Замена текста в документе Word

Как заменить текст в документе Word, пробую - не получается:

        object fileName = Path.Combine(@"D:\Document.doc");
        WordOperation.Application wordApp = new  WordOperation.Application { Visible = false };
        WordOperation.Document aDoc = wordApp.Documents.Open(fileName, ReadOnly: false, Visible: true);
        aDoc.Activate();
        FindAndReplace(wordApp, "xxx", textBox1.Text);
        FindAndReplace(wordApp, "yyy", textBox2.Text);
        aDoc.Close();
        MessageBox.Show("Successfully Accomplished");
    }
    private void FindAndReplace(WordOperation.Application doc, object findText, object replaceWithText)