using作用

SqlConnection conn=null;
try
{
   PrepareEverionment();
   conn = new SqlConnection();
   conn.ConnectionString = "...................";
   conn.Open();
}
finally
{
   conn.Close();
}

这段代码问题是：如PrepareEverionment()发生异常时，conn就任然为null在执行finally中代码时就会出异常，应该用using这样在using中会自动判断conn是否为null