Getting Started with ADO.NET in VS .NET

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Getting Started with ADO.NET
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1: Getting Started with ADO.NET
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Getting Started with ADO.NET
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In this chapter, you'll learn how to: Identify the primary objects that make up Microsoft ADO.NET are and how they interact Create Connection and DataAdapter objects by using the DataAdapter Configuration Wizard Automatically generate a DataSet Bind control properties to a DataSet Load data into a DataSet at run time Like other components of the .NET Framework, ADO.NET consists of a set of objects that interact to provide the required functionality. Unfortunately, this can make learning to use the object model frustrating you feel like you need to learn all of it before you can understand any of it. The solution to this problem is to start by building a conceptual framework. In other words, before you try to learn the details of how any particular object functions, you need to have a general understanding of what each object does and how the objects interact. That's what we'll do in this chapter. We'll start by looking at the main ADO.NET objects and how they work together to get data from a physical data store, to the user, and back again. Then, just to whet your appetite, we'll work through building a set of objects and binding them to a simple data form.
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On the Fundamental Interconnectedness of All Things
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In later chapters in this section, we ll examine each object in the ADO.NET object model in turn. At least in theory. In reality, because the objects are so closely interlinked, it s impossible to look at any single object in isolation.
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A roadmap note like this will point you to the discussion of a property or method that hasn t yet been introduced.
Where it s necessary to use a method or property that we haven t yet examined, I ll use roadmap notes, like the one in the margin next to this paragraph, to point you to the chapter where they are discussed.
The ADO.NET Object Model
The figure below shows a simplified view of the primary objects in the ADO.NET object model. Of course, the reality of the class library is more complicated, but we ll deal with the intricacies later. For now, it s enough to understand what the primary objects are and how they typically interact.
The ADO.NET classes are divided into two components: the Data Providers (sometimes called Managed Providers), which handle communication with a physical data store, and the DataSet, which represents the actual data. Either component can communicate with data consumers such as WebForms and WinForms. Data Providers The Data Provider components are specific to a data source. The .NET Framework includes two Data Providers: a generic provider that can communicate with any OLE DB data source, and a SQL Server provider that has been optimized for Microsoft SQL Server versions 7.0 and later. Data Providers for other databases such as Oracle and DB2 are expected to become available, or you can write your own. (You may be relieved to know that we won t be covering the creation of Data Providers in this book.) The two Data Providers included in the .NET Framework contain the same objects, although their names and some of their properties and methods are different. To illustrate, the SQL Server provider objects begin with SQL (for example, SQLConnection), while the OLE DB objects begin with OleDB (for example, OleDbConnection). The Connection object represents the physical connection to a data source. Its properties determine the data provider (in the case of the OLE DB Data Provider), the data source and database to which it will connect, and the string to be used during connecting. Its methods are fairly simple: You can open and close the connection, change the database, and manage transactions. The Command object represents a SQL statement or stored procedure to be executed at the data source. Command objects can be created and executed independently against a Connection object, and they are used by DataAdapter objects to handle communications from a DataSet back to a data source. Command objects can support SQL statements and stored procedures that return single values, one or more sets of rows, or no values at all.
A DataReader is a fast, low-overhead object for obtaining a forward-only, read-only stream of data from a data source. They cannot be created directly in code; they are created only by calling the ExecuteReader method of a Command. The DataAdapter is functionally the most complex object in a Data Provider. It provides the bridge between a Connection and a DataSet. The DataAdapter contains four Command objects: the SelectCommand, UpdateCommand, InsertCommand, and DeleteCommand. The DataAdapter uses the SelectCommand to fill a DataSet and uses the remaining three commands to transmit changes back to the data source, as required. Microsoft ActiveX In functional terms, the Connection and Command Data Objects objects are roughly equivalent to their ADO (ADO) counterparts (the major difference being the lack of support for server-side cursors), while the DataReader functions like a firehose cursor. The DataAdapter and DataSet have no real equivalent in ADO. DataSets The DataSet is a memory-resident representation of data. Its structure is shown in the figure below. The DataSet can be considered a somewhat simplified relational database, consisting of tables and their relations. It s important to understand, however, that the DataSet is always disconnected from the data source it doesn t know where the data it contains came from, and in fact, it can contain data from multiple sources.
The DataSet is composed of two primary objects: the DataTableCollection and the DataRelationCollection. The DataTableCollection contains zero or more DataTable objects, which are in turn made up of three collections: Columns, Rows, and Constraints. The DataRelationCollection contains zero or more DataRelations. The DataTable s Columns collection defines the columns that compose the DataTable. In addition to ColumnName and DataType properties, a DataColumn s properties allow you to define such things as whether or not it allows nulls (AllowDBNull), its maximum length (MaxLength), and even an expression that is used to calculate its value (Expression). The DataTable s Rows collection, which may be empty, contains the actual data as defined by the Columns collection. For each Row, the DataTable maintains its original, current, and proposed values. As we ll see, this ability greatly simplifies certain kinds of programming tasks.
The ADO.NET DataTable provides essentially the same functionality as the ADO Recordset object, although it obviously plays a very different role in the object model.
The DataTable s Constraints collection contains zero or more Constraints. Just as in a relational database, Constraints are used to maintain the integrity of the data. ADO.NET supports two types of constraints: ForeignKeyConstraints, which maintain relational integrity (that is, they ensure that a child row cannot be orphaned), and UniqueConstraints, which maintain data integrity (that is, they ensure that duplicate rows cannot be added to the table). In addition, the PrimaryKey property of the DataTable ensures entity integrity (that is, it enforces the uniqueness of each row). Finally, the DataSet s DataRelationCollection contains zero or more DataRelations. DataRelations provide a simple programmatic interface for navigating from a master row in one table to the related rows in another. For example, given an Order, a DataRelation allows you to easily extract the related OrderDetails rows. (Note, however, that the DataRelation itself doesn t enforce relational integrity. A Constraint is used for that.)
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