Learning C# 3.0

Querying In-Memory Data

As you’ve seen elsewhere in this book, C# allows you to create classes that are complex, with many different properties, which sometimes are objects of other classes as well. You’ve also seen how to create collections of objects that you can manipulate in different ways. Sometimes that complexity works against you, though. Suppose you have a class that represents shipping orders for a warehouse. You could keep a ton of data in such an object, which would make it very versatile, but what if you just wanted to extract a list of the zip codes where your customers live, for demographic purposes? You could write some code to go through the entire collection of objects and pull out just the zip codes. It wouldn’t be terribly difficult, but it might be time-consuming. If that information were in a database, you could issue a simple SQL query, like you learned about in Chapter 20, but collections can’t be queried like a database…until now. Using LINQ, you can issue a SQL-like query against a collection in your code to get another collection containing just the data you want. An example will help make this clear.

Before you can start, you’ll need a collection to work with, so we’ll define a quick and simple Book class, like so:

public class Book
{
    public string Title { get; set; }
    public string Author { get; set; }
    public string Publisher { get; set; }
    public int PublicationYear { get; set; }
}

This is a very basic class, with three string fields and one int field.