.NET: Make an object and all of its subordinate objects COM visible

Question

I've got a proprietary assembly with a class, which needs to be COM visible, so I can use it with other (non .NET) applications.

Imagine a class like this:

public class CProprietary
{
    public CProprietary2 oSubItem;
}

public class CProprietary2
{
}

These classes are proprietary and not COM visible. I don't have the source code of them. As current solution I am creating my own assembly which provides a inherited, COM visible class.

[ComVisible(true), ClassInterface(ClassInterfaceType.AutoDual)]
public class CComVisibleProprietary : CProprietary
{
}

Now I am able to use the base class (CProprietary) via COM, but the object of the subordinate class (CProprietary2) is still returned non COM visible when accessing CComVisibleProprietary.oSubItem.

So my problem is that I need a way to automatically wrap a class and all of its child classes. Or is there any easy way to inherit the ComVisible attribute to the child classes? Some properties of these proprietary classes also return objects of .NET like System.Windows.Forms.ListBox.ObjectCollection, so they don't belong to the same namespace.

Do you have [assembly:ComVisible(true)] attribute (in AssemblyInfo.cs)? See https://stackoverflow.com/questions/15688395/whats-the-deal-with-comvisible-default-and-public-classes-com-exposure — SpruceMoose, Sep 27 '17 at 12:00
Using [ClassInterface] is not a very good way to expose members. COM uses a hyper pure interface-based paradigm. [ClassInterface] is convenient, it synthesizes an interface from the class declaration, but what is not visible stays invisible. Always favor declaring a public interface explicitly, give it the [ComVisible] attribute. Good for multiple reasons, one is that it solves your problem. You can now include properties and methods that are implemented by the base class. — Hans Passant, Sep 27 '17 at 12:40
@HansPassant: How should I make the properitary classes COM visible without using the ClassInterface? I guess I need to create an interface with a copy of all properties and methods then? I wouldn't favor that. — David Gausmann, Sep 27 '17 at 12:51
The machine doesn't care much about what you favor, consider doing what you need to do to make it work. Not excluding sending that email to ask the author to do it for you. — Hans Passant, Sep 27 '17 at 13:12

score 1 · Accepted Answer · answered Sep 27 '17 at 12:05

The correct way from the 'automation view' would be to create own objects for each embedded object (CProprietary2). Each embedded object may be accessed by using a property in the main object (CProprietary) and returning the interface to the embedded object.

But there is no automatic way to achive this. You must provide corresponding interfaces to all objects.

The syntax in VBScript, when prop is of type CPropriatary, you might access the sub item in this way.

prop.oSubItem.DoSomething

.NET: Make an object and all of its subordinate objects COM visible

1 Answers1