Can a ref class be inherited, if can, how?

Question

I declare a interface in my WinRT project, and a ref class A inherit from the interface, so A is sealed.

Now, I want to declare a ref class B that inherit from A, any ideas?

Answer 1

A sealed class cannot be further inherited. You can remove the sealed from class-A and now class-B can derive from A, but publicly exposed RT types are expected to be sealed. So while you may be able to access A across WinRT, it's not an officially supported scenario and has a few issues associated with it (that's been reported in these forums).

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http://blog.voidnish.com

Answer 2

hi,

thank you for your reply.

but when I remove sealed from class-A, it build with an error C4585 : A WinRT 'public ref class' must either be sealed or derive from an existing unsealed class.

for more information, my code like this:

public interface class ICom
{
   ...
}

public ref class A : public ICom
{
   ...
}

public ref class B sealed : public A
{
   ...
}

maybe it can build success in Win8 build8400 + VS2012 RC, but I build with the error in Win8 build9200 + VS2012 RTM

Answer 3

Yeah, you may have to make it non-public, or introduce a dummy base class that's private just so you can derive A from that and then make it public.  In general WinRT does not encourage old fashioned inheritance chains. It's all about interfaces and implementations (like with COM).

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http://blog.voidnish.com

Answer 4

Any unsealed public ref class must have an unsealed Windows Runtime base class in it's inheritance hierarchy.

e.g.

	public ref class A : public Windows::UI::Xaml::DependencyObject, ICom
	{
	  // ...
	};

The RTM compiler now enforces that all unsealed public ref classes have a base class which is also unsealed.

Answer 5

Yeah, your e.g. is right, thank you.

but I try to derive from Platform::Object, it's also an unsealed WinRT class

e.g.

public ref class A : public Platform::Object, ICom
{
  //...
}

I also get an error C4585 : A WinRT 'public ref class' must either be sealed or derive from an existing unsealed class.

why?

Answer 6

Hi,

Platform::Objectis sealed; it is not a universal base class.
After you have defined an unsealed classMyBasethat inherits fromDependencyObject, other public or private ref classes in your component or app may inherit fromMyBase. I

Please take a look Inheritance section in this document
http://msdn.microsoft.com/en-us/library/windows/apps/xaml/hh699870.aspx

Best regards,
Jesse

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Jesse Jiang [MSFT]
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