Question

I have two IEnumerables called BaseReportDefinitions and InputReportDefinitions. I need to do a left outer join where i want all the InputReportDefinitions and whichever BaseReportDefinitions that match. Both IEnumberables contain ReportDefinition objects that contain ParentName and ReportName properties that need to be used as the join key. I want to return the ReportDefinition object for each (in the case of BaseReportDefinition entry it may be null) in an anonymous object.

I have seen many examples of linq outer joins and outer joins with a static second condition that often gets put into a where condition but nothing that really uses two conditions fully for the join.

Answer 1

var items = inputReportDefinitions.GroupJoin(
              baseReportDefinitions,
              firstSelector => new {
                         firstSelector.ParentName, firstSelector.ReportName
                                   },
              secondSelector => new {
                         secondSelector.ParentName, secondSelector.ReportName
                                   },
              (inputReport, baseCollection) => new {inputReport, baseCollection})
              .SelectMany(grp => grp.baseCollection.DefaultIfEmpty(),
                         (col, baseReport) => new
                                                 {
                                                    Base = baseReport,
                                                    Input = col.inputReport
                                                 });

I believe this ends up being a left outer join. I don’t know how to convert this monstrosity to a query statement. I think if you add AsQueryable() to the end it could be used in Linq-to-SQL, but honestly, I have little experience with that.

EDIT: I figured it out. Much easier to read:

var otherItems = from i in inputReportDefinitions
                         join b in baseReportDefinitions
                         on new {i.ParentName, i.ReportName} 
                         equals new {b.ParentName, b.ReportName} into other
                         from baseReport in other.DefaultIfEmpty()
                         select new
                                    {
                                        Input = i,
                                        Base = baseReport
                                    };