THE MANY FACES OF LINQ

LINQ (Language Integrated Query, pronounced “link”) is a data-selection mechanism designed to give programs the ability to select data in the same way from any data source. Ideally the program would be able to use exactly the same method to fetch data whether it’s stored in arrays, lists, relational databases, XML data, Excel worksheets, or some other data store. Currently the LINQ API supports data stored in relational databases, objects within the program stored in arrays or lists, and XML data.

LINQ queries often let a program make complex data selections with very little code. For example, suppose you’re writing a billing program and you want to list customers with outstanding balances ordered by their balances. You could certainly use Visual Basic code to loop through a customer list to find the customers and then sort the results, but a LINQ query can find and order them in a few simple lines. The result isn’t always as fast as optimized Visual Basic code but it is often much simpler for complicated queries such as this one.

LINQ provides dozens of extension methods that apply to all sorts of data-holding objects such as arrays, dictionaries, and lists. Visual Basic provides a LINQ query syntax that converts SQL-like queries into calls to the LINQ extension methods to select data.

LINQ tools are divided into the three categories summarized in the following list:

• LINQ to Objects refers to LINQ functions that interact with Visual Basic objects such as arrays, dictionaries, and lists. Most of this chapter presents examples using these kinds of objects to demonstrate LINQ concepts.
• LINQ to XML refers to LINQ features that read and write XML data. Using LINQ, you can easily move data between XML hierarchies and other Visual Basic objects.
• LINQ to ADO.NET refers to LINQ features that let you write LINQ-style queries to extract data from relational databases.

The first section in this chapter, “Introduction to LINQ,” provides an intuitive introduction to LINQ. Many of the details about LINQ functions are so complex and technical that they can be hard to understand, but the basic ideas are really quite simple. The introduction gives examples that demonstrate the essential concepts to try to give you an understanding of the basics.

The section “Basic LINQ Query Syntax” describes the most useful LINQ query commands. These let you perform complex queries that select, filter, and arrange data taken from program objects. The next section, “Advanced LINQ Query Syntax,” describes additional LINQ query commands.

“LINQ Functions” describes functions that are provided by LINQ but that are not supported by Visual Basic’s LINQ query syntax. To use these functions, you must apply them to the arrays, dictionaries, lists, and other objects that they extend.

“LINQ Extension Methods” explains how LINQ extends objects such as arrays, dictionaries, and lists. It describes method-based queries and explains how you can write your own extensions to increase the power of method-based queries.

After describing the tools provided by LINQ, most of the rest of the chapter describes the three main categories of LINQ usage: LINQ to Objects, LINQ to XML, and LINQ to ADO.NET. The chapter finishes by describing Parallel LINQ (PLINQ).

LINQ to Objects is a bit easier to cover effectively than LINQ to XML and LINQ to ADO.NET because it doesn’t require that you have any special knowledge beyond Visual Basic itself. To understand LINQ to XML properly, you need to understand XML, which is a complex topic in its own right. Similarly, to get the most out of LINQ to ADO.NET, you need to understand relational databases such as SQL Server, a huge topic about which many books have been written.

Because LINQ to Objects is easiest to cover, this chapter focuses mostly on it, and most of the examples throughout the chapter deal with LINQ to Objects. The final sections of the chapter do provide some information about LINQ to XML and LINQ to ADO.NET, however, to give you an idea of what is possible in those arenas.