Friday, May 10, 2013

Saving OLEObject Content To File

It is possible with OLE to embed files in Excel workbooks and saves them back to disk (I know OLE is not cutting edge technology). To embed some file :
  • Insert ribbon menu
  • Object (in Text)
  • From file tab
  • (Browse to file)
If your file is another Office document, saving it back to disk is trivial. But here I want to embed any file (e.g. txt, xml). Anton post gave me the directions. I pushed further the analysis and eventually reversed engineered the OLEObject MemoryStream content :
0x2 0x0header
string\0file name
string\0file path
0x0 0x0 0x3 0x0(native header ?)
inttemp file path length
string\0temp file path
intcontent length
bytescontent
inttemp file path utf16 length
bytestemp file path utf16
intfile name utf16 length
bytesfile name utf16
intfile path utf16 length
bytesfile path utf16
Note : header is different for non Package OLE objects like Office documents or pdf.

In this sample program, I load some excel workbook and for each embedded ole object, I display its name and its content :
[STAThread]
static void Main(string[] args)
{
    var excel = new Application();
    try
    {
        Workbook workbook = excel.Workbooks.Open(@"D:\Classeur1.xlsx");
        foreach (Worksheet worksheet in workbook.Worksheets)
        {
            foreach (OLEObject ole in worksheet.OLEObjects())
            {
                Console.WriteLine("name : {0}", ole.Name);
                if (ole.progID == "Package")
                {
                    string content = ole.GetContent();
                    if (content != null)
                        Console.WriteLine(content);
                }
            }
        }
        workbook.Close();
    }
    finally
    {
        excel.Quit();
    }
    Console.Write("Press a key...");
    Console.Read();
}
The progID has "Package" value when the embedded content is not standard OLE content. The GetContent extension method gets a MemoryStream from the OLEObject and loads the content from the stream :
static class OLEExtensions
{
    public static string GetContent(this OLEObject ole)
    {
        ole.Copy();
        System.Windows.Forms.IDataObject data = System.Windows.Forms.Clipboard.GetDataObject();
        object obj = data.GetData("Native");
        System.Windows.Forms.Clipboard.SetDataObject("");
        var ms = obj as MemoryStream;
        if (ms != null)
            return ms.GetOLEContent();
        return null;
    }
}
We copy the OLE object to the clipboard to get the MemoryStream. The STAThread attribute is required in Main method to avoid some NullReferenceException when calling GetData method. The GetOLEContent extension method extracts the content from the stream thanks to the reverse engineered stream structure :
static class OLEStreamExtensions
{
    public static int ReadHeader(this MemoryStream ms)
    {
        var header = new byte[2];
        int read = ms.Read(header, 0, header.Length);
        if (read != header.Length)
            throw new FormatException("End of stream while reading header");
        if (header[0] != 2 || header[1] != 0)
            throw new FormatException("Bad header");
        return read;
    }
    public static string ReadString(this MemoryStream ms)
    {
        var sb = new StringBuilder();
        while (true)
        {
            int b = ms.ReadByte();
            if (b == -1)
                throw new FormatException("End of stream while reading string");
            if (b == 0)
                return sb.ToString();
            sb.Append((char)b);
        }
    }
    public static int ReadInt(this MemoryStream ms)
    {
        var bytes = new byte[4];
        int read = ms.Read(bytes, 0, bytes.Length);
        if (read != bytes.Length)
            throw new FormatException("End of stream while reading int");
        return BitConverter.ToInt32(bytes, 0);
    }
    public static byte[] ReadBytes(this MemoryStream ms, int count)
    {
        var bytes = new byte[count];
        int read = ms.Read(bytes, 0, count);
        if (read != count)
            throw new FormatException("End of stream while reading bytes");
        return bytes;
    }
    public static string GetOLEContent(this MemoryStream ms)
    {
        ms.ReadHeader();
        string name = ms.ReadString();
        string path = ms.ReadString();
        int reserved = ms.ReadInt();
        if (reserved != 0x30000)
            throw new FormatException(string.Format("Unexpected reserved bytes : got {0} but expected {1}", reserved.ToString("x"), 0x30000.ToString("x")));
        int tempLength = ms.ReadInt();
        string tempPath = ms.ReadString();
        if (tempPath.Length + 1 != tempLength)
            throw new FormatException(string.Format("Mismatch between temp length {0} and temp full path length {1}", tempLength, tempPath.Length + 1));
        int contentLength = ms.ReadInt();
        byte[] content = ms.ReadBytes(contentLength);
        int delta = sizeof(int) * 3 + (name.Length + path.Length + tempPath.Length) * 2;
        if (ms.Length != ms.Position + delta)
            throw new FormatException("Unexpected end of file");
        return UTF8Encoding.UTF8.GetString(content);
    }
}
This code uses Excel but it might work with any Office document (Word, Powerpoint...). Of course, you should have installed the PIA. I have validated this code with xml and txt files. I use Excel 2007 SP3 MSO.

Edit : You can refactor this code to extract the name and the bytes of each embedded object to be able to save the contents to files :
class OLEContent
{
    #region Fields
    private readonly string name;
    private readonly byte[] content;
    #endregion
    public OLEContent(string name, byte[] content)
    {
        this.name = name;
        this.content = content;
    }
    public string Name { get { return name; } }
    public byte[] Content { get { return content; } }
}
...
public static OLEContent GetOLEContent(this MemoryStream ms)
{
    ...
    return new OLEContent(name, content);
}
...
OLEContent content = ole.GetContent();
if (content != null)
    File.WriteAllBytes(Path.Combine(tempPath, content.Name), content.Content);
...
This new version can also save images (like jpg) to disk.

1 comment:

Tolib Rahimov said...

Great post! You saved my life.