Question

I have a COM interface with a following method definition (IDL notation):

SCODE GetText( [in, out] ULONG* pcwcBuffer,
              [out, size_is(*pcwcBuffer)] WCHAR* awcBuffer );

Typelib marshaling is used for COM+, the type library is registered, other methods of the interface work allright when called through COM+, but not this method.

The server side copies an array of WCHARs into the awcBuffer and its length into pwcBuffer, no buffer overrun ever occurs.

static const wchar_t* Text = L"Sample";
STDMETHODIMP CImpl::GetText( ULONG* bufferLength, WCHAR* buffer )
{
    const int length = wcslen( Text );
    *bufferLength = length;
    memcpy( buffer, Text, length * sizeof( WCHAR ) );
    return S_OK;
}

When the client calls this method through COM+ the buffer contents gets lost. Specifically only the first wide char is preserved – if the server copies “Sample” wide character string, the client only receives “S” string. The return value on the client size is S_OK, the buffer length returned to the client is exactly the same as what the server copied.

I finally switched to BSTR to workaround this problem but it’s really interesting why the whole valid looking construct doesn’t work.

What’s the possible reason of the described behaviour?

Answer 1

IIRC, the typelib marshaller ignores the size_is attribute — thus, only 1 char is marshaled.