Question

I’ve managed to stump myself with LINQ. I am trying to create an editable datagrid in a WPF app using a subset gathered from a LINQ query:

var LookUpEvents = from d in ThisData.Events.Local
                   where d.StartDate.Value.Date <= DateTime.Now.Date &&
                   (d.EndDate.HasValue == false || d.EndDate.Value.Date >= DateTime.Now.Date)
                   select d;
RangeEventGrid.ItemsSource = LookUpEvents;
RangeEventGrid.Items.Refresh();

This query works, and the datagrid is populated however I am unable to edit the datagrid, when trying to this exception is thrown:

"'EditItem' is not allowed for this view."
   at System.Windows.Controls.ItemCollection.System.ComponentModel.IEditableCollectionView.EditItem(Object item)
   at System.Windows.Controls.DataGrid.EditRowItem(Object rowItem)

When loading the full dataset using:

ThisData.Events.Load();
FullEventGrid.ItemsSource = ThisData.Events.Local;

Everything works fine and the data is editable. The XAML used is identical (I have also tried swapping the bound datagrids and the full result remains editable and the query throws the exception still) and the only difference between these is the query. When I try to change the query I end up with a new exception:

The specified type member 'Date' is not supported in LINQ to Entities. Only initializers, entity members, and entity navigation properties are supported.

The query used for that:

var LookUpEvents = from d in ThisData.Events
                   where d.StartDate.Value.Date <= DateTime.Now.Date &&
                   (d.EndDate.HasValue == false || d.EndDate.Value.Date >= DateTime.Now.Date)
                   select d;
LookUpEvents.Load(); //Exception thrown here.
RangeEventGrid.ItemsSource = LookUpEvents;
RangeEventGrid.Items.Refresh();

The really weird thing about that exception (weird to me at least) is that I use DateTime comparison in other queries that do not throw any exceptions, for example this query in another place works fine:

var LookUpSessions = from d in ThisData.Sessions
                     where d.EndTime.Hour >= (DateTime.Now.Hour - 1) && d.StartTime.Hour <= (DateTime.Now.Hour + 2)
                     && d.Event.IsActive == true
                     orderby d.StartTime.Hour, d.StartTime.Minute
                     select d;

Is it not possible to bind a LINQ query result to a DataGrid to be editable? That seems like it’d be a huge oversight if that’s the case. I feel like it’s much more likely I’m just missing something basic since LINQ, WPF and EF are all brand new to me.

Thanks in advance.

Answer 1

To make editing data in the GridView possible you cannot use an IEnumerable<T> or IQueryable<T> as the items source. You need a collection type that implemnts IList and IEnumerable<T> or IQueryable<T> don’t.

A possible solution is that you create an ObservableCollection<T> (that does implement IList) from your LINQ query:

RangeEventGrid.ItemsSource = new ObservableCollection<Event>(LookUpEvents);

This is also the reason why

FullEventGrid.ItemsSource = ThisData.Events.Local;

does work because Local is already of type ObservableCollection<Event>.

Your very first query doesn’t throw an exception (although you are using DateTime.Date) because it is not a LINQ-to-Entities/database query. It is a LINQ-to-Objects query that runs in memory on the Local collection. There is no database query involved.

If you remove Local you run LINQ-to-Entities and LINQ-to-Entities doesn’t support all methods and properties that LINQ-to-Objects does, especially it doesn’t support DateTime.Date (but apparently is does support DateTime.Hour).

To perform the comparison by Date in a LINQ-to-Entities query you can use EntityFunctions:

var today = DateTime.Now.Date;
var LookUpEvents = from d in ThisData.Events
                   where EntityFunction.TruncateTime(d.StartDate) <= today &&
                         (!d.EndDate.HasValue ||
                          EntityFunction.TruncateTime(d.EndDate) >= today)
                   select d;

Or maybe the EntityFunction.DiffDays function is an option as well.