Interface and struct

  • Thread starter Thread starter Bob Clegg
  • Start date Start date
B

Bob Clegg

Hi,
I have a Data Class that declares a Public struct.
It accepts instances of this struct in some of its function calls.
I now want to put an interface between this class and the calling classes.

I would like to move the struct out to the module that is housing the
interface.

But when I do this I get a scoping problem.

The interface declaration of any function that is passing one of these
structs complains that it can't expose a friend type publicly.

I have tried putting the struct into the interface and I have also tried a
public declaration in the module proper and another module.

The only thing that seems to work but doesn't sit comfortably with me is to
declare it public in one of the calling classes.
Any thoughts?
thanks
Bob
 
Hi Bob,

Do you mean you have :

Class Foo
Public Structure Bar
...
End Structure
End Class

and you want to define an Interface, such as :

Public IFace
Function GetBar() as Foo.Bar
End Interface

If so, Foo must be Public.
Alternatively, don't have Bar nested.



Bill.
 
Hi Bill,
The class is right.
The interface declaration is more like
Function UpdateBar(myBar as Bar) as boolean

The problem is that it seems to me that Bar should no longer live in Foo,
now that Foo is hiding behind an interface. ( The reason for this is that
Foo is a data class using SQL Server objects and there is soon going to be
an FooBrother that uses Oracle data objects. Only one of the two will be
compiled depending on the installation.) So where to place Bar?
As a number of calling classes want to use Bar it would seem ideal to me to
place it in a module.
But if I do that then the above declaration fails with can't expose a
friend type.
I have a working solution by getting Bar to bunk down in one of the calling
classes, but it seems tacky.
regards
Bob
 
This may or may not work, just been doing a lot of API reading latley

Could you put an attribute on it suchas
<MarshalAs(UnmanagedType.FunctionPtr)> to keep it from getting GC'd. or is
that the problem?

As far as going out of scope, it shouldn't matter. Also, a structure and a
class are essentially the same thing except that a struct is created on the
stack while a class is created on the heap.

Theres another difference, but its not major. I jsut can't remember it.
 
Hi Bob,

nested classes and structures are only useful for hiding and access to the outer
classes private scoped members. So, you are probably best NOT to nest the
struct.

eg:

Class Foo: Implements IFace
...
End Class

Structure Bar
..
End Sturcture

Interface IFace
..
End Interface

That is, keep them all separate. I have a feeling you are use to structs being
like UDT's in Vb6, which they are in many ways, but they are also more like a
class in many ways too. Most importantly, they do not have to be declared
inside a class, but can be declared just like any class can be.

Bill
 
Hi Bill,
You are on the money.
It is the way I am thinking about it.
Yes. VB6 habits die hard.
The struct can stand alone.
Thanks.
Bob
 
Thanks Bob.

Bob Clegg said:
Hi Bill,
You are on the money.
It is the way I am thinking about it.
Yes. VB6 habits die hard.
The struct can stand alone.
Thanks.
Bob
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