Question

I need to marshal an array of String^ to call a unmanaged function that expects an array of BSTRs.

On MSDN I found the article

How to: Marshal COM Strings Using C++ Interop

with this code sample:

// MarshalBSTR1.cpp
// compile with: /clr
#define WINVER 0x0502
#define _AFXDLL
#include <afxwin.h>

#include <iostream>
using namespace std;

using namespace System;
using namespace System::Runtime::InteropServices;

#pragma unmanaged

void NativeTakesAString(BSTR bstr) {
   printf_s("%S", bstr);
}

#pragma managed

int main() {
   String^ s = "test string";

   IntPtr ip = Marshal::StringToBSTR(s);
   BSTR bs = static_cast<BSTR>(ip.ToPointer());
   pin_ptr<BSTR> b = &bs;

   NativeTakesAString( bs );
   Marshal::FreeBSTR(ip);
}

So I created a new BSTRs’ array and called the Marshal::StringToBSTR() for every String of the array.
Then I created a managed pin_ptr array.

array<pin_ptr<BSTR> >^ gcDummyParameters = gcnew array<pin_ptr<BSTR> >(asParameters->Length);

but I receved the error:

Error   2   error C2691: 'cli::pin_ptr<Type>' : a managed array cannot have this element type

I tried also with a native array:

pin_ptr<BSTR> dummyParameters[100000];

but even in this case I got an error:

Error   1   error C2728: 'cli::pin_ptr<Type>' : a native array cannot contain this managed type 

What else can I do?

Answer 1

Microsoft sample looks strange: there is no need to pin BSTR type because it is unmanaged. Just create BSTR array and fill every member using Marshal::StringToBSTR. Don’t use pin_ptr.