19.5 ADO.NET Object Models

Transcription

1 Ordering information: Visual Basic.NET How to Program 2/e The Complete Visual Basic.NET Training Course DEITEL TM on InformIT: Sign up for the DEITEL BUZZ ONLINE newsletter: DEITEL TM instructor-led training at your site: [Authors Note: This article is an excerpt from Chapter 19, Sections 19.5 and 19.6 of Visual Basic.NET How to Program.] 19.5 ADO.NET Object Models The ADO.NET object model provides an API for accessing database systems programmatically. ADO.NET was created for the.net framework and is the next generation of ActiveX Data Objects (ADO), which was implemented with Microsoft s Component Object Model (COM) framework. The primary ADO.NET namespaces are System.Data, System.Data.OleDb and System.Data.SqlClient. These namespaces contain classes for working with databases and other types of data sources (such as, XML files). Namespace System.Data is the root namespace for the ADO.NET API. Namespaces System.Data.OleDb and System.Data.SqlClient contain classes that enable programs to connect with and modify data sources. Namespace System.Data.OleDb contains classes that are designed to work with any data source, whereas the System.Data.SqlClient namespace contains classes that are optimized to work with Microsoft SQL Server 2000 databases. Instances of class System.Data.DataSet, which consist of a set of Data- Tables and relationships among those DataTables, represent a cache of data data that a program stores temporarily in local memory. The structure of a DataSet mimics the structure of a relational database. An advantage of using class DataSet is that it is disconnected the program does not require a persistent connection to the data source to work with data in a DataSet. The program connects to the data source only during the initial population of the DataSet, then reconnects to store any changes made in the DataSet. Hence, the program does not require an active, permanent connection to the data source. An instance of class OleDbConnection of namespace System.Data.OleDb represents a connection to a data source. An instance of class OleDbDataAdapter connects to a data source through an instance of class OleDbConnection. An OleDb- DataAdapter object populates a DataSet with data from a data source. An instance of class OleDbCommand of namespace System.Data.OleDb represents an arbitrary SQL command to be executed on a data source. A program can use instances of class OleDbCommand to manipulate a data source through an OleDbConnection. The programmer must close the active connection to the data source explicitly when no further changes are to be made. Unlike DataSets, OleDbCommand objects do not cache data in local memory Programming with ADO.NET: Extracting Information from a Database This section presents an example that introduces how to connect to a database, query the database and display the results of the query. The database we use in these examples is a Microsoft Access database called Books (described in the appendix to this article). Every program that uses this database must reference the database s location. Before compiling and running this example, readers must change lines to update the database location on their computers, such that the code specifies the correct location for the database file.

2 Chapter 19 Database, SQL and ADO.NET Connecting to and Querying an Access Data Source The example of Fig performs a simple query on the Books database that retrieves the complete contents of the Authors table and displays the data in a DataGrid (a component from namespace System.Windows.Forms that can display a data source in a GUI). The program connects to the database, queries the database and displays the results. The discussion that follows the example presents the key aspects of the program. [Note: We present the complete Microsoft Visual Studio auto-generated code in Fig so that readers can see the database and GUI code Visual Studio creates for this example.] 1 ' Fig : DisplayTable.vb 2 ' Displaying data from a database table. 3 4 Public Class FrmTableDisplay 5 Inherits System.Windows.Forms.Form 6 7 #Region " Windows Form Designer generated code " 8 9 Public Sub New() 10 MyBase.New() ' This call is required by the Windows Form Designer. 13 InitializeComponent() ' Add any initialization after the 16 ' InitializeComponent call ' fill DataSet1 with data 19 OleDbDataAdapter1.Fill(DataSet1, "Authors") ' bind data in Users table in dataset1 to dgdauthors 22 dgdauthors.setdatabinding(dataset1, "Authors") 23 End Sub ' New ' Form overrides dispose to clean up the component list. 26 Protected Overloads Overrides Sub Dispose( _ 27 ByVal disposing As Boolean) If disposing Then 30 If Not (components Is Nothing) Then 31 components.dispose() 32 End If 33 End If 34 MyBase.Dispose(disposing) 35 End Sub ' Dispose Friend WithEvents dgdauthors As System.Windows.Forms.DataGrid 38 Friend WithEvents OleDbSelectCommand1 As _ 39 System.Data.OleDb.OleDbCommand Friend WithEvents OleDbInsertCommand1 As _ 42 System.Data.OleDb.OleDbCommand Fig Database access and information display (part 1 of 7).

3 3 Database, SQL and ADO.NET Chapter Friend WithEvents OleDbUpdateCommand1 As _ 45 System.Data.OleDb.OleDbCommand Friend WithEvents OleDbDeleteCommand1 As _ 48 System.Data.OleDb.OleDbCommand Friend WithEvents OleDbConnection1 As _ 51 System.Data.OleDb.OleDbConnection Friend WithEvents OleDbDataAdapter1 As _ 54 System.Data.OleDb.OleDbDataAdapter Friend WithEvents DataSet1 As System.Data.DataSet ' Required by the Windows Form Designer 59 Private components As System.ComponentModel.Container ' NOTE: The following procedure is required by the 62 ' Windows Form Designer 63 ' It can be modified using the Windows Form Designer. 64 ' Do not modify it using the code editor. 65 <System.Diagnostics.DebuggerStepThrough()> _ 66 Private Sub InitializeComponent() Me.dgdAuthors = New System.Windows.Forms.DataGrid() 69 Me.OleDbSelectCommand1 = _ 70 New System.Data.OleDb.OleDbCommand() Me.OleDbInsertCommand1 = _ 73 New System.Data.OleDb.OleDbCommand() Me.OleDbUpdateCommand1 = _ 76 New System.Data.OleDb.OleDbCommand() Me.OleDbDeleteCommand1 = _ 79 New System.Data.OleDb.OleDbCommand() Me.OleDbConnection1 = _ 82 New System.Data.OleDb.OleDbConnection() Me.OleDbDataAdapter1 = _ 85 New System.Data.OleDb.OleDbDataAdapter() Me.DataSet1 = New System.Data.DataSet() 88 CType(Me.dgdAuthors, _ 89 System.ComponentModel.ISupportInitialize).BeginInit() CType(Me.DataSet1, _ 92 System.ComponentModel.ISupportInitialize).BeginInit() Me.SuspendLayout() 95 Fig Database access and information display (part 2 of 7).

4 Chapter 19 Database, SQL and ADO.NET 4 96 ' 97 ' dgdauthors 98 ' 99 Me.dgdAuthors.DataMember = "" 100 Me.dgdAuthors.Location = New System.Drawing.Point(8, 8) 101 Me.dgdAuthors.Name = "dgdauthors" 102 Me.dgdAuthors.Size = New System.Drawing.Size(304, 256) 103 Me.dgdAuthors.TabIndex = ' 106 ' OleDbSelectCommand1 107 ' 108 Me.OleDbSelectCommand1.CommandText = _ 109 "SELECT authorid, firstname, lastname FROM Authors" Me.OleDbSelectCommand1.Connection = Me.OleDbConnection ' 114 ' OleDbInsertCommand1 115 ' 116 Me.OleDbInsertCommand1.CommandText = _ 117 "INSERT INTO Authors(authorID, firstname, lastname)" & _ 118 "VALUES (?,?,?)" Me.OleDbInsertCommand1.Connection = _ 121 Me.OleDbConnection Me.OleDbInsertCommand1.Parameters.Add _ 124 (New System.Data.OleDb.OleDbParameter("authorID", _ 125 System.Data.OleDb.OleDbType.Numeric, 0, _ 126 System.Data.ParameterDirection.Input, False, _ 127 CType(10, Byte), CType(0, Byte), "authorid", _ 128 System.Data.DataRowVersion.Current, Nothing)) Me.OleDbInsertCommand1.Parameters.Add _ 131 (New System.Data.OleDb.OleDbParameter("firstName", _ 132 System.Data.OleDb.OleDbType.Char, 50, _ 133 System.Data.ParameterDirection.Input, False, _ 134 CType(0, Byte), CType(0, Byte), "firstname", _ 135 System.Data.DataRowVersion.Current, Nothing)) Me.OleDbInsertCommand1.Parameters.Add _ 138 (New System.Data.OleDb.OleDbParameter("lastName", _ 139 System.Data.OleDb.OleDbType.Char, 50, _ 140 System.Data.ParameterDirection.Input, False, _ 141 CType(0, Byte), CType(0, Byte), "lastname", _ 142 System.Data.DataRowVersion.Current, Nothing)) ' 145 ' OleDbUpdateCommand1 146 ' 147 Me.OleDbUpdateCommand1.CommandText = _ 148 "UPDATE Authors SET authorid =?, firstname =?, " & _ Fig Database access and information display (part 3 of 7).

5 5 Database, SQL and ADO.NET Chapter "lastname =? WHERE (authorid =?)" & _ 150 " AND (firstname =?) AND (lastname =?)" Me.OleDbUpdateCommand1.Connection = Me.OleDbConnection1 153 Me.OleDbUpdateCommand1.Parameters.Add ( _ 154 New System.Data.OleDb.OleDbParameter("authorID", _ 155 System.Data.OleDb.OleDbType.Numeric, 0, _ 156 System.Data.ParameterDirection.Input, False, _ 157 CType(10, Byte), CType(0, Byte), "authorid", _ 158 System.Data.DataRowVersion.Current, Nothing)) Me.OleDbUpdateCommand1.Parameters.Add _ 161 (New System.Data.OleDb.OleDbParameter("firstName", _ 162 System.Data.OleDb.OleDbType.Char, 50, _ 163 System.Data.ParameterDirection.Input, False, _ 164 CType(0, Byte), CType(0, Byte), "firstname", _ 165 System.Data.DataRowVersion.Current, Nothing)) Me.OleDbUpdateCommand1.Parameters.Add _ 168 (New System.Data.OleDb.OleDbParameter("lastName", _ 169 System.Data.OleDb.OleDbType.Char, 50, _ 170 System.Data.ParameterDirection.Input, False, _ 171 CType(0, Byte), CType(0, Byte), "lastname", _ 172 System.Data.DataRowVersion.Current, Nothing)) Me.OleDbUpdateCommand1.Parameters.Add _ 175 (New System.Data.OleDb.OleDbParameter _ 176 ("Original_authorID", _ 177 System.Data.OleDb.OleDbType.Numeric, 0, _ 178 System.Data.ParameterDirection.Input, False, _ 179 CType(10, Byte), CType(0, Byte), "authorid", _ 180 System.Data.DataRowVersion.Original, Nothing)) Me.OleDbUpdateCommand1.Parameters.Add _ 183 (New System.Data.OleDb.OleDbParameter _ 184 ("Original_firstName", _ 185 System.Data.OleDb.OleDbType.Char, 50, _ 186 System.Data.ParameterDirection.Input, False, _ 187 CType(0, Byte), CType(0, Byte), "firstname", _ 188 System.Data.DataRowVersion.Original, Nothing)) Me.OleDbUpdateCommand1.Parameters.Add _ 191 (New System.Data.OleDb.OleDbParameter _ 192 ("Original_lastName", _ 193 System.Data.OleDb.OleDbType.Char, 50, _ 194 System.Data.ParameterDirection.Input, False, _ 195 CType(0, Byte), CType(0, Byte), "lastname", _ 196 System.Data.DataRowVersion.Original, Nothing)) ' 199 ' OleDbDeleteCommand1 200 ' 201 Me.OleDbDeleteCommand1.CommandText = _ Fig Database access and information display (part 4 of 7).

6 Chapter 19 Database, SQL and ADO.NET "DELETE FROM Authors WHERE (authorid =?) AND " & _ 203 "(firstname =?) AND (lastname =?)" Me.OleDbDeleteCommand1.Connection = Me.OleDbConnection1 206 Me.OleDbDeleteCommand1.Parameters.Add _ 207 (New System.Data.OleDb.OleDbParameter("authorID", _ 208 System.Data.OleDb.OleDbType.Numeric, 0, _ 209 System.Data.ParameterDirection.Input, False, _ 210 CType(10, Byte), CType(0, Byte), "authorid", _ 211 System.Data.DataRowVersion.Original, Nothing)) Me.OleDbDeleteCommand1.Parameters.Add _ 214 (New System.Data.OleDb.OleDbParameter("firstName", _ 215 System.Data.OleDb.OleDbType.Char, 50, _ 216 System.Data.ParameterDirection.Input, False, _ 217 CType(0, Byte), CType(0, Byte), "firstname", _ 218 System.Data.DataRowVersion.Original, Nothing)) Me.OleDbDeleteCommand1.Parameters.Add _ 221 (New System.Data.OleDb.OleDbParameter("lastName", _ 222 System.Data.OleDb.OleDbType.Char, 50, _ 223 System.Data.ParameterDirection.Input, False, _ 224 CType(0, Byte), CType(0, Byte), "lastname", _ 225 System.Data.DataRowVersion.Original, Nothing)) ' 228 'OleDbConnection1 229 ' 230 Me.OleDbConnection1.ConnectionString = _ 231 "Provider=Microsoft.Jet.OLEDB.4.0;Password="""";" & _ 232 "User ID=Admin;Data Source=C:\Documen" & _ 233 "ts and Settings\thiago\Desktop\vbhtp2e\examples\" & _ 234 "Ch19\Fig19_27\Books.mdb;Mode=Sha" & _ 235 "re Deny None;Extended Properties="""";" & _ 236 "Jet OLEDB:System database="""";jet OLEDB:Regis" & _ 237 "try Path="""";Jet OLEDB:Database Password="""";" & _ 238 "Jet OLEDB:Engine Type=5;Jet OLEDB:Dat" & _ 239 "abase Locking Mode=1;Jet OLEDB:Global Partial " & _ 240 "Bulk Ops=2;Jet OLEDB:Global Bulk T" & _ 241 "ransactions=1;jet OLEDB:New Database " & _ 242 "Password="""";Jet OLEDB:Create System Databas" & _ 243 "e=false;jet OLEDB:Encrypt Database=False;" & _ 244 "Jet OLEDB:Don't Copy Locale on Compact=" & _ 245 "False;Jet OLEDB:Compact Without Replica " & _ 246 "Repair=False;Jet OLEDB:SFP=False" ' 249 ' OleDbDataAdapter1 250 ' 251 Me.OleDbDataAdapter1.DeleteCommand = _ 252 Me.OleDbDeleteCommand1 253 Fig Database access and information display (part 5 of 7).

7 7 Database, SQL and ADO.NET Chapter Me.OleDbDataAdapter1.InsertCommand = _ 255 Me.OleDbInsertCommand Me.OleDbDataAdapter1.SelectCommand = _ 258 Me.OleDbSelectCommand Me.OleDbDataAdapter1.TableMappings.AddRange _ 261 (New System.Data.Common.DataTableMapping() _ 262 {New System.Data.Common.DataTableMapping("Table", _ 263 "Authors", New System.Data.Common.DataColumnMapping() _ 264 {New System.Data.Common.DataColumnMapping("authorID", _ 265 "authorid"), New System.Data.Common.DataColumnMapping _ 266 ("firstname", "firstname"), _ 267 New System.Data.Common.DataColumnMapping("lastName", _ 268 "lastname")})}) Me.OleDbDataAdapter1.UpdateCommand = _ 271 Me.OleDbUpdateCommand ' 274 ' DataSet1 275 ' 276 Me.DataSet1.DataSetName = "NewDataSet" 277 Me.DataSet1.Locale = _ 278 New System.Globalization.CultureInfo("en-US") ' 281 ' FrmTableDisplay 282 ' 283 Me.AutoScaleBaseSize = New System.Drawing.Size(5, 13) 284 Me.ClientSize = New System.Drawing.Size(320, 273) 285 Me.Controls.AddRange(New System.Windows.Forms.Control() _ 286 {Me.dgdAuthors}) Me.Name = "FrmTableDisplay" 289 Me.Text = "Table Display" 290 CType(Me.dgdAuthors, System.ComponentModel. _ 291 ISupportInitialize).EndInit() CType(Me.DataSet1, System.ComponentModel. _ 294 ISupportInitialize).EndInit() Me.ResumeLayout(False) End Sub ' InitializeComponent #End Region End Class ' FrmTableDisplay Fig Database access and information display (part 6 of 7).

8 Chapter 19 Database, SQL and ADO.NET 8 Fig Database access and information display (part 7 of 7). Before compiling the example, the reader must register the Books database as a data source. In Visual Studio, right click the Data Connections node in the Server Explorer then click Add Connection. In the Provider tab of the window that appears, choose Microsoft Jet 4.0 OLE DB Provider the driver for Access databases. In the Connection tab, click the ellipses button ( ) to the right of the database name textbox to open the Select Access Database window. Locate the folder that contains the Books database, select the Books database and click OK. Then, click the Add Connection window s OK button. Now, the database is listed as a connection in the Server Explorer. Drag the Books database node from the Server Explorer onto the Windows Form. This creates an OleDbConnection to the source, which the Windows Form designer shows as OleDbConnection1. Next, drag an OleDbDataAdapter from the Toolbox s Data subheading onto the Windows Form designer. This displays the Data Adapter Configuration Wizard, which configures the OleDbDataAdapter instance with a custom query for populating a DataSet. Click Next to display a drop-down list of possible connections. Select the connection created in the previous step from the drop-down list and click Next. The resulting screen allows the reader to choose how the OleDbDataAdapter should access the database. Keep the default Use SQL Statement option and click Next. Click the Query Builder button, then select the Authors table from the Add menu then Close that menu. Place a check mark in the *All Columns box from the small Authors window. Click OK then click Finish. Next, we create a DataSet to store the query results. To do so, drag DataSet from the Data tab in the Toolbox onto the form. This displays the Add DataSet window. Choose the Untyped DataSet (no schema) the query with which we populate

9 9 Database, SQL and ADO.NET Chapter 19 the DataSet dictates the DataSet s schema, or structure (i.e., the tables that comprise the DataSet and the relationships among those tables). Finally, add DataGrid dgd- Authors to the Form. Figure includes all of the auto-generated code. Normally, we omit this code from examples, because this code consists solely of GUI components. In this case, however, we must discuss database functionality in the auto-generated code. Furthermore, we use Visual Studio s default naming conventions in this example to show exactly the code Visual Studio creates. Normally, we would change these names to conform to our programming conventions and style. Lines initialize the OleDbConnection for this program. Property ConnectionString specifies the path to the database file on the computer s hard drive. [Note: If you are using our example, you will need to update this string to specify the correct database location on your computer.] An instance of class OleDbDataAdapter populates the DataSet in this example with data from the Books database. The instance properties DeleteCommand (lines ), InsertCommand (lines ), SelectCommand (lines ) and UpdateCommand (lines ) are OleDbCommand objects that specify how the OleDbDataAdapter deletes, inserts, selects and updates data in the database. Each OleDbCommand object must have an OleDbConnection through which the OleDbCommand can communicate with the database. Property Connection is set to the OleDbConnection to the Books database. For OleDbUpdateCommand1, line 152 sets the Connection property, and lines set the CommandText. Although Visual Studio.NET generates most of this program s code, we manually enter code in the FrmTableDisplay constructor (lines 9 23) to populate dataset1 using an OleDbDataAdapter. Line 19 calls OleDbDataAdapter method Fill to retrieve information from the database associated with the OleDbConnection and to place this information in the given DataSet. The second argument to method Fill is a String that specifies the name of the database table from which to Fill the DataSet. Line 22 invokes DataGrid method SetDataBinding to bind the DataGrid to a data source. The first argument is the DataSet in this case, DataSet1 whose data the DataGrid should display. The second argument is a String that represents the name of the table within the data source that we want to bind to the DataGrid. Executing line 22 fills the DataGrid with the information in the DataSet. The information in DataSet1 is used to set the correct number of rows and columns in the DataGrid and to provide the columns with default names.

10 Chapter 19 Database, SQL and ADO.NET 10 APPENDIX: RELATIONAL DATABASE OVERVIEW: Books DATABASE This appendix overviews the tables of the Books database used in this article. The database consists of four tables: Authors, Publishers, AuthorISBN and Titles. The Authors table (described in Fig ) consists of three fields (or columns) that maintain each author s unique ID number, first name and last name. Field Description authorid firstname lastname Author s ID number in the database. In the Books database, this Integer field is defined as an auto-increment field. For each new record inserted in this table, the database increments the authorid value, ensuring that each record has a unique authorid. This field represents the table s primary key. Author s first name (a String). Author s last name (a String). Fig Authors table from Books. The Publishers table (Fig ) consists of two fields, representing each publisher s unique ID and name. Field Description publisherid publishername The publisher s ID number in the database. This auto-incremented Integer field is the table s primary-key field. The name of the publisher (a String). Fig Publishers table from Books. The AuthorISBN table (Fig ) consists of two fields, which maintain ISBNs for each book and their corresponding authors ID numbers. This table helps associate the names of the authors with the titles of their books. ISBN is an abbreviation for International Standard Book Number a numbering scheme by which publishers worldwide give every book a unique identification number. Field Description authorid isbn The author s ID number, which allows the database to associate each book with a specific author. The integer ID number in this field must also appear in the Authors table. The ISBN for a book (a String). Fig AuthorISBN table from Books.

11 11 Database, SQL and ADO.NET Chapter 19 The Titles table (Fig ) consists of seven fields, which maintain general information about the books in the database. This information includes each book s ISBN, title, edition number, copyright year and publisher s ID number, as well as the name of a file that contains an image of the book cover, and finally, each book s price. Field Description isbn title editionnumber copyright publisherid imagefile price ISBN of the book (a String). Title of the book (a String). Edition number of the book (a String). Copyright year of the book (an Integer). Publisher s ID number (an Integer). This value must correspond to an ID number in the Publishers table. Name of the file containing the book s cover image (a String). Suggested retail price of the book (a real number). [Note: The prices shown in this database are for example purposes only.] Fig Titles table from Books. Figure illustrates the relationships among the tables in the Books database. The first line in each table is the table s name. The field whose name appears in italics contains that table s primary key. A table s primary key uniquely identifies each record in the table. Every record must have a value in the primary-key field, and the value must be unique. This is known as the Rule of Entity Integrity. Note that the AuthorISBN table contains two fields whose names are italicized. This indicates that these two fields form a compound primary key each record in the table must have a unique authorid isbn combination. For example, several records might have an authorid of 2, and several records might have an isbn of , but only one record can have both an authorid of 2 and an isbn of Authors authorid firstname lastname AuthorISBN Titles 1 1 authorid isbn isbn Publishers publisherid publishername 1 title editionnumber copyright publisherid imagefile price Fig Table relationships in Books.

12 Chapter 19 Database, SQL and ADO.NET 12 Common Programming Error 19.1 Failure to provide a value for a primary-key field in every record breaks the Rule of Entity Integrity and causes the DBMS to report an error Common Programming Error 19.2 Providing duplicate values for the primary-key field in multiple records causes the database management system (DBMS) to report an error The lines that connect the tables in Fig represent the relationships among the tables. Consider the line between the Publishers and Titles tables. On the Publishers end of the line, there is a 1, and on the Titles end, there is an infinity ( ) symbol. This line indicates a one-to-many relationship, in which every publisher in the Publishers table can have one or more corresponding books in the Titles table. Note that the relationship line links the publisherid field in the Publishers table to the publisherid field in Titles table. In the Titles table, the publisherid field is a foreign key a field for which every entry has a unique value in another table and where the field in the other table is the primary key for that table (e.g., publisherid in the Publishers table). Programmers specify foreign keys when creating a table. The foreign key helps maintain the Rule of Referential Integrity: Every foreign-key field value must appear in another table s primary-key field. Foreign keys enable information from multiple tables to be joined together for analysis purposes. There is a one-to-many relationship between a primary key and its corresponding foreign key. This means that a foreignkey field value can appear many times in its own table, but must appear exactly once as the primary key of another table. The line between the tables represents the link between the foreign key in one table and the primary key in another table. Common Programming Error 19.3 Providing a foreign-key value that does not appear as a primary-key value in another table breaks the Rule of Referential Integrity and causes the DBMS to report an error The line between the AuthorISBN and Authors tables indicates that, for each author in the Authors table, there can be one or more corresponding ISBNs for books written by that author in the AuthorISBN table. The authorid field in the AuthorISBN table is a foreign key of the authorid field (the primary key) of the Authors table. Note, again, that the line between the tables links the foreign key in table AuthorISBN to the corresponding primary key in table Authors. The AuthorISBN table links information in the Titles and Authors tables. Finally, the line between the Titles and AuthorISBN tables illustrates a one-tomany relationship; a title can be written by any number of authors. In fact, the sole purpose of the AuthorISBN table is to represent a many-to-many relationship between the Authors and Titles tables; an author can write any number of books, and a book can have any number of authors.