How to instantiate an object with a generic parameter?

michal kreslik

Hello, I can't figure this one out, so I'll be happy if someone lends me a helping hand here. How do I instantiate an object with a generic type parameter if I don't know what the type parameter will be at run time? Example:

namespace GenericTypeInitialization
{
class Program
{
static void Main()
{
Console.WriteLine("Press 1 for FooA, press 2 for FooB:");
ConsoleKeyInfo key = Console.ReadKey(true);

        Type TypeOfFoo;

        if (key.KeyChar == '1')
        {
            Console.WriteLine("FooA selected.");
            TypeOfFoo = typeof(FooA);
        }
        else if (key.KeyChar == '2')
        {
            Console.WriteLine("FooB selected.");
            TypeOfFoo = typeof(FooB);
        }

        ////
        Bar<TypeOfFoo> MyInstanceOfBar = new Bar<TypeOfFoo>();
    }
}

class FooA {}

class FooB {}

class Bar<TFoo> {}

}

Obviously, the above code doesn't compile. I'm getting an error of The type or namespace name 'TypeOfFoo' could not be found (are you missing a using directive or an assembly reference?) How do I get the type parameter at run time then? I've tried various things, but none worked. Also, you can't instantiate the Bar<> class before '////'. Thanks a lot for any input, Michal

Giorgi Dalakishvili

You will have to use reflection I guess

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Ennis Ray Lynch Jr

You can't. Generics in .NET are evaluated during runtime creating special types, [^] The best solution to your problem is to use inheritance and base classes.

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Le centriste

Generics are designed to be used when you know the type at compile-time (early binding). When you don't know the type until run-time (late binding), you have to use reflection.

----- You seem eager to impose your preference of preventing others from imposing their preferences on others. -- Red Stateler, Master of Circular Reasoning and other fallacies If atheism is a religion, then not collecting stamps is a hobby. -- Unknown God is the only being who, to rule, does not need to exist. -- Charles Baudelaire

michal kreslik

Ok, so how would you use reflection to make the above example work as intended? Thanks, Michal

Le centriste

I reexamined your example, and her is what I suggest. Since you only have 2 well-known concrete classes, you may not need reflection for now, but generics is not the way to go either. Make an interface:

public interface IFoo
{
    void MyMethod(); // Interface defines one method.
}

Then have some implementation, 2 in you example:

public class FooA : IFoo
{
    public void MyMethod() { Console.WriteLine("Hello from FooA"); }
}

public class FooB : IFoo
{
    public void MyMethod() { Console.WriteLine("Hello from FooB"); }
}

Then, using your example code:

class Program
{
    static void Main()
    {
        Console.WriteLine("Press 1 for FooA, press 2 for FooB:");
        ConsoleKeyInfo key = Console.ReadKey(true);            
        IFoo myFoo = null;            

        if (key.KeyChar == '1')            
        {                
            myFoo = new FooA();            
        }            
        else if (key.KeyChar == '2')            
        {                
            myFoo = new FooB();            
        }
        else
        {
            throw new InvalidOperationException("Wrong selection, you must select 1 or 2"); // Always have a watch dog.
        }

        myFoo.MyMethod(); // This prints which foo was created.
    }
}

This example is pretty simplistic, but you get the idea. In this particular case, you didn't need generics nor reflection. In real-life application, this is rarely sufficient. Read on reflection, I am sure there are quite good articles on this site. Good luck.

----- You seem eager to impose your preference of preventing others from imposing their preferences on others. -- Red Stateler, Master of Circular Reasoning and other fallacies If atheism is a religion, then not collecting stamps is a hobby. -- Unknown God is the only being who, to rule, does not need to exist. -- Charles Baudelaire

michal kreslik

Hello, thanks for your reply. Unfortunately, this is not what I need. Interface was my first thought on how to solve this, but I can't use the interface because I have to be able to copy the containing type by value, not by reference. If you declare the object as an interface, you can only copy it as a reference later on (unless you unbox it in which case you - again - need to know its type :) I did not include the error handling in my example as I guess we are solving something else here. Also, as I said, the type should not be instantiated before '////' mark. It's actually being instantiated much later on in the code, but I need to know the type first. I've been working with reflection and generics before. My above example is a much simplified illustration of what I need to do, but the main issue is still finding out the type and passing it along as a parameter. Thanks, Michal

darkelv

class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine((new Bar<FooA>()).ToString());
            Console.WriteLine((new Bar<FooB>()).ToString());
        }
    }

    public abstract class BaseFoo
    {
    }

    public class FooA: BaseFoo
    {
        public FooA()
        {
        }
        public override string ToString()
        {
            return "FooA";
        }
    }

    public class FooB: BaseFoo
    {
        public FooB()
        {
        }
        public override string ToString()
        {
            return "FooB";
        }
    }

    public class Bar<TFoo> where TFoo : BaseFoo, new()
    {
        TFoo tFoo = new TFoo();
        public override string ToString()
        {
            return tFoo.ToString();
        }
    }

Mark Churchill

You'll be looking at TypeBuilder.MakeGenericType unfortunately I think...

Mark Churchill Director Dunn & Churchill Free Download:
Diamond Binding: The simple, powerful, reliable, and effective data layer toolkit for Visual Studio.

michal kreslik

Thanks for your post, but it doesn't solve the problem of instantiating the class with a generic parameter on demand during run time. Also, the generic object can't inherit the abstract class because as I said, it has to be copiable by value, not by reference. Michal

michal kreslik

Hello, Mark, thanks, I've looked into TypeBuilder, but this class is used to build the objects on the fly. I don't need to BUILD the obect on the fly, I already have the objects well defined beforehand. I just need a reference for that object type. So it looks like this cannot be accomplished in C#.

Mark Churchill

You could have an IDictionary and preinitialize it with {typeof(Foo), typeof(Bar< Foo >)}, {typeof(Deh), typeof(Bar< Deh >)...}

Mark Churchill Director Dunn & Churchill Free Download:
Diamond Binding: The simple, powerful, reliable, and effective data layer toolkit for Visual Studio.

modified on Thursday, February 07, 2008 6:13:04 AM