How do I (or if I can't) use Variants on simple DLLs?

Question

I want to expose some functionality of a internal object as a DLL - but that functionality uses variants. But I need to know: I can export a function with Variant parameters and/or return - or is better to go to an string-only representation?

What is better, from language-agnostic POV (the consumer is not made with Delphi - but all will run in Windows)?

Answer 1

You could use OleVariant, which is the variant value type that is used by COM. Make sure not to return it as a function result as stdcall and complex result types can easily lead to problems.

A simple example library DelphiLib;

uses
  SysUtils,
  DateUtils,
  Variants;

procedure GetVariant(aValueKind : Integer; out aValue : OleVariant); stdcall; export;
var
  doubleValue : Double;
begin
  case aValueKind of
    1: aValue := 12345;
    2:
    begin
      doubleValue := 13984.2222222222;
      aValue := doubleValue;
    end;
    3: aValue := EncodeDateTime(2009, 11, 3, 15, 30, 21, 40);
    4: aValue := WideString('Hello');
  else
    aValue := Null();
  end;
end;

exports
  GetVariant;

---

How it could be consumed from C#:

public enum ValueKind : int
{
   Null = 0,
   Int32 = 1,
   Double = 2,
   DateTime = 3,
   String = 4
}

[DllImport("YourDelphiLib",
           EntryPoint = "GetVariant")]
static extern void GetDelphiVariant(ValueKind valueKind, out Object value);

static void Main()
{
   Object delphiInt, delphiDouble, delphiDate, delphiString;

   GetDelphiVariant(ValueKind.Int32, out delphiInt);
   GetDelphiVariant(ValueKind.Double, out delphiDouble);
   GetDelphiVariant(ValueKind.DateTime, out delphiDate);
   GetDelphiVariant(ValueKind.String, out delphiString);
}

Answer 2

As far as I know there is no problems to work with Variant variable type in other languages. But it will be great if you export the same functions for different variable types.