return array of object from C# to C in COM

Question

I am using COM to communicate between C++ and C#.

I have the following classes in C#

1. Email Address

   /// <summary>
   /// Email Address
   /// </summary>
   public class EmailAddress
   {
       /// <summary>
       /// SMTP Address
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]        
       public string Address;
   
       /// <summary>
       /// Name
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string Name;
   }
   

2. Mailbox

   /// <summary>
   /// MailBox Entity
   /// </summary>   
   public struct MailBoxEntity
   {
       /// <summary>
       /// SMTP Address
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string SMTPAddress;
   
       /// <summary>
       /// Mailbox Display Name
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string Name;
   
       /// <summary>
       /// Mailbox Server Name
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string sServerName;
   }
   

3. EmailEntity (yet to implement IEmailEntity which will contain properties for each field)

   /// <summary>
   /// Class for Email Entity
   /// </summary>
   public class EmailEntity : IEmailEntity
   {
       /// <summary>
       /// BccRecipients
       /// </summary>
       [MarshalAs(UnmanagedType.ByValArray)]
       public EmailAddress[] BccRecipients;
   
       /// <summary>
       /// Body
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string Body;
   
       /// <summary>
       /// CcRecipients
       /// </summary>
       [MarshalAs(UnmanagedType.ByValArray)]
       public EmailAddress[] CcRecipients;
   
       /// <summary>
       /// Culture
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string Culture;
   
       /// <summary>
       /// DateTimeCreated
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string DateTimeCreated;
   
       /// <summary>
       /// DateTimeReceived
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string DateTimeReceived;
   
       /// <summary>
       /// DateTimeSent
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string DateTimeSent;
   
       /// <summary>
       /// FromAddress
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string FromAddress;
   
       /// <summary>
       /// FromName
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string FromName;
   
   
       /// <summary>
       /// HasAttachments
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string HasAttachments;
   
       /// <summary>
       /// Id
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string Id;
   
       /// <summary>
       /// Importance
       /// </summary>
      [MarshalAs(UnmanagedType.BStr)]
       public string Importance;
   
       /// <summary>
       /// LastModifiedName
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string LastModifiedName;
   
       /// <summary>
       /// LastModifiedTime
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string LastModifiedTime;
   
       /// <summary>
       /// MimeContent
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string MimeContent;
   
       /// <summary>
       /// ParentFolderId
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string ParentFolderId;
   
       /// <summary>
       /// Original Mailbox
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public MailBoxEntity OriginalMailBox;
   
   
       /// <summary>
       /// ParentFolderName
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string ParentFolderName;
   
       /// <summary>
       /// ReceivedByAddress
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string ReceivedByAddress;
   
       /// <summary>
       /// ReceivedByName
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string ReceivedByName;
   
       /// <summary>
       /// Size
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string Size;
   
       /// <summary>
       /// Subject
       /// </summary>
       [MarshalAs(UnmanagedType.BStr)]
       public string Subject;
   
       /// <summary>
       /// ToRecipients
       /// </summary>
       [MarshalAs(UnmanagedType.ByValArray)]
       public EmailAddress[] ToRecipients;
   
   }
   

How do I access access an array of EmailEnity objects (in C++) returned from a function in C# and read the fields of the EmailEntity class. Please note that EmailEntity class contains members like EmailAddress[] which itself is an array of objects. I was planning to implement interface IEmailEntity that contains properties to access fields of EmailEntity and use it to access fields in C++. Is that the right way ?

How do I access complex fields members like array of EmailAddress (EmailAddress[] ToRecipients) in C++.

Please suggest.

Thanks

Answer 1

You could simplify the .NET Code like this (remove all MarshalAs that should be done automatically):

[ComVisible(true)]
public class MyRootClass : IMyRootClass // some class to start with
{
    public IEmailEntity[] GetEntities()
    {
        List<IEmailEntity> list = new List<IEmailEntity>();
        for(int i = 0; i < 10; i++)
        {
            EmailEntity entity = new EmailEntity();
            List<IEmailAddress> addresses = new List<IEmailAddress>();
            addresses.Add(new EmailAddress { Name = "Joe" + i });
            entity.BccRecipients = addresses.ToArray();
            entity.Body = "hello world " + i;
            list.Add(entity);
        }
        return list.ToArray();
    }   
}

[ComVisible(true)]
public interface IMyRootClass
{
    IEmailEntity[] GetEntities();
}

public class EmailEntity : IEmailEntity
{
    public IEmailAddress[] BccRecipients { get; set; }
    public string Body { get; set; }
}

public class EmailAddress : IEmailAddress
{
    public string Address { get; set; }
    public string Name { get; set; }
}

[ComVisible(true)]
public interface IEmailAddress
{
    string Address { get; set; }
    string Name { get; set; }
}

[ComVisible(true)]
public interface IEmailEntity
{
    IEmailAddress[] BccRecipients { get; set; }
    string Body { get; set; }
    // to be continued...
}

To use it with C++, you need to register the DLL and build a .TLB (Type Library file) as explained in a similar answer here: Implement a C# DLL COM File In Unmanaged C++ Program

Then, you can access these classes in C++, like this:

#include "stdafx.h"
#import  "c:\MyPathToTheTlb\YourAssembly.tlb" // import the COM TLB

using namespace YourAssembly;

int _tmain(int argc, _TCHAR* argv[])
{
  CoInitialize(NULL);
  IMyRootClassPtr ptr(__uuidof(MyRootClass));
  CComSafeArray<IUnknown*> entities = ptr->GetEntities(); // CComSafeArray needs atlsafe.h in the PCH
  for(int i = entities.GetLowerBound(0); i <= entities.GetUpperBound(0); i++)
  {
    IEmailEntityPtr entity;
    entities.GetAt(i).QueryInterface(&entity);
    _bstr_t body = entity->Body;
    printf("%S\n", body.GetBSTR());

    CComSafeArray<IUnknown*> recipients = entity->BccRecipients;
    for(int j = recipients.GetLowerBound(0); j <= recipients.GetUpperBound(0); j++)
    {
      IEmailAddressPtr address;
      recipients.GetAt(j).QueryInterface(&address);
      _bstr_t name = address->Name;
      printf(" %S\n", name.GetBSTR());
    }
  }
  CoUninitialize();
}