Dr. Dobb's Journal - February 2008 - (Page 55)

d02hoff_p4ma 12/7/07 1:44 PM Page 55 by Kevin Hoffman Core Technology Continuous LINQ Extending and enhancing C# 3.5 with language extensions and LINQ If you haven’t been paying a lot of attention to the barrage of new Microsoft technology, LINQ is one of the key new features of the .NET Framework 3.5. LINQ, short for “Language Integrated Query,” lets you express filtering, sorting, grouping, and aggregation tasks using your language of choice with a syntax that looks pretty familiar to anyone with experience writing SQL queries for relational databases. Kevin is a Research Developer at Liquidnet, an equities trading system, where he has been researching Web 2.0, social software, and .NET 3.0. He can be contacted at alothien@gmail.com. No matter what kind of application you’re writing or what industry the application is for, there are some basic tasks that almost every programmer needs to do. At some point, you will likely have a list of data you need sorted, filtered, grouped, or aggregated in some fashion. Before LINQ, this involved the tedium of creating a loop to iterate through all of the list items, then adding the items that match a filter to the output collection, making sure to preserve the proper sort order. Using LINQ, you can now write statements that look like this, dramatically simplifying the code: var results = from customer in CustomerList where customer.Age > 21 orderby customer.LastName select customer; Through the magic of the language extensions feature in .NET 3.5, this query will essentially turn into something that looks like this: IEnumerable results = CustomerList.Where( f => f.Age > 21).OrderBy( o=> o.LastName ).Select( s => s); Don’t worry if this syntax looks ugly to you—that’s the entire purpose behind embedding language shortcuts for LINQ queries directly in C# and VB.NET. What you see in the preceding code sample are language extensions and lambda functions. The Where language extension to IEnumerable lets you filter pretty much any list, so long as the items in the list implement IEquatable . On its own, LINQ is a fantastic addition to the arsenal of any developer writing virtually any kind of application. The true power of the .NET Framework 3.5 comes with your ability to enhance the functionality of LINQ using language extensions. Language extensions let you create your own overridden implementations for Where, OrderBy, GroupBy, and the like. Continuous LINQ (CLINQ) is just one sample of such an extension to LINQ that was created using this facility. When building database applications, the general model is you write a query to obtain some data. Data can then be modified and then sent back to the database. In general, however, you do not expect to get continuous updates from the database as data in the underlying tables change. With most relational databases, the performance hit from doing such a thing would bring the database to a halt and probably impact the application as well. Other applications need to be quickly updated in response to continuously changing data. An example from the financial industry would be an application that listens to market data ticks. Using the current implementation of LINQ, you could write a query like this: var tickData = from tick in MarketData.Ticks where tick.Symbol == "AAPL" orderby tick.Ask descending select tick; February 2008 l www.ddj.com l Dr. Dobb’s Journal 55 http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - February 2008

Dr. Dobb's Journal - February 2008 Contents Hmmmm Alia Vox Developer Diaries Developer’s Notebook South American Software Development Conversations Inside Visual Studio 2008 BibPort: Creating Bibliographic References Continuous LINQ The ZK Framework Static Testing C++ Code The Agile Edge Effective Concurrency Swaine’s Flames

Dr. Dobb's Journal - February 2008

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