Is there a default default property?

Asked Dec 03 '18 at 20:29

Active Dec 03 '18 at 20:38

Viewed 108 times

I'm exporting a C# class to VB6 through COM interop. It seems to have a default COM property without me ever having set one!

The C# class looks like:

[ComVisible(true), ClassInterface(ClassInterfaceType.AutoDual)]
class Foo
{
     public int I { get; set; }
     public bool B { get; set; }
     public int Value { get; set; }  // <--- the culprit
     public string S { get; set; }
}

_{(I know why should I not use autodual - please overlook that, its useful for this stage of development)}

The property Value is behaving as the default in VB6. It is shown this way in object browser. But notice that I have not set a DispId attribute to indicate the default.

Also, in OLEView you can see that in the IDL of the TLB for the assembly the Value property has attribute [id(00000000), propget]. This is equivalent to DISPID_VALUE. So, for some reason the compiler or TLBEXP or something is explicitly making Value into the default.

Why is this happening? I actually don't want a default property and its existence was masking a coding mistake which "just worked", basically by accident.

edited Dec 03 '18 at 20:38

asked Dec 03 '18 at 20:29

StayOnTarget

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In VB6 a lot of things just work by accident. – Olivier Jacot-Descombes Dec 03 '18 at 21:10
Tlbexp.exe does that. View the type library with oleview.exe, File > View TypeLib. [dispid] auto-generation is not documented, but this is not horribly insensible. Use the [DispId] attribute to override. – Hans Passant Dec 03 '18 at 21:51
Are you sure the property was not configured using the "Procedure Attributes" dialog box? – Simon Mourier Dec 03 '18 at 23:08
@SimonMourier isn't that for VB6 code, not for C# code? Is there such a thing in VS2015 also? – StayOnTarget Dec 04 '18 at 12:51
@HansPassant I guess does it look for a property named "Value" then? – StayOnTarget Dec 04 '18 at 12:52
It does not have anything else to look for, so yes. Could be interesting if you give the class an indexer btw. – Hans Passant Dec 04 '18 at 12:59
Oops I got that the other way around. – Simon Mourier Dec 04 '18 at 13:02
@SimonMourier no worries, thanks! – StayOnTarget Dec 04 '18 at 13:05

Is there a default default property?

0 Answers0