Home/ Questions/Q 772011
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-14T18:44:38+00:00

I have an ATL COM object that I am using from C#. The interface

  • 0

I have an ATL COM object that I am using from C#. The interface currently looks like:

interface ICHASCom : IDispatch{
    [id(1), helpstring("method Start")] HRESULT Start([in] BSTR name, [out,retval] VARIANT_BOOL* result);
...
    [id(4), helpstring("method GetCount")] HRESULT GetCount([out,retval] LONG* numPorts);
...

    [id(7), helpstring("method EnableLogging")] HRESULT EnableLogging([in] VARIANT_BOOL enableLogging);
};

That is, it’s a very simple interface. I also have some events that I send back too.
Now, I would like to add something to the interface. In the ATL I have some results, which are currently structs and look like
struct REPORT_LINE
{
string creationDate;
string Id;
string summary;
};
All the members of the struct are std::string. I have an array of these that I need to get back to the C#. What’s the best way to do this?

I suspect someone is going to say, “hey, you can’t just send std::string over COM like that. If so, fine, but what’s the best way to modidfy the struct? Change the std::string to BSTR? And then how do I,
1) Set up the IDL to pass an array of structs (structs with BSTR or std::string)
2) If I must use SAFEARRAYS, how do I fill the SAFEARRAYS with the structs.

I’m not familiar with COM except for use with simple types.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    a user defined structure is incompatible with the automation interface. You can probably work out a nested array or two dimensional safe array of BSTRs, but a more maintainable solution would be wrapping the structure as an automation object with 3 properties, then wrap the array as a collection that has an enumerator.

    Neither IDL nor Automation define byte alignment for a struct. So you can have compatibility problems if your COM server has different struct alignment with the client. e.g. VB has a 4-byte alignment, while the #import in Visual C++ default to a 8-byte alignment. If you have a slightest chance in the future to use the interface in scripting, avoid using structs.

    Suggested reading:

    • 0