Convert Word document to base64 C # string

I am trying to get a base64 view of an active Word document while it is still open in Word and I get the following error in ReadAllBytes ():

The process cannot access the file "file path" used by another process

public string GetEncodedTemplate()
        {
            //Convert a Word document 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 understand that the error occurs because the specified document is still open in Word, my question is: is it still possible to get a base64 representation of the document without resorting to saving a temporary file?

I am using C # .NET 4.0 AND MS Office 2010