As Keyword, conversion of derived class to base class type

Raybarg

Here is example code I made for this question:

using System;
namespace TestingAs
{
class baseClass
{
public string foo
{
get;
set;
}
}
class derivedClass : baseClass
{
public string bar
{
get;
set;
}
}

class Program
{
    static void Main(string\[\] args)
    {
        derivedClass derived = new derivedClass();
        baseClass b = derived as baseClass;
        Console.WriteLine("type is {0}", b.GetType());
        Console.ReadLine();
    }
}

}

I also tried just casting baseclass b = (baseClass)derived; which gives the same result: type is TestingAs.derivedClass Why b is not type of baseClass? How would I "convert" my derived class into base class type? Thank you in advance!

Abhishek Sur

This is the same concept of Reference Copying. In runtime the pointer of the object is set to derived class object. Thus it will always hold the derivedclassobject even if you cast it to base class object. :)

Abhishek Sur

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PIEBALDconsult

Convert.ChangeType ? Why would you want to?

OriginalGriff

Think about it. When you derive a class from baseClass, you are adding information to baseClass. When you try to convert your derived class to a baseClass, you would have to throw information away. You could not recover the information by re-casting your item to a derivedClass, because otherwise your program would have to behave very differently with the following :

derivedClass d = new derivedClass();
baseClass b = d as baseClass;
derivedClass d2 = (derivedClass) b;

and

baseClass b = new baseClass();
derivedClass d2 = (derivedClass) b;

If the definitions were in (say) different methods to the casts, it could get very difficult to work out what is going on. Since wherever you can use a baseClass, you can use any derived class, C# is not keen on throwing away information, which could not be recovered by a re-cast back to the derived class.

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Raybarg

Does it matter why I want to? My app is using .dll's which are returning class arrays. My previous problem was that the application was completed and working well when it was required to use custom data in a way I found "good" solution to change my application to use derived class from the class type returned from the .dll layer, it just worked as long as I did not need to use same class array objects as parameters in .dll layer method calls.

PIEBALDconsult

I didn't quite follow all that, but I still see no reason to downgrade the instances.