Inheritance question

xkrja

I've referenced a dll made in VB.Net and now I need to inherit from some of the classes in the dll. Below is an example of how I did that:

class Tag : DataProvider.Trend.Tag
{
}

Now if I do like this:

DataProvider.Trend.Tag myBaseTag = new DataProvider.Trend.Tag();
Tag myTag = myBaseTag as Tag;

this results in that 'myTag' gets the value null. Why is that? How can I assign the value of myBaseTag to myTag? Thanks for help!

Lost User

Tag myTag = new Tag(); DataProvider.Trend.Tag myBaseTag = myTag as DataProvider.Trend.Tag; Because myTag inherite from DataProvider.Trend.Tag.

xkrja

Thanks for the reply. But the problem for me is that I call methods in the dll I mention and they return the type 'DataProvider.Trend.Tag' BUT I need to cast that to type 'Tag'. I don't want to cast 'Tag' to 'DataProvider.Trend.Tag' (the reason is that 'Tag' so that it is accepted by a Web Service). Thanks again.

Christian Graus

xkrja wrote:

DataProvider.Trend.Tag myBaseTag = new DataProvider.Trend.Tag(); Tag myTag = myBaseTag as Tag;

Of course that does not work. You can cast down, you cannot cast up. You need to read a basic book on OO

Christian Graus Driven to the arms of OSX by Vista. Read my blog to find out how I've worked around bugs in Microsoft tools and frameworks.

DaveyM69

That's not going to work 'out of the box', you need to provide an explicit/implicit conversion method if you want to upcast.

Dave
Generic BackgroundWorker - My latest article!
BTW, in software, hope and pray is not a viable strategy. (Luc Pattyn)
Why are you using VB6? Do you hate yourself? (Christian Graus)

Mirko1980

You can cast an object of type A to a type B that inherits from A only when the object was already declared as B. What you want to achieve is possible by creating a new object of type Tag and then initializing all its properties by using values from the 'DataProvider.Trend.Tag' original object (you can create a copy constructor for this).

dojohansen

Say we have

class A { protected string field1; }
class B : A { protected string field2; }
class C : B { protected string field3; }

void foo(A a) {}
void bar(B b) {}
void foobar(C c) {}

Now consider what instances of these objects *actually* will be inside your hardware. (We can safely ignore the fact that A extends object for the purpose of looking at why your attempted cast doesn't make sense.) First a word about references: A reference is what we might also call a "managed pointer": Like a pointer, it simply contains the address where the data is stored, but unlike a pointer, we cannot obtain this address and the memory manager can move stuff around and update pointers as needed without us knowing anything about it. Since string is a reference type, an instance of A is a word of memory (4 bytes on a 32-bit system) containing a reference to string data. An instance of B however is two words of data, containing two references, to field1 and field2 data. And C of course is three words, laid out in memory like an instance of B appended with the additional reference to field3 data. What you want to do is the equivalent of trying to use an instance of A and pass it to foobar, which takes a parameter of type C. Since an A is just the word for the field1 reference, foobar might access memory outside the bounds of the object and it could screw up other live objects (placed next to the instance of A in memory). So clearly the compiler shouldn't let you do that. If you wanted the compiler to just go ahead and create an instance of C for you when you make the cast, it wouldn't know what to do with field2 and field3. There is no way *in general* to know that the MotorBike class (descendant type) works just fine with only the data it inherited from the Vehicle class. If there were, we could just go like this:

AmazingWebSerive s = (AmazingWebService)(new object());

Sadly, this doesn't work. :) Instead, it's you as a programmer who may know this and provide a way to construct a B with only A data, or a C with only A data, and so on. Then you decide what the defaults should be for the data that doesn't exist in A.

class A { protected string field1; }
class B : A { protected string field2; }
class C : B
{
protected string field3;
public C(A a) { field1 = a.field1; }
}

Now if I have an A and I want to "use it as a C" I can simply go C c = new C(a); and work from there. BUT... if the reason you derived your own Tag class was be