Querying non-generic collections with LINQ to Objects
We will soon publish excerpts of the LINQ in Action book. One of them will probably show you how to query non-generic collections. Before it becomes available, let me show you an overview of what it presents.
When you read and hear about LINQ, you almost always see
queries that work on the generic collections provided by the
.NET Framework. In fact, you can use LINQ to Objects with
any type that implements IEnumerable<T>. This
means that LINQ to Objects will work with your own generic
collection types or generic collections from other
frameworks.
A problem you may encounter is that
not all .NET collections actually implement
IEnumerable<T>. In fact, only strongly-typed
collections implement this interface. Arrays, generic lists
and dictionaries are strongly-typed: you can work with an
array of integers, a list of strings or a dictionary of
Book objects. The non-generic collections do not
implement IEnumerable<T>, but implement
IEnumerable. Does this mean that you won’t be able to
use LINQ with ArrayList or DataSet objects, for example?
Fortunately, solutions exist. In the following code sample, we use Cast<Book> to convert a non-generic ArrayList collection into a typed-collection of Book objects. This allows us to use the Where or Select operators for example:
ArrayList arrayList = new ArrayList();
arrayList.Add(new
Book { Title="LINQ in Action" });
arrayList.Add(new
Book { Title="LINQ for Fun" });
arrayList.Add(new Book
{ Title="Extreme LINQ" });
var titles =
from book in arrayList.Cast<Book>()
where book.Title.Contains("Action")
select
book.Title;
The Cast operator casts the elements of a source
sequence to a given type.
It’s interesting to note that
you can also use a feature of C# query expressions. In a C#
query expression, an explicitly-typed iteration variable
translates to an invocation of Cast. Here how to use
Cast transparently by declaring the
book iteration variable as a Book:
var titles =
from Book book in arrayList
where book.Title.Contains("Action")
select book.Title;
Stay tuned to learn more in our upcoming excerpts. Of course, you can also get the full book immediately!
Cross-posted from http://LinqInAction.net