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How to: Marshal COM Strings Using C++ Interop

This topic demonstrates how a BSTR (the basic string format favored in COM programming) can be passed from a managed to an unmanaged function, and vice versa. For interoperating with other strings types, see the following topics:

The following code examples use the managed, unmanaged #pragma directives to implement managed and unmanaged functions in the same file, but these functions interoperate in the same manner if defined in separate files. Files containing only unmanaged functions do not need to be compiled with /clr (Common Language Runtime Compilation).

The following example demonstrates how a BSTR (a string format used in COM programming) can be passed from a managed to an unmanaged function. The calling managed function uses StringToBSTR to obtain the address of a BSTR representation of the contents of a .NET System.String. This pointer is pinned using pin_ptr to ensure that its physical address is not changed during a garbage collection cycle while the unmanaged function executes. The garbage collector is prohibited from moving the memory until the pin_ptr goes out of scope.

// 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);
}

The following example demonstrates how a BSTR can be passed from an unmanaged to an unmanaged function. The receiving managed function can either use the string in as a BSTR or use PtrToStringBSTR to convert it to a String for use with other managed functions. Because the memory representing the BSTR is allocated on the unmanaged heap, no pinning is necessary, because there is no garbage collection on the unmanaged heap.

// MarshalBSTR2.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 managed

void ManagedTakesAString(BSTR bstr) {
   String^ s = Marshal::PtrToStringBSTR(static_cast<IntPtr>(bstr));
   Console::WriteLine("(managed) convered BSTR to String: '{0}'", s);
}

#pragma unmanaged

void UnManagedFunc() {
   BSTR bs = SysAllocString(L"test string");
   printf_s("(unmanaged) passing BSTR to managed func...\n");
   ManagedTakesAString(bs);
}

#pragma managed

int main() {
   UnManagedFunc();
}
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