如何转换IEnumerable <CustomType> 列出，应用分组逻辑，然后转换回IEnumerable <CustomType> ？

Question

I want to take a list of items from a table, apply grouping logic(grouping by assignment type, start and end time), and then convert back to starting type.

  IEnumerable<Assignment> assignments = UnitOfWork.AssignmentRepository.Get(filter: a => a.Start.CompareTo(start) >= 0
                    && a.End.CompareTo(end) <= 0,
                    orderBy: o => o.OrderBy(a => a.Start));

    List<Assignment> pre_alerts = new List<Assignment>();

 foreach (Assignment a in assignments)
            {
                foreach (AssignmentType t in assignmentTypes)
                {  // determine if the assignment's type has a minimum required
                    if (a.AssignmentTypeID == t.AssignmentTypeID && t.Minimum>0)
                    {

                        // get the minimum assignments requierd
                        int min_required = t.Minimum;

                        DateTime s = a.Start;
                        DateTime e = a.End;
                        AssignmentType type = a.AssignmentType;

                        IEnumerable<Assignment> minimum_assignments = UnitOfWork.AssignmentRepository.Get(filter: m => m.Start.CompareTo(s) >= 0 && m.End.CompareTo(e) <= 0 && m.AssignmentTypeID == a.AssignmentTypeID, orderBy: o => o.OrderBy(m => m.Start));

                        int min_actual = minimum_assignments.Count();

                        if (min_actual < min_required)
                        { 
                        // add a single alert for time range and assignment type
                           pre_alerts.Add(a);
                        }
                    }
                }
            }
 var alerts = pre_alerts.GroupBy(x => new Assignment{ x.AssignmentTypeID, x.Start, x.End }).Select(k => k);

how do I convert alerts to IEnumerable<Assignment> ?

Answer 1

When using Select you are already returning it as an IEnumerable. If you want it to be of your type then:

var alerts = pre_alerts.GroupBy(x => new Assignment{ x.AssignmentTypeID, x.Start, x.End }).Select(k => k.Key);

The result of a GroupBy is an IGrouping so by Selecting just k you are returning an IEnumerable<IGrouping<Assignment...>>

But it feels to me that if that is the case then there is no reason to use the GroupBy but just Select to create the assignments and then distinct