使用LINQ对数据进行分组不起作用

Question

I want to query a table and group by field BillPeriodId and I'm implementing it in the way the example in this article was done. Below is the method doing the grouping

public List<BillPeriodLine> GetLinesGroupedByPeriod()
{
    var linesGrouped = _lineRepository.AllIncluding(p => p.BillPeriod)
                                          .GroupBy(pl => pl.BillPeriodId)
                                          .Select(g => new Group<int, BillPeriodLine>
                                          {
                                              Key = g.Key,
                                              Values = g
                                          });
    return new List<BillPeriodLine>(linesGrouped);
}

The Discoverable Type

public class Group<T, K>
{
    public K Key;
    public IEnumerable<T> Values;
}

But it underlines g.Key with the following error message:

Cannot implicitly convert type 'int' to 'BillTracker.Entities.BillPeriodLine'

It's exactly the way it's done in the example except I'm using lamda expressions. It gives same error using LINQ statements. I don't know what I'm doing wrong. Sompne point me to the right direction please?

Answer 1

you have T and K around the wrong way (or vice versa )

new Group<int, BillPeriodLine>

...

public class Group<T, K>
{
    public K Key;
    public IEnumerable<T> Values;
}

Cannot implicitly convert type 'int' to 'BillTracker.Entities.BillPeriodLine'

The message says it perfectly, you are trying to push a square peg in a round hole

Answer 2

While Michael Randall's answer is perfectly sound regarding the error message, this is only part of the issue.

I guess that you'd like to display the grouped BillPeriodLine s in some sort of table as shown in the link you provided. Anyway, you can't simply return a List<BillPeriodLine> and assume it to be grouped magically, neither can you create a List<BillPeriodLine> from an IEnumerable<Group<int, BillPeriodLine>> - which is the type of linesGrouped . Presumably what you were trying to do, is to return a List<Group<int, BillPeriod>>

public List<Group<int, BillPeriodLine>> GetLinesGroupedByPeriod()
{
    var linesGrouped = _lineRepository.AllIncluding(p => p.BillPeriod)
                                      .GroupBy(pl => pl.BillPeriodId)
                                      .Select(g => new Group<int, BillPeriodLine>
                                      {
                                          Key = g.Key,
                                          Values = g
                                      });
    return linesGrouped.ToList();
}

This List<Group<int, BillPeriodLine>> can be used from a view as shown in the example: The view is iterating over the List<Group<int, BillPeriodLine>> . For each Group<int, BillPeriodLine> it creates a header for the group and then iterates over all items in Group<int,BillPeriodLine>.Values creating a line for each BillPeriodLine .

Furthermore (as Michael stated) you'll have to swap the type parameters of your Group<T,U> class (or switch the types where you create the Group<int, BillPeriodLine> , but I'd prefer the former, since I'd consider the key coming first a bit more intuitive).