Releasing an OLE IStorage file handle in C#

Question

I'm trying to embed a PDF file into a Word document using the OLE technique described here: https://learn.microsoft.com/en-us/archive/blogs/brian_jones/embedding-any-file-type-like-pdf-in-an-open-xml-file

I've tried to implement the C++ code provided in C# so that the whole project's in one place and am almost there except for one roadblock. When I try to feed the generated OLE object binary data into the Word document I get an IOException.

IOException: The process cannot access the file 'C:\Wherever\Whatever.pdf.bin' because it is being used by another process.

There is a file handle open the .bin file ("oleOutputFileName" below) and I don't know how to get rid of it. I don't know a huge amount about COM - I'm winging it here - and I don't know where the file handle is or how to release it.

Here's what my C#-ised code looks like. What am I missing?

    public void ExportOleFile(string oleOutputFileName, string emfOutputFileName)
    {
        OLE32.IStorage storage;
        var result = OLE32.StgCreateStorageEx(
            oleOutputFileName,
            OLE32.STGM.STGM_READWRITE | OLE32.STGM.STGM_SHARE_EXCLUSIVE | OLE32.STGM.STGM_CREATE | OLE32.STGM.STGM_TRANSACTED,
            OLE32.STGFMT.STGFMT_DOCFILE,
            0,
            IntPtr.Zero,
            IntPtr.Zero,
            ref OLE32.IID_IStorage,
            out storage
        );

        var CLSID_NULL = Guid.Empty;

        OLE32.IOleObject pOle;
        result = OLE32.OleCreateFromFile(
            ref CLSID_NULL,
            _inputFileName,
            ref OLE32.IID_IOleObject,
            OLE32.OLERENDER.OLERENDER_NONE,
            IntPtr.Zero,
            null,
            storage,
            out pOle
        );

        result = OLE32.OleRun(pOle);

        IntPtr unknownFromOle = Marshal.GetIUnknownForObject(pOle);
        IntPtr unknownForDataObj;
        Marshal.QueryInterface(unknownFromOle, ref OLE32.IID_IDataObject, out unknownForDataObj);
        var pdo = Marshal.GetObjectForIUnknown(unknownForDataObj) as IDataObject;

        var fetc = new FORMATETC();
        fetc.cfFormat = (short)OLE32.CLIPFORMAT.CF_ENHMETAFILE;
        fetc.dwAspect = DVASPECT.DVASPECT_CONTENT;
        fetc.lindex = -1;
        fetc.ptd = IntPtr.Zero;
        fetc.tymed = TYMED.TYMED_ENHMF;

        var stgm = new STGMEDIUM();
        stgm.unionmember = IntPtr.Zero;
        stgm.tymed = TYMED.TYMED_ENHMF;
        pdo.GetData(ref fetc, out stgm);

        var hemf = GDI32.CopyEnhMetaFile(stgm.unionmember, emfOutputFileName);
        storage.Commit((int)OLE32.STGC.STGC_DEFAULT);

        pOle.Close(0);
        GDI32.DeleteEnhMetaFile(stgm.unionmember);
        GDI32.DeleteEnhMetaFile(hemf);
    }

UPDATE 1: Clarified which file I meant by "the .bin file".
UPDATE 2: I'm not using "using" blocks because the things I want to get rid of aren't disposable. (And to be perfectly honest I'm not sure what I need to release to remove the file handle, COM being a foreign language to me.)

Answer 1

I found the answer and it's pretty simple. (Probably too simple - it feels like a hack but since I know so little about COM programming I'm just going to go with it.)

The storage object had multiple references on it, so just keep going until they're all gone:

var storagePointer = Marshal.GetIUnknownForObject(storage);
int refCount;
do
{
    refCount = Marshal.Release(storagePointer);
} while (refCount > 0);