Re: About Reflecting and generics - C# Discussion Boards

BobJanova14-Feb-12 23:20

14-Feb-12 23:20

I can almost guarantee that you don't want to do what you think you do. If you create a type via reflection, you can't use it in a normal compile-time way because the type is not known until runtime. That means you need to do everything about it through reflection, which is slow.

I still don't have a good understanding of what exactly you're trying to do, but generally you want to use a List<BaseClass> or List<Interface>, where all the classes you're picking between at runtime inherit from/implement the class/interface in question. Then you can use compile time method binding (i.e. write normal code) against the class/interface, and use the magic of OO (polymorphism) to get different functionality.

In this case it appears you already have the base classes, and in fact you just want the (compile time) class MyClass to be written in terms of Car and Moto. (I hope that MyClass is a dummy name! Make sure your classes have meaningful names.) I'm not even sure you need generics here. Remember that if you have

class MyClass {
 void SomeMethod(Car car, Moto moto){ ... }
}

... you can pass any instance of a subclass of Car and/or Moto to it. The same applies if you're using a generic class at some point, for example if your implementation includes a List<Car> – you can still put an AudiA8 into that list, as long as that class inherits from Car, even if the class is loaded at runtime.

I suspect what you actually want is something like

class MyClass {
 public Car Car { get; set; }
 public Moto Moto { get; set; }

 ...
}

... and then to set the properties to instances of subclasses of Car and Moto as chosen at runtime.

PozzaVecia14-Feb-12 23:29

14-Feb-12 23:29

not sure I have explained my problem.

I have string and I need to inizialize the class
So from may excel i get "AudiA8", "ZXR"

And I need to inizialize:
MyClass.SomeMethod(AudiA8,ZXR) //from string to class

BobJanova15-Feb-12 0:50

15-Feb-12 0:50

You have not explained your problem. And you still haven't.

Here's the part you may not be getting. If AudiA8 is a Car (i.e. it inherits from Car, or implements ICar), then you can pass it to a method taking a car. That is, you can do

MyClass.SomeMethod(new AudiA8(), new ZXR());

... given

class AudiA8 : Car {
 ...
}

class ZXR : Moto {
 ...
}

and

class MyClass {
 static void SomeMethod(Car car, Moto moto){ ... }
}

You don't need any runtime shenanigans with MyClass to make that work. You can still do this even if you're creating the cars/motos at runtime:

Car CreateCar(string classname){
 return (Car)Activator.CreateInstance(classname);
}

Moto CreateMoto(string classname){
 return (Moto)Activator.CreateInstance(classname);
}

void DoSomething(){
 MyClass.SomeMethod(
    CreateCar("YourNamespace.Cars.AudiA8"), 
    CreateMoto("YourNamespace.Motos.ZXR")
 );
}

PozzaVecia15-Feb-12 1:24

15-Feb-12 1:24

MyClass.SomeMethod(new AudiA8(), new ZXR());

... given

class AudiA8 : Car {
 ...
}
 
class ZXR : Moto {
 ...
}

yes but from excel i have a string "AudiA8" and not new AudiA8()

myClass<car, mot>

...generic need car as a type but not as string, but I have only string as input

BobJanova15-Feb-12 1:59

15-Feb-12 1:59

Did you bother reading my previous post all the way to the bottom where it shows you how to instantiate a type from a string?

PozzaVecia15-Feb-12 2:30

15-Feb-12 2:30

Yes sorry, I'm not able to fit the solution in my case

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace TestGeneric
{
    class Program
    {
        static void Main(string[] args)
        {
            //Input are string they comes from excel! I can customize the string but is a string
            string myCar = "carA";
            string myMoto = "motoB";

            //here the problem:

            // i SHOULD INIZIALIZE mPricer using string "carA" and "carB". 
            // I need to obtain an istance of mPricer<carA,motoB>  starting from strings

            mPricer< (car) Activator.CreateInstance(myCar), (moto) Activator.CreateInstance(myMoto)> Pr = new  mPricer< (car) Activator.CreateInstance(myCar), (moto) Activator.CreateInstance(myMoto)>();
           
        }
    }

    //dummy class for example.. It is only example to show it must have two generics
    public class mPricer<T, G> 
        where T:car, new()
        where G : moto, new()
    {
        double total;
        public mPricer()
        {
            T c = new T();
            G m = new G();
            total = c.GetPrice() + m.GetCost();        
        }

        public void ShowTotal() 
        {
            Console.WriteLine(total);
        }
    }

    public abstract class car { abstract public double GetPrice();}
    public class carA : car { public override double GetPrice() { return 11; } }
    public class carB : car { public override double GetPrice() { return 12; } }

    public abstract class moto { abstract public double GetCost(); }
    public class motoA : moto { public override double GetCost() { return 7; } }
    public class motoB : moto { public override double GetCost() { return 6; } }


}

modified 15-Feb-12 9:04am.

BobJanova15-Feb-12 2:37

15-Feb-12 2:37

There is no reason why mPricer should construct these objects within its own constructor. Construct them outside and pass them in, statically typed as car and moto, as in my example.

// I need to obtain an istance of mPricer<carA,motoB> starting from strings

No, no you do not, and until you understand this you're going to keep tying yourself in knots. You need to obtain an instance of mPricer the car and moto instances of which are instantiated from strings. I already demonstrated how to do this.

You seem to show a fundamental lack of understanding of the difference between types and instances, and frankly until you get a grip on that you shouldn't be dabbling with this sort of runtime creation.

GParkings15-Feb-12 2:52

15-Feb-12 2:52

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
 
namespace TestGeneric
{
    class Program
    {
        static void Main(string[] args)
        {            
            string myCar = "carA";
            string myMoto = "motoB";
 
            mPricer Pr = new  mPricer(myCar,myMoto);
           
        }
    }
 
    public class mPricer 
    {
        double total;
        public mPricer(string carId, string motoId)
        {
            Car c = (car) Activator.CreateInstance(myCar);
            Moto m = (moto) Activator.CreateInstance(myMoto);
            total = c.GetPrice() + m.GetCost();        
        }
 
        public void ShowTotal() 
        {
            Console.WriteLine(total);
        }
    }
 
    public abstract class car { abstract public double GetPrice();}
    public class carA : car { public override double GetPrice() { return 11; } }
    public class carB : car { public override double GetPrice() { return 12; } }
 
    public abstract class moto { abstract public double GetCost(); }
    public class motoA : moto { public override double GetCost() { return 7; } }
    public class motoB : moto { public override double GetCost() { return 6; } }
 

}

That, i think, should do it ... you don't need a generic class

Pedis ex oris
Quidquid latine dictum sit, altum sonatur

GParkings15-Feb-12 2:54

15-Feb-12 2:54

or better yet....

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
 
namespace TestGeneric
{
    class Program
    {
        static void Main(string[] args)
        {            
            string myCar = "carA";
            string myMoto = "motoB";

            Car c = (car) Activator.CreateInstance(myCar);
            Moto m = (moto) Activator.CreateInstance(myMoto);
 
            mPricer Pr = new  mPricer(c,m);
           
        }
    }
 
    public class mPricer 
    {
        double total;
        public mPricer(Car c, Moto m)
        {
            total = c.GetPrice() + m.GetCost();        
        }
 
        public void ShowTotal() 
        {
            Console.WriteLine(total);
        }
    }
 
    public abstract class car { abstract public double GetPrice();}
    public class carA : car { public override double GetPrice() { return 11; } }
    public class carB : car { public override double GetPrice() { return 12; } }
 
    public abstract class moto { abstract public double GetCost(); }
    public class motoA : moto { public override double GetCost() { return 7; } }
    public class motoB : moto { public override double GetCost() { return 6; } }
 
 
}

it helps to think about the interface exposed by a type ... regardless of which runtime car class you have you always want to call GetPrice ... we require that all car classes expose a getPrice method so this is either declared on the base class or interface. since this base class contains all the common functionality required it is sufficient for use throughout mPricer

generics would only be required if you wanted to expose the car/moto instances and didn't want to require a cast to their runtime types but, in an interface driven design you should probably ask yourself WHY you need the runtime type

Pedis ex oris
Quidquid latine dictum sit, altum sonatur

PozzaVecia15-Feb-12 3:09

15-Feb-12 3:09

Oky thanks a lot.
The problem is that mPrice is a hudge class and I can not change it.

public class mPricer<T, G>

Contrains are:
input as string
ublic class mPricer<t, g="">

So I think is not possible to solve my problem without changing the class

GParkings15-Feb-12 3:25

15-Feb-12 3:25

aye, if you cant modify mPricer you are in the unfortunate situation of being stuck with someone elses shoddy design ... we've all been there and you have my sympathy.

Pedis ex oris
Quidquid latine dictum sit, altum sonatur

BobJanova15-Feb-12 3:40

15-Feb-12 3:40

Well, if the class is broken and you can't change it, then you're pretty screwed. You should have mentioned this in the original question :P.

How much can you 'not change it'? Is this because you don't have the source, or because someone has told you you can't? If the latter, ignore them and don't tell them, and make the trivial change to call a factory instead of using new() directly.

PozzaVecia15-Feb-12 10:17

15-Feb-12 10:17

Are you sure you can compile it?
Activator.CreateIstance() doesn't accept string

BobJanova15-Feb-12 21:57

15-Feb-12 21:57

You might be right there, answering from memory doesn't always work.

Try

Type carType = Type.GetType("MyNamespace.Cars.Car");
Car car = (Car)Activator.CreateInstance(carType);

PozzaVecia16-Feb-12 0:02

16-Feb-12 0:02

Thanks, really I was missing some reference in my project.
Thanks for your replay It is very usefull. I'm finding a solution using your path..

GParkings15-Feb-12 22:54

15-Feb-12 22:54

TBH, i just copied those lines from the OPs example without thinking, however there is an overload for createInstance that takes assembly name and type name as strings.. so i minor tweak Wink | ;)

Pedis ex oris
Quidquid latine dictum sit, altum sonatur

PozzaVecia16-Feb-12 0:05