Re: How to pass strings from VC++.exe to C#.dll

From:

"Giovanni Dicanio" <giovanniDOTdicanio@REMOVEMEgmail.com>

Newsgroups:

microsoft.public.vc.mfc

Date:

Wed, 17 Sep 2008 19:09:07 +0200

Message-ID:

<#mwB3gOGJHA.4936@TK2MSFTNGP03.phx.gbl>

<raghudr@gmail.com> ha scritto nel messaggio
news:526599dc-e8b0-4ac7-8e5a-4c9fb8b83db3@k7g2000hsd.googlegroups.com...

Now i have to return a string from C#.dll which is a COM component and
receive it in VC++.exe.I am not sure on how to do this

[...]

i changed the syntax to

public string ShowDialog(StringBuilder str)
{

string dest ="C:\\RAGHU\\rag.xml";
return dest;
}

OK, so this is the C# method returning the string (exposed to C++ using
COM), is it correct?

In Vc++ .exe

int _tmain(int argc, _TCHAR* argv[])
{

WCHAR pretval;

if (SUCCEEDED(hRes))
{
pretval = pDotNetCOMPtr->ShowDialog(str);//giving error in this
s tatement
}

So, I think that you are using #import directive from C++ to access the C#
COM component, correct?

If so, pretval in that case must not be a WCHAR, but a _bstr_t (representing
a COM BSTR).
So, please try this:

  // Get string from C# COM component
  _bstr_t s = pDotNetComPtr->ShowDialog( str );

In this way, you can have your string passed from C# to C++ using COM.

I would like to repeat to make it clear: the default method to pass strings
in COM is using *BSTR*. There are wrapper classes for BSTR, like CComBSTR
and _bstr_t. _bstr_t is usually used if you use the #import directive.

_bstr_t Class:
http://msdn.microsoft.com/en-us/library/zthfhkd6(VS.80).aspx

In general, if you have a COM component implemented in C#, with an
ITestClass::GetSomeString() method that returns a string, like this:

<code>

using System;
using System.Runtime.InteropServices;

namespace ComCSharp
{
    [Guid("A64F7325-BB44-4624-A78A-62F60CA69638")]
    public interface ITestClass
    {
        [DispId(1)]
        string GetSomeString();
    }

    public class TestClass : ITestClass
    {
        public string GetSomeString()
        {
            string s = "Hello from C# COM component.";
            return s;
        }

    }
}

</code>

You can call that from C++ using code like this:

<code>

#include "stdafx.h"

#import "ComCSharp.tlb"

using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
    CoInitialize(NULL);

    ComCSharp::ITestClassPtr p;
    p.CreateInstance( __uuidof( ComCSharp::TestClass ) );

    // Call method GetSomeString() from COM component (written in C#);
    // this method returns a BSTR that is displayed in C++ console.
    _bstr_t s = p->GetSomeString();

    wcout << L"String from C#: " << s << endl;

    CoUninitialize();

    return 0;
}

</code>

I tested that on VC2008.

HTH,
Giovanni