try this
private string ReadDocFile(object sPath, ref string sDocContent)
{
Microsoft.Office.Interop.Word.ApplicationClass wordApp = new Microsoft.Office.Interop.Word.ApplicationClass();
object missing = System.Reflection.Missing.Value;
Word._Document aDoc = null;
aDoc = wordApp.Documents.Open(ref sPath, ref missing, ref readOnly, ref missing, ref missing, ref missing,_
ref missing, ref missing, ref missing, ref missing, ref missing, ref isVisible, ref missing,_
ref missing, ref missing, ref missing);
aDoc.Activate();
aDoc.ActiveWindow.Selection.WholeStory();
aDoc.ActiveWindow.Selection.Copy();
IDataObject data = Clipboard.GetDataObject();
sDocContent = data.GetData(DataFormats.UnicodeText).ToString();
wordApp.Documents.Close(ref isNoSave, ref missing, ref missing); ;
wordApp.Quit(ref isNoSave, ref missing, ref missing);
return (string)sDocContent;
}