Linq grouping into list

Question

I have the following select query in linq:

var something = from te in taskEvidences join e in evidences on te.EvidenceId equals e.Id join tr in taskRequirements on te.TaskListId equals tr.TaskListId join r in newSelectableModule.Requirements on tr.RequirementListId equals r.Requirement.Id select new { Evidence = e, RequirementIndices = r.Index };

Currently it selects an Evidence object along with several Index (int) values, so for example I might get 5 records back, all with the same Evidence object and 5 different indices.

What I want to do it just return a single record with the Evidence object and a List<int> of the indices. I attempted to use grouping, but I keep getting errors about the type can't be inferred from the usage. This is one such attempt:

group new {e, r} by new {e} into g select new { Evidence = g.Key, RequirementIndices = g.SelectMany(x => x.r.Index) };

The error occurs around the SelectMany being assigned to the RequirementIndices property. I've tried several suggests I've found online, but none of them have helped. I assume it is a small error on my part, but I'm going code blind now!

Update:

Exact error:

The type arguments for method 'Enumerable.SelectMany(IEnumerable, Func>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

So what's the exact error? A minimal reproducible example would make it a lot easier to help you. — Jon Skeet
– Jon Skeet, Commented Mar 3, 2017 at 12:16
SelectMany is to flatten a list of lists. Joining tables doesn't nest lists into lists. What if you change it to Select? BTW, r seems to be part of the grouping. — Jeroen van Langen
– Jeroen van Langen, Commented Mar 3, 2017 at 12:17
@JeroenvanLangen You were right, I only needed a Select rather than SelectMany! Told you I was going code blind! — XN16
– XN16, Commented Mar 3, 2017 at 13:11

XN16 · Accepted Answer · 2017-03-03 13:10:54Z

As @JeroenvanLangen suggested in the comment on my question, I didn't need SelectMany only a Select:

var something = from te in taskEvidences join e in evidences on te.EvidenceId equals e.Id join tr in taskRequirements on te.TaskListId equals tr.TaskListId join r in newSelectableModule.Requirements on tr.RequirementListId equals r.Requirement.Id group new { e, r } by new { e } into g select new { Evidence = g.Key, RequirementIndices = g.Select(x => x.r.Index).ToList() };

Sergey Kalinichenko · Accepted Answer · 2017-03-03 13:19:29Z

You should be able to produce the same grouped result by avoiding a join in the top level:

var something = from te in taskEvidences join e in evidences on te.EvidenceId equals e.Id select new { Evidence = e, RequirementIndices = ( from tr in taskRequirements join r in newSelectableModule.Requirements on tr.RequirementListId equals r.Requirement.Id where te.TaskListId equals tr.TaskListId select r.Index ).ToList() };

Now the list is selected through a correlated subquery with a join, which eliminates creation of "duplicates" of the parent record. It should have the same performance as the original query.

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