SQLite - 选择两个日期之间的预订

Question

我正在尝试将 SQLlite 用于使用 Xamarin.Forms 创建的旅馆预订系统。我想搜索给定 ID 和房间类型的两个日期之间存在的预订记录。

// Count number of bookings between these dates at the hostel for the specified room type
int count = App.Database.CheckValidBooking(hostel_ID, hostel_room_type, DPickerFrom.Date, DPickerTo.Date).Count();

public List<PaidBookings> CheckValidBooking(int ID, string type, DateTime period_start, DateTime period_end)
{
    return database.Query<PaidBookings>("SELECT * FROM PaidBookings WHERE ID = ? AND Type = ? AND checkIn >= ? AND checkOut <= ?;", ID, type, period_start, period_end);
}

执行时出现SQL语句出错：

public List<PaidBookings> CheckValidBooking(int ID, string type, DateTime period_start, DateTime period_end)
{
    return database.Query<PaidBookings>("SELECT * FROM PaidBookings WHERE ID = ? AND Type = ? AND checkIn >= ? AND checkOut <= ?;", ID, type, period_start, period_end);
}

System.NullReferenceException：对象引用未设置为对象的实例。

我该如何解决这个问题？谢谢。

Answer 1

你应该把你的变量放在“你的变量”之间

试试这个：

database.Query<PaidBookings>(String.Format("SELECT * FROM PaidBookings WHERE ID = '{0}' AND Type = '{1}' AND checkIn >= '{2}' AND checkOut <= '{4}';", ID, type, period_start, period_end));

就像@Shawn 提到的那样，这是一种不好的做法，通常你应该使用如下命令参数：

SqlCommand cmd = new SqlCommand();
cmd.CommandText =String.Format("SELECT * FROM PaidBookings WHERE ID = @ID AND Type = @Type AND checkIn >= @checkIn AND checkOut <= @checkOut;");

//  add parameter to command object
cmd.Parameters.Add(new SqlParameter("@ID", yourId));

....

// execut the commend
 cmd.ExecuteNonQuery();