Linq：将对象从一个列表添加到另一种类型的列表（MVC，C＃）

Question

I have a "FindItemsResults <Appointment> list" which contains appointment objects.

I want to select "Start" property value and of all appointment objects from this List and save these values in an other list of type DateTime ... List<DateTime> avaliableBookingDays

List<DateTime> avaliableBookingDays = new List<DateTime>();
FindItemsResults<Appointment> appointments = calendar.FindAppointments(cView);
IEnumerable<DateTime> res = from x in appointments
                            select new DateTime { (x.Start};
avaliableBookingDays.AddRange(res);

My LINQ exepression fails:

IEnumerable<DateTime> res = from x in appointments
                          select new DateTime { (x.Start};

I get error, like that:

Cannot inittialize type "DateTime" with a collection inittializer because it do not implement "System.Collections.IEnumerable"

The above-mentioned LINQ must do this:

foreach (var appObj in appointments)
{
    avaliableBookingDays.Add(appObj.Start);
}

Answer 1

If you want to project items in sequence to some other type, use Enumerable.Select :

var avaliableBookingDays = appointments.Select(a => a.Start).ToList();

Note that there is no equivalent of ToList() operator in query syntax. So the best you can do is get IEnumerable<DateTime> result and convert it to list later:

IEnumerable<DateTime> res = from x in appointments
                            select x.Start;

var avaliableBookingDays = res.ToList();

Also keep in mind - DateTime is a value type . You don't need to create new instance of this type to get a copy of appointment start time - copy will be created automatically.

Answer 2

Your code is not even going to compile

IEnumerable<DateTime> res = from x in appointments
                          select new DateTime { (x.Start};

because it has syntax errors ! In your select part, you are projecting to a new DateTime object and you have just an opening ( , which does not make any sense!

I think you were getting the mentioned error when you had the closing bracket as well like

IEnumerable<DateTime> res = from x in appointments
                          select new DateTime { (x.Start)};

Here your code is trying (assume it was valid) to project the items from appointments collection to another collection of DateTime with some invalid syntax ! There is no need to use the new keyword here as you are not trying to create an object of a view model class/annonymous object since all you need is the Start property value.

Solution

Since Start is the DateTime type property, you want to select only that.

List<DateTime> avaliableBookingDays = (from x in appointments 
                                              select x.Start).ToList();

Or

List<DateTime> avaliableBookingDays = appointments.Select(a=>a.Start).ToList();