target);

you have to define
IGenericMapper<out T>
to support your scenario, but this applies other limitations.
Simply IGenericMapper<IModel> != IGenericMapper<ActualModel> even if ActualModel : IModel
In most scenarios it makes sense to have a base interface that isn't a generic. See IList<T> for example, it implements IList.
You can then choose on implementation to explicitly implement interface members. See List<T>.GetEnumerator() : IEnumerable
Do not use generics under the assumption that you can eliminate casting everywhere. I've tried it C# simply doesn't have the required features for this.
I would suggest an IGenericMapper interface as base interface for IGenericMapper<T> then do your generic code on IGenericMapper and finally (and at that point you already have the type) cast it back to the specific type.

Any Downsides to making a method generic where the method return type includes an interface?

Consider a method like:
IEnumerable<IFoo> DoSomethingRequiringIFooInterface(IEnumerable<IFoo> foos)
where the method uses the IFoo interface (perhaps for filtering or sorting), but it's not creating new objects implementing IFoo.
Wouldn't it always be better to make the method generic like:
IEnumerable<TFoo> DoSomethingRequiringIFooInterface<TFoo>(IEnumerable<TFoo> foos)
where TFoo : IFoo
so that the client can preserve type? Or are there some downsides (or some cases) where you wouldn't want to do this?
Thanks!

One possible downside is if you want to overload the method:
void F(IFoo x) {}
void F(IBar x) {}
works fine. But
void F<T>(T x) where T : IFoo {}
void F<T>(T x) where T : IBar {}
doesn't compile, because the two methods now have the same signature.

You wouldn't want to do this if the returned values could be a different type of object implementing IFoo than the objects being passed in. The generic version is appropriate if all of the elements returned need to have at least the same level of type specificity as the elements passed in. So, if your function always returns SpecialFoo if provided SpecialFoo, the generic method is more appropriate, as it's more tightly constrained.

Some drawbacks could be possible if you are using Reflection to change or analyze your code, than you have to have more complex code to do this is as complexity of code you are chewing with reflection is bigger.

I would prefer generic method because you can always specify method with IFoo, and have same result as with nongeneric method:
DoSomethingRequiringIFooInterface<IFoo>(...);
And also you give better type safety options to user baecause he can do
(if Foo1 : IFoo) :
DoSomethingRequiringIFooInterface<Foo1>(...);
And exactly know that he is getting IEnumerable<Foo1>.
So this means that generic method is better because gives user more options and has no usabllity drawbacks except for more text he has to write.

This has a downside when you try using it with two different IFoo objects. like:
DoSomethingRequiringIFooInterface({ new AFoo(), new BFoo() });
using the first version this will work. using the second it won't.

c# generics on a method with the constraint that the type must be of "this" type

I have a C# class hierarchy with a common base type and two derived types. I want to declare an abstract method on the base class something like this :
public abstract IEnumerable<T> GetSiblings<T>() where T : MyBaseClass
... and I want this method to be implemented in the derived classes such that T is the type of that derived type, for each of the derived types, ie, in derived class A:
public override IEnumerable<A> GetSiblings<A>() { ... }
... and in derived class B ...
public override IEnumerable<B> GetSiblings<B>() { ... }
Put another way, each derived class must implement the method so that it returns an IEnumerable of items of the same type. Is there any way to implement this in C# ?

Well, you can hardly call a method generic if it only accepts a parameter of a single type, and your method signatures will have different return types which isn't allowed. Why don't you define an interface for all of these classes and simply return an IEnumerable<IMyClass>?

You can't do this because the return types are different. Simple as that. The reason is if you create an instance of A and stuff it into your base class(cast it) then the return type will be wrong.
You might be able to to use new instead but that might break your hierarchy.

This is not supported by the type system. It's a common enough problem, represented often as
class Animal<T> where T : Animal<T> { }
class Cat : Animal<Cat> { } // what you desire
class Dog : Animal<Cat> { } // what is possible yet not desired
But not a problem that has as yet been acted upon by the appropriate parties (be it the framework providers or C# team, not sure who).
Until it passes the critical "worth it" test as determined by costs (and opportunity costs) versus benefits, you'll have to work around it.

I found the solution. Apparently in C# 4.0, generic parameter types can be covariant, so what I've posted above will work. C# 3.5 or lower, and it doesn't work. Took a lot of Googling.

How can you require a constructor with no parameters for types implementing an interface?

Is there a way?
I need all types that implement a specific interface to have a parameterless constructor, can it be done?
I am developing the base code for other developers in my company to use in a specific project.
There's a proccess which will create instances of types (in different threads) that perform certain tasks, and I need those types to follow a specific contract (ergo, the interface).
The interface will be internal to the assembly
If you have a suggestion for this scenario without interfaces, I'll gladly take it into consideration...

Not to be too blunt, but you've misunderstood the purpose of interfaces.
An interface means that several people can implement it in their own classes, and then pass instances of those classes to other classes to be used. Creation creates an unnecessary strong coupling.
It sounds like you really need some kind of registration system, either to have people register instances of usable classes that implement the interface, or of factories that can create said items upon request.

You can use type parameter constraint
interface ITest<T> where T: new()
{
//...
}
class Test: ITest<Test>
{
//...
}

Juan Manuel said:
that's one of the reasons I don't understand why it cannot be a part of the contract in the interface
It's an indirect mechanism. The generic allows you to "cheat" and send type information along with the interface. The critical thing to remember here is that the constraint isn't on the interface that you are working with directly. It's not a constraint on the interface itself, but on some other type that will "ride along" on the interface. This is the best explanation I can offer, I'm afraid.
By way of illustration of this fact, I'll point out a hole that I have noticed in aku's code. It's possible to write a class that would compile fine but fail at runtime when you try to instantiate it:
public class Something : ITest<String>
{
private Something() { }
}
Something derives from ITest<T>, but implements no parameterless constructor. It will compile fine, because String does implement a parameterless constructor. Again, the constraint is on T, and therefore String, rather than ITest or Something. Since the constraint on T is satisfied, this will compile. But it will fail at runtime.
To prevent some instances of this problem, you need to add another constraint to T, as below:
public interface ITest<T>
where T : ITest<T>, new()
{
}
Note the new constraint: T : ITest<T>. This constraint specifies that what you pass into the argument parameter of ITest<T> must also derive from ITest<T>.
Even so this will not prevent all cases of the hole. The code below will compile fine, because A has a parameterless constructor. But since B's parameterless constructor is private, instantiating B with your process will fail at runtime.
public class A : ITest<A>
{
}
public class B : ITest<A>
{
private B() { }
}

Juan,
Unfortunately there is no way to get around this in a strongly typed language. You won't be able to ensure at compile time that the classes will be able to be instantiated by your Activator-based code.
(ed: removed an erroneous alternative solution)
The reason is that, unfortunately, it's not possible to use interfaces, abstract classes, or virtual methods in combination with either constructors or static methods. The short reason is that the former contain no explicit type information, and the latter require explicit type information.
Constructors and static methods must have explicit (right there in the code) type information available at the time of the call. This is required because there is no instance of the class involved which can be queried by the runtime to obtain the underlying type, which the runtime needs to determine which actual concrete method to call.
The entire point of an interface, abstract class, or virtual method is to be able to make a function call without explicit type information, and this is enabled by the fact that there is an instance being referenced, which has "hidden" type information not directly available to the calling code. So these two mechanisms are quite simply mutually exclusive. They can't be used together because when you mix them, you end up with no concrete type information at all anywhere, which means the runtime has no idea where to find the function you're asking it to call.

So you need a thing that can create instances of an unknown type that implements an interface. You've got basically three options: a factory object, a Type object, or a delegate. Here's the givens:
public interface IInterface
{
void DoSomething();
}
public class Foo : IInterface
{
public void DoSomething() { /* whatever */ }
}
Using Type is pretty ugly, but makes sense in some scenarios:
public IInterface CreateUsingType(Type thingThatCreates)
{
ConstructorInfo constructor = thingThatCreates.GetConstructor(Type.EmptyTypes);
return (IInterface)constructor.Invoke(new object[0]);
}
public void Test()
{
IInterface thing = CreateUsingType(typeof(Foo));
}
The biggest problem with it, is that at compile time, you have no guarantee that Foo actually has a default constructor. Also, reflection is a bit slow if this happens to be performance critical code.
The most common solution is to use a factory:
public interface IFactory
{
IInterface Create();
}
public class Factory<T> where T : IInterface, new()
{
public IInterface Create() { return new T(); }
}
public IInterface CreateUsingFactory(IFactory factory)
{
return factory.Create();
}
public void Test()
{
IInterface thing = CreateUsingFactory(new Factory<Foo>());
}
In the above, IFactory is what really matters. Factory is just a convenience class for classes that do provide a default constructor. This is the simplest and often best solution.
The third currently-uncommon-but-likely-to-become-more-common solution is using a delegate:
public IInterface CreateUsingDelegate(Func<IInterface> createCallback)
{
return createCallback();
}
public void Test()
{
IInterface thing = CreateUsingDelegate(() => new Foo());
}
The advantage here is that the code is short and simple, can work with any method of construction, and (with closures) lets you easily pass along additional data needed to construct the objects.

Call a RegisterType method with the type, and constrain it using generics. Then, instead of walking assemblies to find ITest implementors, just store them and create from there.
void RegisterType<T>() where T:ITest, new() {
}

I don't think so.
You also can't use an abstract class for this.

I would like to remind everyone that:
Writing attributes in .NET is easy
Writing static analysis tools in .NET that ensure conformance with company standards is easy
Writing a tool to grab all concrete classes that implement a certain interface/have an attribute and verifying that it has a parameterless constructor takes about 5 mins of coding effort. You add it to your post-build step and now you have a framework for whatever other static analyses you need to perform.
The language, the compiler, the IDE, your brain - they're all tools. Use them!

No you can't do that. Maybe for your situation a factory interface would be helpful? Something like:
interface FooFactory {
Foo createInstance();
}
For every implementation of Foo you create an instance of FooFactory that knows how to create it.

You do not need a parameterless constructor for the Activator to instantiate your class. You can have a parameterized constructor and pass all the parameters from the Activator. Check out MSDN on this.