Problem with casting

[snowbunny]

OK, I've not done this before so excuse me for being thick!

I have a class 'Client' which is stored in an external dll and I need to extend, just to add extra methods to it.

External.Client cl = new External.Client();

my local class, also called Client, is derived from this:

public class Client : External.Client
{
public void Save()
{
...
}
}

What I need to do is cast from an External.Client to my own local Client:

Client thiscl = (Client)cl;

but this throws a runtime error :

Unable to cast object of type 'External.Client' to type 'MyNamespace.Client'.

How can I do this?

Thanks

[r035198x]

OK, I've not done this before so excuse me for being thick!

I have a class 'Client' which is stored in an external dll and I need to extend, just to add extra methods to it.

External.Client cl = new External.Client();

my local class, also called Client, is derived from this:

public class Client : External.Client
{
public void Save()
{
...
}
}

What I need to do is cast from an External.Client to my own local Client:

Client thiscl = (Client)cl;

but this throws a runtime error :

Unable to cast object of type 'External.Client' to type 'MyNamespace.Client'.

How can I do this?

Thanks

The cast won't work because the two objects are inconvertible.

[Plater]

There is a way to do what you want, and I wish I knew it. (I've been looking for it myself in a mannor of speaking)

They do it with the various Stream objects, Image->Bitmap and WebRequest->HttpWebRequest.


What you could do as a possible work-around, is have your custom class NOT extend that class, but have a public/private instance of it, and have the constructor take in the object.

[snowbunny]

The cast won't work because the two objects are inconvertible.

Thanks. Useful.

So.... at the risk of sounding bolshy.....if all I'm doing is taking one class and extending it with methods, not properties, how *should* I make it convertible?

[snowbunny]

There is a way to do what you want, and I wish I knew it. (I've been looking for it myself in a mannor of speaking)

They do it with the various Stream objects, Image->Bitmap and WebRequest->HttpWebRequest.


What you could do as a possible work-around, is have your custom class NOT extend that class, but have a public/private instance of it, and have the constructor take in the object.

Thanks, that's more useful :)
Unfortunately, I'm in the situation where I'd have to cast from each to the other, so that's probably not going to help.

There ya go, I tought this would be a simple "oi, thickie" type thing. Someone please tell me it is!

[r035198x]

Thanks, that's more useful :)
Unfortunately, I'm in the situation where I'd have to cast from each to the other, so that's probably not going to help.

There ya go, I tought this would be a simple "oi, thickie" type thing. Someone please tell me it is!

I can imagine your excitement at the responses.

If B extends A, then A objects are not convertible to B objects. It's a basic inheritance rule issue.

[Plater]

Well do you NEED to cast to it, or can you get away with just running functions on it's public functions/properties?

Because then you could just make a static class that has your new functions that also take in that original class as a parameter?


If you need at the internals, you might have to re-write those DLL functions.


Unless you can figure something cheeky out with a UNION construct?

[r035198x]

Well do you NEED to cast to it, or can you get away with just running functions on it's public functions/properties?

Because then you could just make a static class that has your new functions that also take in that original class as a parameter?


If you need at the internals, you might have to re-write those DLL functions.


Unless you can figure something cheeky out with a UNION construct?

Or just write a method in your class that takes the parent class object as argument and manually copies all the stuff you want to an object of your class and returns that object of your class type.

[alan4cast]

(read the problem backwards - never mind)

[snowbunny]

Or just write a method in your class that takes the parent class object as argument and manually copies all the stuff you want to an object of your class and returns that object of your class type.

Yeah, I'd thought of doing that, but figured there must be a more elegant solution.
I'm *sure* I've read somewhere that you can cast A > B *and B > A, albeit with loss of data. Oh well, back to the drawing board!

[r035198x]

Yeah, I'd thought of doing that, but figured there must be a more elegant solution.
I'm *sure* I've read somewhere that you can cast A > B *and B > A, albeit with loss of data. Oh well, back to the drawing board!

No you cannot. You can only cast objects if they are *convertible*.
A subclass object is convertible to an object of it's base class but not vice versa.