private void button3_Click(object sender, EventArgs e)
    {
      DBNull q = DBNull.Value;
      RRR.QQQ(q);
      string w = "";
      RRR.QQQ(w);
      float y = 5.4F;
      RRR.QQQ(y);
    }
    public static class RRR
    {
      public static void QQQ(DBNull r) { MessageBox.Show("dbnull"); }
      public static void QQQ(string r) { MessageBox.Show("string"); }
      public static void QQQ(object r) { MessageBox.Show("other"); }
    }