passing params to SP with condition

STR= "SELCT * from tbl1 ";
 if (drpyrsal.SelectedValue != "--Select--")
            {
                STR= STR+ " and Year([frmdt])=" + drpyrsal.SelectedValue + "  ";
            }
            if (drpmonth.SelectedValue != "--Select--")
            {
                STR= STR+ "and Month([frmdt])=" + drpmonth.SelectedValue + "  ";
            }

and execute it with dataset

how can i do the same with SP

  SqlCommand cmd = new SqlCommand("usp_SP1", con); // Stored procedure
            cmd.CommandType = CommandType.StoredProcedure;
 cmd.Parameters.AddWithValue("@yr", drpyrsal.SelectedValue);
            cmd.Parameters.AddWithValue("@mon", drpmonth.SelectedValue);