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The class has been made static. You haven t seen the word static before, but it means always there. In the programs you ve written up until now, you ve had to create instances of classes using new. When a class is made static, it means that there s always one and only one instance of that class present when the program is running. When a C# program starts up, before the code that you ve written is given control, any static classes are created automatically. This means that there s no need to ever create an instance of the Program class by using new because it s always there when your program starts. The next line of the program declares a method called Main in the Program class:

cn.Close()

If the SQL statement contains action queries such as an INSERT, a DELETE, or an UPDATE statement they re correctly ignored by the NextResult method because they don t return any resultsets. Notice that you cannot use the CommandBehavior.Sin; gleResult option when invoking the ExecuteReader method on a query that contains multiple SELECT statements; otherwise, all queries after the first one will be ignored. That concludes the first part of my ADO.NET discussion, which covered using the Con; nection, Command, and DataReader objects. Even though the name and syntax of these objects and their methods differ from classic ADO, you ll probably agree that few things have changed. Using ADO.NET in connected mode is much like using ADO (except that you can t count on keysets and dynamic cursors). However, a discon; nected scenario is a different matter, as I ll explain in the following chapter.

In the preceding chapter, you saw how to work with ADO.NET in connected mode, processing data coming from an active connection and sending SQL commands to one. ADO.NET in connected mode behaves much like classic ADO, even though the names of the involved properties and methods (and their syntax) are often different. You ll see how ADO.NET differs from its predecessor when you start working in dis connected mode. ADO 2.x permits you to work in disconnected mode using client-side static recordsets opened in optimistic batch update mode. This was one of the great new features of ADO that proved to be a winner in client/server applications of any size. As a matter of fact, working in disconnected mode is the most scalable technique you can adopt because it takes resources on the client (instead of on the server) and, above all, it doesn t enforce any locks on database tables (except for the short-lived locks that are created during the update operation). In this chapter, I ll cover most of the features of ADO.NET in disconnected mode, except those that are related to XML for example, strongly typed DataSet classes and the XmlDataDocument class which I ll discuss in the next chapter. Also, because my main goal is to teach what each ADO.NET object does, all the samples in this chapter are based on handwritten code. At the end of the next chapter, once you re familiar with the basic concepts, I ll show you how you can rely on Visual Studio .NET and data binding to create functional database applications in minutes.

After you save a publication for the rst time, you can save changes simply by clicking the Save button on the Standard toolbar. The new version of the publication then overwrites the previous version. Tip By default, Publisher saves the publication you are working on every 10 minutes, in case the program stops responding or your computer shuts down unexpectedly. To adjust the time interval between saves, click Options on the Tools menu in an open publication, click the Save tab, and change the Save AutoRecover setting to the interval you want.

To keep the code as concise as possible, all the code samples in this chapter assume that the following Imports statements are used at the file or project level:

Imports Imports Imports Imports Imports Imports Imports System.Data System.Data.Common System.Data.OleDb System.Data.SqlClient System.Data.Odbc System.IO System.Text.RegularExpressions

static void Main(string[] args) { // content of the Main method goes here }

class, for implementing dictionary-like objects; and the NameObjectCollectionBase class, for creating collections whose elements can be indexed by either their string key or index.

You can create a regular read/write, strongly typed collection by inheriting a class from CollectionBase. This base collection provides all the members that are generic, such as Count, RemoveAt, and Clear, and you can complete the collection by providing only the members that take or return objects of the specific type that the collection is expected to contain. For example, you can build an initial version of the InvoiceLine-Collection class by adding the Add, Insert, and Remove methods, and the Item property: