VB6 to C# interop: Cannot directly set a property of type 'object'

Question

Given the following C# class:

namespace ComTest
{
    [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
    [Guid("A1D11BE5-40A1-4566-A1CA-418ABC76017C")]
    public interface IThing
    {
        [DispId(1)]
     void SetValue( object input );
        [DispId(2)]
     object Value {get; set;}
    }

    [ComVisible(true)]
    public class Thing: IThing
    {
     internal object PValue;

     public void SetValue(object input)
     {
      PValue = input;
     }

     public object Value
     {
      get
      {
       return PValue;
      }
      set
      {
       PValue = (object)value;
      }
     }

    }
}

Can anyone explain the following behaviour in VB6?

Dim oThing as Thing
Dim input as Variant, output as Variant
input = "Some text"
Set oThing = new Thing
oThing.SetValue(input)    ' works fine
output = oThing.Value     ' works fine
Set oThing.Value = input  ' fails with runtime error '424': Object required

[new] I set the assembly to be COM-visible and checked the project properties box marked "Register for COM interop" (Build tab).

As you may guess, the example is a cut down minimal version of the actual class I am using. I compared the generated TLB info for the actual class and the Thing class (below). The lines marked ** are found in the Thing class but not the actual class (the actual coclass contains [default] dispinterface IThing), and this extra generated interface _Thing appears to change the behaviour in VBA, in two ways: (1) the properties will no longer AutoComplete, (2) the direct assignment to Value now works as the contributors have found. Any thoughts?

// Generated .IDL file (by the OLE/COM Object Viewer)
// 
// typelib filename: ThingLib.tlb
[
  uuid(92093AA7-870E-498A-8B80-97545D221E24),
  version(1.0),
  helpstring("AAA Testing COM interop")
]
library ThingLib
{
    // TLib :     // TLib : mscorlib.dll : {BED7F4EA-1A96-11D2-8F08-00A0C9A6186D}
    importlib("mscorlib.tlb");
    // TLib : OLE Automation : {00020430-0000-0000-C000-000000000046}
    importlib("stdole2.tlb");

    // Forward declare all types defined in this typelib
    dispinterface IThing;
**  interface _Thing;

    [
      uuid(7FC255DD-F82F-4B39-8755-9680A97033B5),
      version(1.0),
        custom({0F21F359-AB84-41E8-9A78-36D110E6D2F9}, "ComTest.IThing")    

    ]
    dispinterface IThing {
        properties:
        methods:
            [id(0x00000001)]
            void SetValue([in] VARIANT input);
            [id(0x00000002), propget]
            VARIANT Value();
            [id(0x00000002), propputref]
            void Value([in] VARIANT rhs);
    };

    [
      uuid(D96FB9C7-A0AF-35D3-A0F6-A07A9ED47984),
      version(1.0),
        custom({0F21F359-AB84-41E8-9A78-36D110E6D2F9}, "ComTest.Thing")
    ]
    coclass Thing {
**      [default] interface _Thing;
        interface _Object;
        dispinterface IThing;
    };

**    [
**      odl,
**      uuid(C309BDD0-239F-38AA-A057-254F8E01BD4B),
**      hidden,
**      dual,
**      oleautomation,
**        custom({0F21F359-AB84-41E8-9A78-36D110E6D2F9}, "ComTest.Thing")    
**    ]
**    interface _Thing : IDispatch {
**    };
};

Answer 1

Jeremy, I tried your code and it works fine (although I used Excel VBA since I do not have VB6 installed). There is a typo in your code where

object Value (get; set;)

should be

object Value {get; set;}

but I doubt that it has anything to do with your error since it should give you a compile error. Did you try the code exactly as you put it here?

Answer 2

I also tried your COM interface implementation with a VB6 client without any problems (with the exception of the typo Jakob mentioned in the Value property).

Answer 3

Change your code to:

Dim oThing as Thing
Dim input as Variant, output as Variant
input = "Some text"
Set oThing = new Thing
oThing.SetValue(input)    ' works fine
output = oThing.Value     ' works fine
Set oThing.Value = input  ' Change this line

And everything should work. (You have to Set objects in VB6, you are trying to Let an object, which can only be done for atomic types).