Dynamic Cast

[Wildhorn]

void Whatever(Type ClassType, ArrayList myArrayList)
{
ClassType myClass = new ClassType();
myClass = (ClassType)myArrayList[0];
}
 

How can I make this work? ClassType is the type of an homemade class.

[tlhintoq]

A) Who says that doesn't work? You haven't said it produces an error.
B) What is in your ArrayList? We have no idea if you are trying to convert a Cat to a Dog or a String to an int

How about making a constructor in your custom class that takes the information stored in your array elements?

[Wildhorn]

It doesnt work, else I would not be asking for help. It just doesnt want to compile. It refuse to take a variable Type for a cast.

In my arraylist, like I said, are homemade class.

[tlhintoq]

The syntax for the cast is legal.
I'm sure you realize this because you know you can cast a float to an int, in this way for example

float Yogi = 0.15f;
int bear = (int)Yogi;

But the compiler doesn't have a crystal ball. If you have a custom class of "Widget" and an arraylist containing "Gadget"s - The compiler just doesn't know *how* it should cast that.

You may need to write a custom cast for your different classes.
http://msdn.microsoft.com/en-us/libr...05(VS.80).aspx

Usually I'm working with classes that inherit from a common class, so VIsual Studio can generally do that impliticly (no custom cast function)

class Mammal
class Dog : Mammal
class Cat : Mammal

Dog myDoberman = new Dog();
Cat myTabby = (Cat)myDoberman;// Legal since they are equal classes of same parent

[jkmyoung]

I think you want Invoke: eg http://msdn.microsoft.com/en-us/libr...(v=VS.71).aspx

.. I don't understand why you're creating an object and then shortly thereafter overwriting it.

            ConstructorInfo constructorMethod = ClassType.GetConstructor(new Type[0]);
            object c = constructorMethod.Invoke(new object[0]);

//Assumes you have a public constructor with no arguments.

If you're going to run methods on it, you need to use Type.GetMethod(s)() to get the methods you want to run. Is this really necessary, or could you just use some sort of polymorphism instead, as suggested above?

[Wildhorn]

This was just an example. I would not write you the whole thing because it does 400 lines long. But I need to dynamically cast something as a type I receive as parameter. Your thing for constructor is nice, but I need to cast something that already have variables saved into it.

about:

class Mammal
class Dog : Mammal
class Cat : Mammal

Dog myDoberman = new Dog();
Cat myTabby = (Cat)myDoberman;// Legal since they are equal classes of same parent

That's not it. I dont know that it is a Cat. It could be a Cat, a Dog, an Elephant, an Alien. I dont know beforehand what the cast will be.

It would be more:

class Mammal
class Dog : Mammal
class Cat : Mammal
Type myType = typeof(Cat);

MyType myTabby = (myType)myDoberman;

Which doesnt compile.

[jkmyoung]

Why do you need the cast? What are you doing that afterwards that needs myTabby to be treated as a dynamic type? Any other function can call typeof on the object to determine what type it is anyways.

There's probably an easier way to do it.

[Wildhorn]

Because I essentially do the same treatment to 3 different class data, so I do not want to have to write 3 times 400 lines of code when only 1 cast need to be changed.

[Christian Binder]

If you have the following:
MyType myTabby = (myType)myDoberman;

what are you going to do with myTabby?
If you don't know the type of object you're getting,
how would you manipulate it's data or execute it's methods?
(e.g. Cat can do other things than Dog can do and has other Attributes
Both could do the action Walk() sinc they are both mammals but a dog would be able to do the action ClimbUpTree()
)

So if you just want to do basic things to this objects, you can cast it to their base-class (e.g. (Mammal)myObject; )

Maybe you can tell what you want to achieve with your program and speak more special about your classes and data and not only about mammals which I think isn't the subject of your program.

[Monomachus]

@Wildhorn
I would say you got a lot of possibilities.
1. You can do an interface ISomething and then make a helper which works with ISomething and inherit all those classes from ISomething.

2. Make an abstract class, inherit all those classes from abstract class. Which is not very different from interface but could help because you put all the code which is common in abstract class.

3. Add to constructor of those classes a parameter which will indicate the helper which should work with all those classes.

Dunno, I just think you should really know what you need to do.

[jkmyoung]

Modularization is a little tough when you're starting out.
1. Make a list of all the things that you plan to do with all 3 of your classes. Eg maybe there's a toString() method, some sort of descriptor methods, like length or size or colour.

2. Write a class that has all of these common functions.

At this point I am not sure if you even need more than 1 class.

[tlhintoq]

Are your three different classes all inherited from the same type?
I do this with UserControls for industrial scanners.

Class ScannerUI : UserControl
Class BrandAlpha: ScannerUI
Class BrandBravo: ScannerUI

In this way I have no problems doing a case exactly as you have described.

[Wildhorn]

No they do not inherit from the same type, but they all have string variables that need to be treated the same way.

Anyway, I got a very "dirty" solution that I do not like much but that will do it for the moment. I made a function that receive the class object and with a switch case return an object of the good type and it works now, but it piss me off that I cant on the fly cast stuff when language like LUA allow such task (you dont even need to cast anything, LUA knows what you want).

[tlhintoq]

A) Change it so they do all inherit from the same type
B) Write a custom cast for your class as recommended earlier

[jkmyoung]

Maybe make a 4th 'utility' class that handles the strings?

Object ob;
String str;
 
switch (TypeOf(ob)){
case TypeOf(Dog):
  str = (Dog)ob.Tag;
  break;
case TypeOf(Cat):
  str = (Cat)ob.Tray;
  break;
case TypeOf(Moose):
  str = (Moose)ob.Antler;
  break;
}
 
Utility.HandleString(str);
 

Actually, not sure if you can use switch on Types, but just an if-else if chain.

[Monomachus]

@Wildhorn
In .NET 4.0 you can do dynamic cast. So please see the specs for dynamic keyword.
Also please do notice that C# is a static/strongly-typed language and by adding DLR (Dynamic Language Runtime) it hasn't changed into a dynamic language.
To see why is C# statically typed please visit the link "Why is C# statically typed?"

[hype261]

If you have to cast then your design is probably wrong. From what I understand of your problem is that you want to store multiple classes in the same array and then cast them out to use them. Even if you get this to work you will probably run into run time problems due to improper casting. Like others have suggested you should define an interface if you want multiple classes to have the same behavior even if they don't derive from the same type.

[jkmyoung]

Could you post sample LUA code that does what you want?

This is mostly for interest, but also to see if there is another way to do it.