How to access a C# public struct via COM with Delphi 7

Question

In Delphi 7, I'm trying to read some C# .Net public struct constants using COM.

I can't seem to get the syntax right.

Delphi code

I haven't found any way to write code that will even compile against extracted TLB.pas file. This code fails to compile with the error: "Object or class required"

   S := ItemFieldIdentifier.AverageMarketPrice;

So, even though the C# programmer tells me that this isn't an instantiatable object, the TLB has a constructor for this type, so I figured I try instantiating. This code failed to compile with the error "Undeclared identifier":

   var
     ItemFieldIdentifier1: ItemFieldIdentifier;
     ...
     ItemFieldIdentifier1 :=  CoItemFieldIdentifier.Create;
     S := ItemFieldIdentifier1.AverageMarketPrice;

Is there a way for me to access these constants via COM, or does the C# programmer need create a instantiatable object that I can create and then access?

C# code

    public struct ItemFieldIdentifier
    {
        ...
                    public static readonly string AverageMarketPrice = "AMP";
        ...

Then, in C#, they have codes a com interop layer

    public class ItemFieldIdentifier
    {
       public string AverageMarketPrice
       {
            get
            {
                 return xxx.Public.ItemCat.ItemFieldIdentifier.AverageMarketPrice; // <-- this is getting the value from the struct.
    }

TLB File

  DIID__ItemFieldIdentifier: TGUID = '{667FB47F-6394-4ED6-842B-7581184B4138}';
  IID__ItemFieldIdentifier_2: TGUID = '{FBF4AD9B-38B5-3856-ADAC-57DB3198BBE7}'
  CLASS_ItemFieldIdentifier: TGUID = '{3A9E41E3-A509-483B-A212-6A507EA29B5B}';

  _ItemFieldIdentifier = dispinterface;
  _ItemFieldIdentifier_2 = interface;
  _ItemFieldIdentifier_2Disp = dispinterface; 

    ItemFieldIdentifier = _ItemFieldIdentifier_2;

    // *********************************************************************//
    // DispIntf:  _ItemFieldIdentifier
    // Flags:     (4096) Dispatchable
    // GUID:      {667FB47F-6394-4ED6-842B-7581184B4138}
    // *********************************************************************//
  _ItemFieldIdentifier = dispinterface
    ['{667FB47F-6394-4ED6-842B-7581184B4138}']
    ...
    property AverageMarketPrice: WideString readonly dispid 45;
    ...

  end;

    // *********************************************************************//
    // Interface: _ItemFieldIdentifier_2
    // Flags:     (4432) Hidden Dual OleAutomation Dispatchable
    // GUID:      {FBF4AD9B-38B5-3856-ADAC-57DB3198BBE7}
    // *********************************************************************//
      _ItemFieldIdentifier_2 = interface(IDispatch)
        ['{FBF4AD9B-38B5-3856-ADAC-57DB3198BBE7}']
      end;


    // *********************************************************************//
    // Interface: _ItemFieldIdentifier_2
    // Flags:     (4432) Hidden Dual OleAutomation Dispatchable
    // GUID:      {FBF4AD9B-38B5-3856-ADAC-57DB3198BBE7}
    // *********************************************************************//
      _ItemFieldIdentifier_2 = interface(IDispatch)
        ['{FBF4AD9B-38B5-3856-ADAC-57DB3198BBE7}']
      end;

    // *********************************************************************//
    // DispIntf:  _ItemFieldIdentifier_2Disp
    // Flags:     (4432) Hidden Dual OleAutomation Dispatchable
    // GUID:      {FBF4AD9B-38B5-3856-ADAC-57DB3198BBE7}
    // *********************************************************************//
      _ItemFieldIdentifier_2Disp = dispinterface
        ['{FBF4AD9B-38B5-3856-ADAC-57DB3198BBE7}']
      end;

    // *********************************************************************//
    // The Class CoItemFieldIdentifier provides a Create and CreateRemote method to          
    // create instances of the default interface _ItemFieldIdentifier_2 exposed by              
    // the CoClass ItemFieldIdentifier. The functions are intended to be used by             
    // clients wishing to automate the CoClass objects exposed by the         
    // server of this typelibrary.                                            
    // *********************************************************************//
      CoItemFieldIdentifier = class
        class function Create: _ItemFieldIdentifier_2;
        class function CreateRemote(const MachineName: string): _ItemFieldIdentifier_2;
      end;

This doesn't make a lot of sense. The .tlb you posted won't compile. Multiple definitions of _ItemFieldIdentifier_2. Could you double check what you posted is accurate. — David Heffernan
– David Heffernan, Commented Feb 24, 2014 at 22:32
Thanks for taking a stab at this, David. I know how important it is to post real code that compiles (especially when I hope you'll take a look at it! :-)) but the entire .pas file was too big to post here. I posted what I thought was an validly pruned version. Guess not though. Remy got me going though. Thanks again. — Tom
– Tom, Commented Feb 25, 2014 at 2:12

Remy Lebeau · Accepted Answer · 2014-02-24 23:42:20Z

1

So, even though the C# programmer tells me that this isn't an instantiatable object, the TLB has a constructor for this type, so I figured I try instantiating. This code failed to compile with the error "Undeclared identifier"

That is odd, because ItemFieldIdentifier is clearly defined (it is an alias for _ItemFieldIdentifier_2).

CoItemFieldIdentifier.Create() returns an _ItemFieldIdentifier_2, so you might try declaring your variable as that. However, _ItemFieldIdentifier_2 does not have any members declared. So, assuming ItemFieldIdentifier_2 derives from ItemFieldIdentifier on the C# side, you might try doing a cast from _ItemFieldIdentifier_2 to _ItemFieldIdentifier on the Delphi side:

var
  ItemFieldIdentifier1: _ItemFieldIdentifier;
...
ItemFieldIdentifier1 := CoItemFieldIdentifier.Create as _ItemFieldIdentifier;
S := ItemFieldIdentifier1.AverageMarketPrice;

Or:

var
  ItemFieldIdentifier1: _ItemFieldIdentifier_2;
...
ItemFieldIdentifier1 := CoItemFieldIdentifier.Create;
S := (ItemFieldIdentifier1 as _ItemFieldIdentifier).AverageMarketPrice;

answered Feb 24, 2014 at 23:42

Remy Lebeau

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1 Comment

David Heffernan Over a year ago

Something is very wrong if this is the solution. Is the .tlb importer getting it wrong. If so I would deal with the issue by fixing the interface declarations so that the code could be written without a cast.

Remy Lebeau · Accepted Answer · 2014-02-24 23:46:58Z

0

The vtRecord OLE type is not handled by Delphi 7. This type matches C# struct type.

I suspect you should use a C# class marked as COMVisible=True as a data object, to be accessed from Delphi.

Or you may try to generate the TLB .pas file using a more recent TLBEXP tool. I found out the latest versions are able to handle such struct types correctly.

edited Feb 24, 2014 at 23:46

Remy Lebeau

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answered Feb 24, 2014 at 23:33

Arnaud Bouchez

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1 Comment

Tom Over a year ago

Good suggestion, Arnaud, to try a more recent version of Delphi. I just tried that, hopeful, but it didn't resolve the problem. Thanks for the suggestion though.

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