Return string from c++ dll - interop services

[Jason]

Hi,

I am having some trouble returning a string from a c++ dll. I tend to get
junk data back and I am not sure of the method, my code so far:

Function is declared in c++ dll 'Test' and exported like this:

extern "C" __declspec( dllexport ) BSTR getbagstr();
extern "C" __declspec( dllexport ) BSTR getbagstr() {
g.getbag()->setdisplaybag( "ZZZZZZZZZZZZZZ ZZZZZZZZZZZZZZ" );
const char * buff = g.getbag()->getdisplaybag( ).c_str();
return SysAllocStringB yteLen (buff , lstrlen(buff));
}

I want use the function via interop services - p/invoke - to display the
string in a simple c# program:

[DllImport("Test ")]
private static extern IntPtr getbagstr();

public static string getBagString()
{
return Marshal.PtrToSt ringAuto(getbag str());
}

The data I am returning is not correct. All display bag does is store and
return the string as below. I am returning int data correctly, but strings
are proving difficult. Anybody able to help?

void Bag::setdisplay bag(const string & str) {
displaybag = str;
}

string Bag::getdisplay bag() { return displaybag; }

[Mattias Sjögren]

public static string getBagString()

{
return Marshal.PtrToSt ringAuto(getbag str());
}

SysAllocStringB yteLen returns a BSTR containing ANSI characters. So
PtrToStringAuto will only work on Win9x platforms. Try changing it to

IntPtr ptr = getbagstr();
string tmp = Marshal.PtrToSt ringAnsi(ptr);
Marshal.FreeBST R(ptr);
return s;
Mattias

--
Mattias Sjögren [C# MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
Please reply only to the newsgroup.

[Jason]

On Sun, 03 Sep 2006 23:21:35 +0200, Mattias Sjögren wrote:

> public static string getBagString()
{
return Marshal.PtrToSt ringAuto(getbag str());
}

SysAllocStringB yteLen returns a BSTR containing ANSI characters. So
PtrToStringAuto will only work on Win9x platforms. Try changing it to

IntPtr ptr = getbagstr();
string tmp = Marshal.PtrToSt ringAnsi(ptr);
Marshal.FreeBST R(ptr);
return s;

Thanks Mattias, this worked. I have been checking through the p/invoke
examples on your website which is an excellent resource. However, I
wondered if you could provide an example for this code:

extern "C" __declspec( dllexport ) __stdcall void setdisplaybag(L PSTR
tiles);
extern "C" __declspec( dllexport ) __stdcall void setdisplaybag(L PSTR
tiles) {
//set display bag code
}

Assuming the above is the exported c++ function, how should C# pass in the
BSTR safely? How should the BSTR be freed or should it?

[DllImport("Test ")]
private static extern void setdisplaybag(I ntPtr ptr);

public static void SetDisplayBag(s tring str)
{
IntPtr tmp = Marshal.StringT oBSTR(str);
setdisplaybag(t mp);
//Marshal.FreeBST R(tmp);
}

[Mattias Sjögren]

>Assuming the above is the exported c++ function, how should C# pass in the

>BSTR safely?

I see nothing about BSTRs in the function signatures, so I would
simply declare it as

static extern void setdisplaybag(s tring tiles);

Mattias

--
Mattias Sjögren [C# MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
Please reply only to the newsgroup.

[Jason]

On Tue, 05 Sep 2006 00:05:07 +0200, Mattias Sjögren wrote:

I see nothing about BSTRs in the function signatures, so I would
simply declare it as

static extern void setdisplaybag(s tring tiles);

Ok fine, but when I input this i get strange output:

settiles("ZZZZZ ZZZZYYYYYYYYYYY YYY");
Console.WriteLi ne(getbagstr()) ;

console output:

v►☺YYYYYYYY YYYYYZZZZZZZZZ

Where are the extra characters coming from at the beginning and what is
needed to avoid them? Thanks in advance.

[Mattias Sjögren]

Jason,

>Where are the extra characters coming from at the beginning and what is
needed to avoid them? Thanks in advance.

I don't know without seeing all the C++ code. It doesn't happen here.
I wrote the folowing test code and it roundtrips the string correctly.

// C++ DLL
#include <windows.h>
#pragma comment(lib, "oleaut32.l ib")

char buf[100];

extern "C"
{
__declspec(dlle xport) BSTR __stdcall getbagstr()
{
return SysAllocStringB yteLen(buf, lstrlen(buf));
}

__declspec(dlle xport) void __stdcall setdisplaybag(c har *tiles)
{
lstrcpyn(buf, tiles, 100);
}

}

// C# client
using System;
using System.Runtime. InteropServices ;

class Test
{
[DllImport("test .dll")]
static extern IntPtr getbagstr();

[DllImport("test .dll")]
static extern void setdisplaybag(s tring tiles);

static void Main()
{
setdisplaybag(" ZZZZZZZZZYYYYYY YYYYYYYY");
IntPtr p = getbagstr();
Console.WriteLi ne(Marshal.PtrT oStringAnsi(p)) ;
Marshal.FreeBST R(p);
}
}
Mattias

--
Mattias Sjögren [C# MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
Please reply only to the newsgroup.