Word API: Prevent Conversion of File

I'm opening word documents with the Documents.Open method in the Microsoft.Office.Interop.Word Namespace. This works fine, except that when I open a file that isn't开发者_StackOverflow a word document, it's automatically converted to be a word document. I'd like to find a way to either raise an exception of the document isn't a word document, detect if the document is, or is not, a word document before opening it, or detect if the document was converted after it is opened and converted. Does anyone have any ideas about how to accomplish this?

A simple test would be to check for the magic number in the file header of the document before trying to open the document with Word.

Binary Word documents (.doc) are compound documents and start with 0xcfd0, where as OpenXML documents (.docx) start with the string "PK".

static bool HasComoundDocumentSignature(string filename)
{
    using (BinaryReader br = new BinaryReader(File.Open(filename, FileMode.Open)))
    {
        UInt16 magicNumber = br.ReadUInt16();      
        return magicNumber == 0xcfd0;
    }
}

static bool HasZipSignature(string filename)
{
    using (BinaryReader br = new BinaryReader(File.Open(filename, FileMode.Open)))
    {
        UInt16 magicNumber = br.ReadUInt16();  
        return magicNumber == 0x4b50;
    }
}

static bool HasWordSignature(string filename)
{
    return HasCompoundDocumentSignature(filename) 
        || HasZipSignature(filename); 
}