Convert Word document to base64 string C#

I'm trying to get the base64 representation of an active Word document while it's still open in Word and am getting the following error in ReadAllBytes():

The process cannot access the file 'file path' it is being used by another process

public string GetEncodedTemplate()
        {
            //Convert a Word document's base64 representation
            string base64 = String.Empty;
            _application.ActiveDocument.Save();

            string docPath = _application.ActiveDocument.FullName;
            byte[] binarydata = File.ReadAllBytes(docPath);
            base64 = System.Convert.ToBase64String(binarydata, 0, binarydata.Length);
            return base64;
        }

I do understand that the error occurs because the specified document is still open in Word, my question is - Is it still somehow possible to get the base64 representation of the document without resorting to saving to a temp file?

I'm using C# .NET 4.0 And MS Office 2010