Practical .NET

Selecting Groups in LINQ

Some questions can only be answered by organizing your data into groups and then finding groups that contain particular members or have particular properties.

Once you've used LINQ to organize your objects into smaller collections with Group/By/Into you can answer lots of interesting questions: A shipping department, for example, might want a list of their "special" days: especially busy days or days have important shipments. In an earlier column I walked through the details of how to use the Group/By/Into clauses in LINQ statements (in both C# and Visual Basic because Group works very differently in the two languages) to organize your data into multiple, smaller collections.

These LINQ queries both generate a collection called ShipOrders. Each item in the ShipOrders collection is a collection of SalesOrder objects where every SalesOrder shares the same ShipDate value (to put it another way, these are collections of orders that ship on the same day):

var res = from so in db.SalesOrders
          group so by so.ShipDate 
  into ShipOrders

Dim res = From so in db.SalesOrders
          Group so By so.ShipDate
  Into ShipOrders = Group

You can select the groups you want by following that final Into clause with a Where clause. You might, for example, want only to know which days have a lot of shipments -- that might be groups with five or more SalesOrders being shipped:

var res = from so in db.SalesOrders
          group so by so.ShipDate 
   into ShipOrders
   where ShipOrders.Count() > 5

Dim res = From so in db.SalesOrders
          Group so By so.ShipDate
   Into ShipOrders = Group
   Where ShipOrders.Count > 5

Alternatively, you might want to know which days that have an especially valuable order going out -- groups that contain at least one order with a total value greater than $5,000:

var res = from so in db.SalesOrders
          group so by so.ShipDate 
   into ShipOrders
   where TotaledOrders.Max(so => so.TotalValue) > 5000

Dim res = From so in db.SalesOrders
          Group so By so.ShipDate
          Into ShipOrders = Group
   Where TotaledOrders.Max(Function(so) so.TotalValue) > 5000

Now, isn't that special?

About the Author

Peter Vogel is a system architect and principal in PH&V Information Services. PH&V provides full-stack consulting from UX design through object modeling to database design. Peter tweets about his VSM columns with the hashtag #vogelarticles. His blog posts on user experience design can be found at http://blog.learningtree.com/tag/ui/.

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