Is there a default default property?

Question

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.

Answer 1

As stated by Hans Passant in comments:

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.

[Tlbexp does seem to look for a property named "Value" to use for this]. Could be interesting if you give the class an indexer btw.