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Supporting LINQ Queries Calling Your Methods

You have a method like this that returns a collection of premium Customer objects:

Public Function GetPremiumCustomers() As List(of Customer)
  Dim db As New dbContextObject
  Return From c In db.Customers
         Where c.IsPremium = True
         Select c
End Function

The problem is that other developers will want to use your method to further refine this query:

res = (From c in GetPremiumCustomers()
       Where c.City = "Regina"
       Select c).ToList()

If you're not careful the GetPremiumCustomers will retrieve every premium customer when the developer using your method only wants some of the premium customers (the ones in Regina). Retrieving more data than you'll ever want is a sure way to slow down your application. Wouldn't it be great if the LINQ query inside your method could be combined with the LINQ query using your method when the SQL statement is generated?

You can give .NET the opportunity to do exactly that -- to collapse both queries into a single SQL statement that retrieves only the premium customers from Regina -- if you just declare your GetPremiumCustomers method as returning the IQueryable interface. All you need to do is write your function's signature:

Public Function GetPremiumCustomers() As IQueryable(of Customer)
  Return From c In db.Customers
         Where c.IsPremium = True
         Select c

to return a result of IQueryable.

Posted by Peter Vogel on 03/16/2016 at 12:07 PM


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