Question

I’ve thrown myself headfirst into C# and .Net 2.0 using Linq, and I’m having a few problems debugging some of the problems, namely the following:

I have a ComboBox control (cmbObjects) I want to populate with a set of objects retrieved using Linq. I’ve written a helper method to populate a List<T> generic:

class ObjectProvider {      public static List<T> Get<T>(bool includeNull) where T : class, new()     {         List<T> list = new List<T>();         LutkeDataClassesDataContext db = ConnectionManager.GetConnection();         IQueryable<T> objects = db.GetTable<T>().AsQueryable();          if (includeNull) list.Add(null);          foreach (T o in objects) list.Add(o);          return list;     }      public static List<T> Get<T>() where T : class, new()     {         return Get<T>(false);     } } 

I verified the results when calling the function with true or false – the List does contain the right values, when passing true, it contains null as the first value, followed by the other objects.

When I assign the DataSource to the ComboBox however, the control simply refuses to display any items, including the null value (not selectable):

cmbObjects.DataSource = ObjectProvider.Get<Car>(true); 

Passing in false (or no parameter) does work – it displays all of the objects.

Is there a way for me to specify a ‘null’ value for the first object without resorting to magic number objects (like having a bogus entry in the DB just to designate a N/A value)? Something along the lines of a nullable would be ideal, but I’m kind of lost.

Also, I’ve tried adding new T() instead of null to the list, but that only resulted in an OutOfMemoryException.

Answer 1

Okay, it seems the DataSource becomes invalid if you try to add a null value. The solution was to just add the items via a simple foreach loop with an empty string at the start instead of assigning the List<>.