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#

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;
}

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;
}

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

Community · Accepted Answer · 2017-05-23 12:33:12Z

5

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();
}

edited May 23, 2017 at 12:33

CommunityBot

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answered Apr 30, 2013 at 10:06

Simon Mourier

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6 Comments

Hans Passant Over a year ago

You'll need to use the [InterfaceType] attribute to make this code work. Right now, the interfaces are only accessible late-bound, the default is ComInterfaceType.InterfaceIsIDispatch.

Simon Mourier Over a year ago

@HansPassant - This code works fine as is. I believe the default is dual.

Hans Passant Over a year ago

You are right, color me purple. Looks like I'll have to edit some answers here and there :(

Gags Over a year ago

I get debug assertion failed. Expression psaSrc!=0 in atlsafe.h . Is it beacuse null is being returned entity->BccRecepients

Simon Mourier Over a year ago

Well this could happen depending on what you actually return. My sample code puts an array in BccRecipients so I don't get a NULL.

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