COM interop: Variable number of parameters

Question

I am developing an C# libraries, that I need to call from COBOL (unmanaged) code. So I am using COM interop for these purposes.

I have created an layer "ModuleLanucher" that mediates the communication. I have registered this ModuleLanucher library by these commands:

regasm ModuleLanucher.dll /codebase
TLBEXP Modulelanucher.dll

Everything works fine if I have concrete number of parameters in function foo. (mentioned below)

Unfortunately my function foo needs variable number of arguments. This is sketch of my code:

namespace NMSPC
{
   class ModuleLanucher
   {
      void foo(string moduleName, params object[] args)
      {
        //call some dlls with args as parameteres
      }
   }
}

From COBOL code I'm calling COM function foo:

VCCOMServer is class "$OLE$NMSPC.ModuleLanucher"

invoke VCCOMServer "new" returning anInstance
invoke anInstance "foo" using MODULENAME, PARAM01, PARAM02, PARAM03, PARAM04

When I try to call foo method from this COBOL code, I get this error:

Exception 65538 not trapped by the class oleexceptionmanager. Description: "OLE Parameter count mismatch" (8002000E): Invalid number of parameters

Is it even posiible to have COM object function with variable number of arguments?

Thank you for any suggestions

Answer 1

Variable argument lists are not supported by COM. I think the COM part of your method is converted into:

void foo(string moduleName, object[] args) //note the missing params...

To call this method from COBOL, you need to pass an array of objects as the 2nd argument.

If you know the maximum #parms on forehand, an alternative can be that you define your method with this maximum #parms, where you specify them as optional to COM.

Missing parameters are passed as Type.Missing.