.NET to COM/ActiveX - params/ParamArray not supported for VB98/VB6 and earlier

Describe the bug

We want to provide a .NET Framework library for VB98/VB6 developers and take advantage of ParamArray usage.

The standard translation from .NET params

[Description("Add item(s)")]
void Add(String item, params Object[] moreItems)

to IDL/TLB by TlbExp.exe is

[id(0x1), vararg, helpstring("Add item(s)")]
void Add([in] BSTR item, [in] SAFEARRAY(VARIANT) moreItems)

Which is seen in VB98/VB6 object catalog as

Sub Add(item As String, ParamArray moreItems() As Variant)

But this COM/ActiveX method signature is accessible with MSScriptControl.ScriptControl only, not with VB98/VB6. The compiler notices an error of incompatibility:

For VB98/VB6 the following IDL/TLB method signature is required (reference of SAFEARRAY):

[id(0x1), vararg, helpstring("Add item(s)")]
void Add([in] BSTR item, [in] SAFEARRAY(VARIANT)* moreItems)

Our current workaround is to change the method signature in .NET:

[Description("Add item(s)")]
void Add(String item, [In] ref Object[] moreItems)

which get generated in IDL/TLB with no vararg, but SAFEARRAY reference:

[id(0x1), helpstring("Add item(s)")]
void Add([in] BSTR item, [in] SAFEARRAY(VARIANT)* moreItems)

Then we need manually to do the following process:

Open the exported TLB in oleview.exe
Save the IDL
Change the attributation by adding the vararg
Compile the IDL with midl.exe

You can see this in the sample project:

TypeLibParamArrayVB6.IDL line 59 and
TypeLibParamArrayVB6.VB98.IDL line 59

To Reproduce

Compile the project from NETSphereSoft/Bug-Samples/dotnet/sdk/44460-TypeLib-ParamArray-VB6
You might to change paths in the post build events
Create a VB98/VB6 project, add the type library (*.tlb) and use the Add method of Class1 in your project
Try to compile
Now take the provided TypeLibParamArrayVB6.VB98.IDL (can be build with TypeLibParamArrayVB6.VB98.bat)
Rename the type library TypeLibParamArrayVB6.VB98.tlb to TypeLibParamArrayVB6.tlb and place it to your TypeLibParamArrayVB6.dll
Use this changed type library now in you VB98/VB6 references
Use the AddVB98 method of Class1 in your project now
Try to compile => should not lead in an error
You should be able to use it in the following ways:
Call class1.AddVB98("Test 1")
Call class1.AddVB98("Test 1", "Test 2")
Call class1.AddVB98("Test 1", "Test 2", "Test 3")

Possible solution

Add a ParamArrayRef attribute for placing at the parameter or method additionally to the standard params usage

[Description("Add item(s)")]
void Add(String item, [ParamArrayRef] params Object[] moreItems)

[Description("Add item(s)")]
[ParamArrayRef]
void Add(String item, params Object[] moreItems)

And export then the IDL/TLB by TlbExp.exe with the requested method signature:

[id(0x1), vararg, helpstring("Add item(s)")]
void Add([in] BSTR item, [in] SAFEARRAY(VARIANT)* moreItems)

Alternately a solution with existing InAttribute and OutAttribute could be to get the same method signature like VB98/VB6 does:

[Description("Add item(s)")]
void Add(String item, [In, Out] params Object[] moreItems)

to lead into

[id(0x1), vararg, helpstring("Add item(s)")]
void Add([in] BSTR item, [in, out] SAFEARRAY(VARIANT)* moreItems)

And a much simplier could be (currently prevented from compiler):

[Description("Add item(s)")]
void Add(String item, [ParamArray] ref Object[] moreItems)

to lead into

[id(0x1), vararg, helpstring("Add item(s)")]
void Add([in] BSTR item, [in, out] SAFEARRAY(VARIANT)* moreItems)

Further technical details

Microsoft Visual Studio Professional 2019 Version 16.11.37 VisualStudio.16.Release/16.11.37+34931.43 Microsoft .NET Framework Version 4.8.09037

dotnet / sdk