Conditionally Grouping List Items

Question

In the following code I create two lists based on a particular property which determines what I group by on. I then concatenate the two lists into a single list.

    var naFundCodeGroups = (from sar in aidItems      
                            where sar.SuFundCode.Substring(0, 2) != "SA"                                     
                            group sar by sar.NaFundCode into sa
                            select sa).ToList();

    var suFundCodeGroups = (from sar in aidItems
                            where sar.SuFundCode.Substring(0, 2) == "SA"
                            group sar by sar.SuFundCode
                            into sa
                            select sa).ToList();

    var fundCodeGroups = naFundCodeGroups.Concat(suFundCodeGroups).ToList();

Is there a more elegant way to do this?
For instance making the group by conditional somehow in a single statement.

Thanks for any help on this.

Answer 1

If there are no common values between the sets of SuFundCode and NaFundCode then sticking with the query comprehension syntax, this should work:

var fundCodeGroups = (from sar in aidItems
   group sar by (sar.SuFundCode.Substring(0, 2) == "SA" ?
      sar.SuFundCode : sar.NaFundCode)
   into sa
   select sa).ToList();

Or using the more compact (and in this case more readable, IMO) fluent/lambda syntax:

var fundCodeGroups = aidItems
   .GroupBy(sar => sar.SuFundCode.Substring(0, 2) == "SA" ?
      sar.SuFundCode : sar.NaFundCode)
   .ToList();

Otherwise, any of these should work, although only the last returns the same kind of grouping as your original:

var fundCodeGroups1 = (from sar in aidItems
   group sar by new { IsSA = sar.SuFundCode.Substring(0, 2) == "SA",
      Code = (sar.SuFundCode.Substring(0, 2) == "SA" ? sar.SuFundCode : sar.NaFundCode) }
   into sa
   select sa).ToList();

var fundCodeGroups2 = aidItems
   .GroupBy(sar => new { IsSA = sar.SuFundCode.Substring(0, 2) == "SA",
      Code = (sar.SuFundCode.Substring(0, 2) == "SA" ? sar.SuFundCode : sar.NaFundCode) })
   .ToList();

var fundCodeGroups3 = aidItems
   .GroupBy(sar => sar.SuFundCode.Substring(0, 2) == "SA")
   .SelectMany(g => g.Key ? g.GroupBy(i => i.SuFundCode) : g.GroupBy(i => i.NaFundCode))
   .ToList();

I'm not sure that any of these of offer better clarity or performance than your original solution, though.