Marshalling BSTRs from C++ to C# with COM interop

Question

I have an out-of-process COM server written in C++, which is called by some C# client code. A method on one of the server\'s interfaces returns a large BSTR to the client, a

Answer 1

anelson has covered this pretty well, but I wanted to add a couple of points;

• CoTaskMemAlloc is not the only COM-friendly allocator -- BSTRs are recognized by the default marshaller, and will be freed/re-allocated using SysAllocString & friends.

• Avoiding USES_CONVERSION (due to stack overflow risks -- see anelson's answer), your full code should be something like this [1]

(note that A2BSTR is safe to use, as it calls SysAllocString after conversion, and doesn't use dynamic stack allocation. Also, use array-delete (delete[]) as BuildResponse likely allocates an array of chars)

• The BSTR allocator has a cache that can make it appear as though there is a memory leak. See http://support.microsoft.com/kb/139071 for some details, or Google for OANOCACHE. You could try disabling the cache and see if the 'leak' goes away.

[1]

HRESULT MyClass::ProcessRequest(BSTR request, BSTR* pResponse)
{
    char* pszResponse = BuildResponse(CW2A(request));
    *pResponse = A2BSTR(pszResponse);
    delete[] pszResponse;
    return S_OK;
}