abstarct class basic question [modified]

[Mirko1980]

You can use generics like this:

abstract class b<T> where T: b, new()
{
...
public T Create() { return new T(); }
}

class D1 : b<D1>
{
...
}
class D2 : b<D2>
{
...
}

So, you can do:

D1 myClassD1 = new D1();
D1 newClassD1 = D1.Create();

But, you can't do anymore:

b myClassAb = new D1;
D1 newClassAb = myClassAb.Create();

because you'd have to specify the generic type for class b.

[Steven Pinto2000]

using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace general { class Program { static void Main(string[] args) { D1 myClass = new D1(); D1 newClass = myClass.Create(); //this is what I would like b myClassAb = new D1(); b newClassAb = myClassAb.Create<D1>(); newClass.KM = 1.23; } } abstract class b { //work in this way !!! public T Create<T>() where T:new() { return new T(); } } class D1 : b { public double KM; public D1() { KM = 1.0; } // public D1 Create() { return new D1(); } } class D2 : b { public double KM; public D2() { KM = 2.0; } // public D2 Create() { return new D2(); } } } here is one solution using generics

modified on Wednesday, June 1, 2011 9:53 AM

[Luc Pattyn]

Actually, you don't need Visual Studio at all. What you are meaning to say is "this requires .NET 4.0" :)

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[PozzaVecia]

Sorry probably I did not explain very well the problem. I need to have avaiable methods in newClassAb //this is what I would like b myClassAb = new D1(); b newClassAb = myClassAb.Create<D1>(); //This is what I need to see K from newClassAb Console.WriteLine(newClassAb.K)>//but I can't see .K avaiable Thanks for your time

modified on Wednesday, June 1, 2011 7:32 AM

[BobJanova]

There's a very simple way, if you know you want a D1: call new D1() not myD1Instance.Create(). I'm guessing this is a simplified example, because it simply doesn't make sense to do things this way in most cases. What is your actual problem? Accepting for the moment that you need a Create method at all: If you know that an instance of b is actually a D1 (for example within the D1 class itself) then you can cast the result of Create safely. Casting is not necessarily bad (it's safer than the use of dynamic, I would say). For example, in some similar code to what I have at the moment for cloning objects in an inheritance tree:

class b {
private int someField;

public b Copy() {
b newB = Create();
CopyTo(newB);
return newB;
}

protected virtual void CopyTo(b newB){
// copy properties into the target
newB.someField = someField;
}

class D1 : b {
private double derivedField;

protected override void CopyTo(b newD1){
// This method will only be called if we are a D1,
// and therefore the result of Create is also a D1,
// and therefore the parameter is also a D1
D1 target = (D1)newD1; // ... so this must succeed
target.derivedField = derivedField; // ... and we can access D1's members
}
}

Outside the class tree, it's unlikely that you know for sure at compile time which of the classes you have an instance of (otherwise you wouldn't need the abstract base class at all), so you can normally only cast the result inside the class, or using if(my is D1) type tests. And if you find yourself writing those you probably just found somewhere that an abstract method should be provided on the base class.

[BobJanova]

Now you can't call Create on an arbitrary b instance, which renders it pointless.

[BobJanova]

In response to your modification: KM should clearly be defined on the base class. It should probably also be a property.

abstract class b
{
    public double KM { get; protected set; }  // C# 2.0 automatic property syntax
    public b Create() { return Activator.CreateInstance(GetType()); }
}

class D1:b 
{
    public D1() { KM = 1.0; }
}
class D2 : b 
{
    public D2() { KM = 2.0; }
}

[PozzaVecia]

Console.WriteLine(b.//<big>b.KM is still not avaiable!!</big>}

namespace general
{

class Program
{
static void Main(string[] args)
{

//this is what I would like
b myClassAb = new D1();
b newClassAb = myClassAb.Create();
Console.WriteLine(b.//K is still not avaiable!!}
}

abstract class b
{
public double KM { get; protected set; } // C# 2.0 automatic property syntax
public b Create() { return (b)Activator.CreateInstance(GetType()); }
}

class D1 : b
{
public D1() { KM = 1.0; }
}
class D2 : b
{
public D2() { KM = 2.0; }
}

[BobJanova]

You should be able to use newClassAb.KM.

[PozzaVecia]

Great!!!! It works...Thanks a lot:cool:

[PIEBALDconsult]

:thumbsup:

[Mirko1980]

I said that at the end of my post. I agree you are losing many advatages of polymorphism, but, at least, you can reuse the Create login in multple classes.

[Steven Pinto2000]

what is K is it KM well i can see KM try it once more because there was some error in my code

[PozzaVecia]

b myClassAb = new D1();
b newClassAb = myClassAb.Create<D1>();
newClassAb.KM = 1.23;

apparently return this error" Error 1 'general.b' does not contain a definition for 'KM' and no extension method 'KM' accepting a first argument of type 'general.b' could be found (are you missing a using directive or an assembly reference?)" The working solution example, thanks to BobJanova is:

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

namespace general
{

class Program
{
static void Main(string[] args)
{

//this is what I would like
b myClassAb = new D1();
b newClassAb = myClassAb.Create();
Console.WriteLine(newClassAb.KM);
}
}

abstract class b
{
public double KM { get; protected set; } // C# 2.0 automatic property syntax
public b Create() { return (b)Activator.CreateInstance(GetType()); }
}

class D1 : b
{
public D1() { KM = 1.0; }
}
class D2 : b
{
public D2() { KM = 2.0; }
}
}

[_Erik_]

I am sorry but, what is the point in creating an instance of a class to just create another instance of the same class? I mean, since Create method is not static it would just work as a constructor so, why don't you just use a constructor? For example, what you want to do is this:

D1 obj = new D1();
D1 obj2 = D1.Create();

And what I am saying is... why not?

D1 obj = new D1();
D1 obj2 = new D1();

I guess you want a static Create method in your base class, don't you? In this case you will have to make your Create method static and use generics:

abstract class BaseClass
{
public static T Create<T>() where T : BaseClass, new()
{
return new T();
}
}

class DerivedClass : BaseClass { }

This way you could use it like this:

DerivedClass obj = BaseClass.Create<DerivedClass>();

[PozzaVecia]

I was looking for a way to avoid to write the same piece of code in each derived class since they basically do the same thing. I Would like to create a method in the base class. But this should allow you to use methods of derived class Is it?

class D1:b
{
..
public D1 Create() { return new D1(); }
}
class D2 : b
{
..
public D2 Create() { return new D2(); }
}

So my goal is not to do

D1 obj = new D1();
D1 obj2 = D1.Create();

but to do

b myClassAb = new D1();
b newClassAb = myClassAb.Create();
Console.WriteLine(newClassAb.KM); //i.e. I need to access to proprieties, methods, etc of class

[_Erik_]

Member 4282287 wrote:

I need to access to proprieties, methods, etc of class

That does not depend on how you create the objects, I mean, your Create method will not help you to achieve what you want. If you can access KM property of a D1 object which you have declared as its base class type (b in this case), you can becouse KM property is definied within the "b" class interface, but not becouse you have made such a strange Create method. So in your sample:

b myClassAb = new D1();
b newClassAb = myClassAb.Create();
Console.WriteLine(newClassAb.KM)

You don't need the Create method at all. Just use the constructor and one instance of the object:

b myClassAb = new D1();
Console.WriteLine(myClassAb.KM)

[PozzaVecia]

ok thanks understood. Really the starting example was a bit different ... i.e. I needed a method avaiable only in derived class. During the forum this method ... become a proprierties.. Apparently I understood. You are right of course. Thanks for your time