ado.net
TRANSCRIPT
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A Hand Book on
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TABLE OF CONTENTS
1. The ADO.Net Model - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - -- -- - - - - - - 3
2. The ADO.net Command Object - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 6
3. The ADO.net Connection Object - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 9
4. The ADO.net Datareader - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 14
5. The ADO.net Dataset Object Part 1 - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - 16
6. The ADO.net Dataset Object Part 2- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 20
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THE ADO.NET MODEL
ADO.Net is the latest in a series of technologies from Microsoft which focus on the
connection of applications to databases of one sort or another. From the DAO which (and
is) was the native mode of connection for MSAccess, through the short-lived RDO, and the
now comparatively long-in-the-tooth ADO, this is the next generation of technology. And,
although it is not likely that there will not be some future add-ons, enhancements, and
upgrades, it appears that this structure of database connectivity is a keeper.
It is not a COM technology, so it can be used on other platforms in addition to Windows,
and agnostic when it comes to the brand of database it facilitates connection to. In
addition, it allows more extensive support to the XML paradigm.
The .Net platform will continue to allow you to use the older ADO connection technology,
but, under most circumstances, this is a poor choice because of the performance penalty ,
which comes from using the unmanaged code in the COM object.
ADO.Net requires some new methods of accomplishing some of the simple tasks of
interacting with data. For example, server-side cursors and are not supported any more
because of the increased overhead and the potentially large number of lacks required on
the server. Accordingly, the only connection s allowed are forward only, read- read-only
result sets, and disconnected result sets. There are rumors of server side cursors being
planned for future releases, probably due to the loud complaint from the developer
community. However, there are a number of techniques and tools provided which greatly
lessen the need for server side cursors, so by the time of the next release, there may be
less need for them.
To gain access to the ADO.Net class libraries, you must add the following statements to
the top of your source files:
Imports System.Data
Imports System.Data.OleDb* or, if you are connecting to SQLServer
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Imports System.Data.SqlClient
There is also support for the ODBC connections through Imports System.Data.ODBC
These commands expose the objects needed to connect to the data source.
Data Retreival
Like ADO, ADO.Net uses a connection object to point to external data. Under the .Net
model, a connection is opened, data is retrieved, and then the connection is closed. The
closing of the connection is necessary to free up resources. The connection string (the
part of the comment which identifies the source of the data, as well as access to it
through username and password) is identical to the connection string grammar under the
old model ADO.
The first way to access data is after you have defined and opened the connection, invoke
the command object providing it with a SELECT statement, or storedprocedure name with
parameters. The Data Reader will allow the application to gain access to the returned
resultset. An ExecuteReader method will allow a line by line reading of the data retrieved.
However, be aware that this is a forward only dataset – once a line is read, unless you
save its contents somewhere, somewhere the data can be lost. The only way to make it
available again is to re-establish the connection and read it again.
The second method opens a connection, retrieves a recordset, then stores that recordset
in an object called a DataSet. The DataSet acts and functions like a local database, storing
the data retrieved – even from multiple sources. It can even link and establish
relationships between multiple tables. At the conclusion of the data retrieval, the
connection is closed, so that in processing the DataSet is completely disconnected from
the data source(s).
The mapping of data between the DataSet and the outside data sources is handled by the
DataAdapter object. In addition to keeping track of the connections to the data sources,
the DataAdapter also facilitates the updating, deleting, and insertion of data back to the
source.
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XML
XML is the native format for ADO.Net. It is so tightly integrated that you can define and
read schemas, and can seamlessly exchange data in the XLM format, both reading and
writing with any application on any platform.
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THE ADO.NET COMMAND OBJECT
In a connected mode environment, after a connection is established with the data, the
data is manipulated and returned using the command object. The command object is
passed a SQL query SELECT statement, which is run by one of the Execute methods. The
three Execute methods are ExecuteNonQuery (for updates or deletes or appends),
Execute Reader (for returning datasets to the client), and ExecuteScalar (returns a single
value).
The command objects themselves are either of SqlCommand, OleDbCommand, or
ODBCCommand types. And the principal properties are the command text (the sql
statement, and the connection object previously created. ) Other properties are the
CommandType, Transaction, CommandTimeout, Parameters, and UpdatedRowSource.
Dim cmd as New SqlCommand
cmd.connection=conn
cmd.CommandText=”SELECT * FROM tblBooks;”
Dim dr as SqlDataReader = cmd.ExecuteReader()
The preceding code will execute a SQL command and put the results into a DataReader
after having made and opened a connection. Had the SQL statement been an INSERT
statement, the appropriate method would have been ExecuteNonQuery(), rather than
ExecuteReader(). There is an additional Execute XMLReader() method which is new in
SQL2000, and is used for processing SELECT queries from an XML dataset.
ExecuteScalar() is another method used for returning a single value from a dataset.
Parameters
In the OleDb model, you can define parameters in the CommandText using question
marks.
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SELECT * from tblBooklist where YearPublished = ?;
Then you need to create and define the parameter prior to passing them into the
command object.
Dim Parm as New OleDBParameter(“YearPublished”, OleDbType.Integer)
Parm.Value=1994
cmd.Parameters.Add(parm)
You can even have ADO.Net populate the list of parameters for you using the
DeriveParameters method.
Dim cmd as New SqlCommand(“up_getshotgun”, conn)
cmd.CommandType=CommandType.StoredProcedure
SqlCommandBuilder.DeriveParameters(cmd)
Debug.writeline(cmd.parameters.count & “parameters”)
For i = 0 to cmd.parameters.count-1
Debug.writeline(cmd.parameters(i).ParameterName)
Next
Stored Procedures
Stored Procedures are handled using the parameters in the previous section and setting
them with the Parameters.Add method. The only difference from the previous code is the
calling of the stored procedure itself. This is accomplished using the CommandType
property, rather than the CommandText property.
Dim cmd as New SqlCommand(“up_getshotgun”, conn)
cmd.CommandType=CommandType.StoredProcedure
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cmd.Parameters.Add(“@Brand”,”Remington”)
Dim dr as SqlDataReader=cmd.ExecuteReader()
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THE ADO.NET CONNECTION OBJECT
Either in connected or disconnected mode, the first thing one needs to do is to connect to
the database(s). This is accomplished in ADO.net by creating a connection object that
points to the subject database.
The properties of the connection object are: Connection string
A string used to connect to the database.
Connection Timeout
The number of seconds till a connection times out (Read
Only)
Database
Returns the database name as specified in connection
string (Read Only)
DataSource
Returns the source attribute as specified in connection
string (Read Only)
ServerVersion
Returns version of connected server.
State
Returns state of current database in integers. Values can
be Closed, Connecting, Open, Executing, Fetching,
Broken
Provider
Returns the value of provider attribute as specified in
connection string (Read Only) (OleDb Only)
PacketSize
Returns size in bytes of network packets (SQL Server
only)
WorkstationID Identifies client, as specified in connection string (Read
Only)
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In the above table, the only property that is NOT read only is the connection string. Some
folks say that it is the connection string that is the most difficult aspect of ADO and
ADO.Net. If so, it is an easily learned one. A typical connection string consists of 4 items:
The Provider, which specifies the name of the underlying OLEDB provider. Appropriate
values are SQLOLEDB (for SQLServer), Microsoft.Jet.OLEDB.4.0 (for Microsoft Access) and
MSDORA (for Oracle);
The Data Source attribute, which shows the location of the database. It can be a path on
a network, or the IP address of a machine on the net;
The UserID and Password, which grant access permission to the database;
The Initial Catalog, which specifies the name of the database in the data source.
Here are some common configurations:
For SQL Server –
Data Source=Jupiter;Initial Catalog=pubs;User Id=ElmerFudd;Password=wabbitt;
Server=Jupiter;Database=pubs;Trusted_Connection=True;Connection
Timeout=10
Data Source=200.192.23.155;Network Library=Wiley3301;Initial
Catalog=pubs;User ID=ElmerFudd;Password=wabbitt;
C#:
using System.Data.SqlClient;
objqlConnection oSQLConn = new
SqlConnection();oSQLConn.ConnectionString=connectstring;;oSQLConn.Open();
obj VB.NET:
Imports System.Data.SqlClient
Dim objSQLConn As SqlConnection = New SqlConnection()
objSQLConn.ConnectionString="connectstring"
objSQLConn.Open()
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For Oracle:
Provider=OraOLEDB.Oracle;Data Source=mydatabase;User
Id=ElmerFudd;Password=wabbitt;
Provider=OraOLEDB.Oracle;Data Source= mydatabase;OSAuthent=1;
C#:
using System.Data.OracleClient;
OracleConnection objOracleConn = new OracleConnection();
objOracleConn.ConnectionString = my connectionstring;
objOracleConn.Open();
VB.NET:
Imports System.Data.OracleClient
Dim objOracleConn As OracleConnection = New OracleConnection()
objOracleConn.ConnectionString = myconnectionstring
objOracleConn.Open()
For MS Access:
Provider=Microsoft.Jet.OLEDB.4.0;Data Source=\pathname\biblio.mdb;User
Id=ElmerFudd;Password=wabbitt;
Notice that the last instruction in the code using the method ‘open()’. After the connection
has been made, and the data retrieved, you need to close the connection using the
connection method ‘close()’. This should be done within an ‘if’ statement which first
checks whether the connection is, in fact, open:
If (objConnection.state and ConnectionState.Open) <>0 Then
objConnection.Close
End If
Note that the state property is ‘0’ if the connection is already closed. Testing for a closed
connection is necessary to prevent an error when you are invoking the ‘close’ method.
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The connection objects methods are:
Open Opens connection
Close Closes connection
BeginTransaction Begins database transaction
ChangeDatabase Changes the name of database connected to
CreateCommand Creates a command object
GetOleDbSchemaTable Returns schema tables and associated restricted
columns
ReleaseObjectPool Shared method which allows closing of connection pool
when last connection is closed
Exception Handling
All ADO connection procedures should be protected with a Try/Catch Block. When dealing
with a connection to another server, this is especially important to let your users know
that it was the connection that failed, rather than the application code.
Try
connSQLNorthwind.ConnectionString = _
"Server=Jupiter;Database=pubs;Trusted_Connection=True;Connection Timeout = 10"
Catch ExSQL As System.Data.SqlClient.SqlException
Dim strErrorMsg As String
Dim strerror As System.Data.SqlClient.SqlError
For Each strerror In ExSQL.Errors
Select Case strerror.Number
Case 17
strErrorMsg = "Missing server"
Case 4060
strErrorMsg = "Missing database"
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Case 18456
strErrorMsg = "Missing user name or password"
Case Else
strErrorMsg = strerror.Message
End Select
MessageBox.Show(sErrorMsg, "SQL Server Error: " & strerror.Number,
MessageBoxButtons.OK MessageBoxIcon.Error)
Next
Catch ExcpInvOp As System.InvalidOperationException
MessageBox.Show("Close the connection first!", _
"Invalid Operation MessageBoxButtons.OK, MessageBoxIcon.Error)
Catch Excp As System.Exception ' generic exception handler
MessageBox.Show(Excp.Message, "Unexpected Exception
MessageBoxButtons.OK, MessageBoxIcon.Error)
End Try
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THE ADO.NET DATAREADER
The Data Reader is a simple object – a use one time and throw away container for holding
the results of an ExecuteReade()r method of the command object. The Data Reader’s
principal method is the Read() method, which reads one record from the result set.
Since in a client server environment, it is considered good programming practice to be
frugal in retrieving records, the DataReader at any one time will contain relatively small
amounts of data. For instance, in a typical form procedure, the trip to the server may only
return a single record, or perhaps a single record with associated child records needed to
populate one screen.
When multiple records are retrieved, they are read from the data reader in loop structure
one record at a time.
Do while dr.Read
Process stuff
Loop
Each record is processed and/or stored in turn, because the DataReader has ho no
capacity for returning to any record after the application has passed it. The only way to
revisit a record in the DataReader is to re-connect and request the data again.
In more sophisticated applications, the DataReader may contain multiple records which
may be read and mapped into a DataGrid. The Grid may be equipped with edit, append
and delete buttons to allow changing of the data and eventual uploading of the changed
results back to the server through the command object. It is also possible to use the
GetSchemaTable() method to acquire characteristics of the data columns and built a local
DataTable on the client – either virtually or in an auxiliary Access, or XML table. The
System.Data.DataTable() method is used to built the structure. A key property is the Item
property, which will allow you to get field names like the fields collection in a DAO
recordset.
It is also possible to retrieve multiple resultsets at the same time into a DataReader. This
is frequently the result of a batch SQL Server stored procedure with multiple SELECT
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statements. The NextResult method allows you to advance to the next recordset within
the DataReader.
Do while dr.NextResult
Do while dr.Read
Process stuff
Loop
Loop
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THE ADO.NET DATASET OBJECT PART 1
The DataSet is an in-memory database – a collection of virtual tables with means to
populate, examine, modify and deleting delete the tables and the data within them. You
can also define and manipulate the relationships between the tables and tables, create
and enforce constraints to maintain those relationships. Surprisingly, the data that
populates the tables within a DataSet can and frequently does come from different data
sources, leading to very powerful flexibility in connecting and maintaining data in various
server sources.
The primary purpose of a DataSet is to allow the user to operate on data from a server in
a disconnected mode. The temporary separation of data from the server is a desirable
thing because it frees memory on the server, as well as connection and network
resources. The DataSet allows data to be ‘borrowed’ from the server in relatively small
usable chunks. The DataSet then ‘manages’ the connection between the data and allows
the user to edit, update, delete, append, or select portions of that data.
The DataSet is first and foremost a collection of table objects. These table objects can be
imported from various and diverse data sources such as SQL Server, Oracle, XML files,
and the like, or they can be entered programmatically or through a convenient User
Interface (UI) and then added to the DataSet programmatically. The most useful methods
are to Accept Changes, AcceptChanges, RejectChanges, Merge, Clone, a few functions for
reading and writing XML and XML schema, and – most important of all- the GetChanges
method, which gets a DataSet which contains all of the data changes made to the current
one since it was loaded.
The table objects look like tables in a regular database, though they are virtual tables,
which reside only in memory. The table objects have collections of DataColumns,
DataRows, Constraints, DataRelations, Property Collections, DataRowView, and a
PrimaryKey, which may be a collection of DataColumns.
DataTable Class Members
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Constructors
Visibility Constructor Parameters
public DataTable ( )
public DataTable ( String tableName )
Properties
Visibility Name Value Type Accessibility
public CaseSensitive Boolean [ Get , Set ]
public ChildRelations DataRelationCollection [ Get ]
public Columns DataColumnCollection [ Get ]
public Constraints ConstraintCollection [ Get ]
public Container IContainer [ Get ]
public DefaultView DataView [ Get ]
public DesignMode Boolean [ Get ]
public DisplayExpression String [ Get , Set ]
public ExtendedProperties PropertyCollection [ Get ]
public HasErrors Boolean [ Get ]
public Locale CultureInfo [ Get , Set ]
public MinimumCapacity Int32 [ Get , Set ]
public Namespace String [ Get , Set ]
public ParentRelations DataRelationCollection [ Get ]
public Prefix String [ Get , Set ]
public PrimaryKey DataColumn [ Get , Set ]
public Rows DataRowCollection [ Get ]
public Site ISite [ Get , Set ]
public TableName String [ Get , Set ]
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Methods
Visibility Name Parameters Return Type
public AcceptChanges ( ) Void
public BeginInit ( ) Void
public BeginLoadData ( ) Void
public Clear ( ) Void
public Clone ( ) DataTable
public Compute ( String expression,
String filter )
Object
public Copy ( ) DataTable
public EndInit ( ) Void
public EndLoadData ( ) Void
public GetChanges ( DataRowState
rowStates )
DataTable
public GetChanges ( ) DataTable
public GetErrors ( ) DataRow
public ImportRow ( DataRow row ) Void
public LoadDataRow ( Object values ,
Boolean
fAcceptChanges )
DataRow
public NewRow ( ) DataRow
public RejectChanges ( ) Void
public Reset ( ) Void
public Select ( String
filterExpression,
String sort,
DataViewRowState
recordStates )
DataRow
public Select ( String
filterExpression )
DataRow
public Select ( ) DataRow
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public Select ( String
filterExpression ,
String sort )
DataRow
public ToString ( ) String
Events
Multicast Name Type
multicast ColumnChanged DataColumnChangeEventHandler
multicast ColumnChanging DataColumnChangeEventHandler
multicast Disposed EventHandler
multicast RowChanged DataRowChangeEventHandler
multicast RowChanging DataRowChangeEventHandler
multicast RowDeleted DataRowChangeEventHandler
multicast RowDeleting DataRowChangeEventHandler
Rows are added programmatically using the DataRows object:
Dim dr as DataRow=tblEmployee.NewRow()
dr(“dcFirstName”)=”Elmer”
dr(“dcLastName”)=”Fudd”
tblEmployee.Rows.Add(dr)
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THE ADO.NET DATASET OBJECT PART 2
DataRow Class Members
Properties
Visibility Name Value Type Accessibility
public HasErrors Boolean [ Get ]
public Item ( Int32 columnIndex )
( DataRowVersion version )
Object [ Get ]
public Item ( DataColumn column ) Object [ Get , Set ]
public Item ( DataColumn column ) (
DataRowVersion version )
Object [ Get ]
public Item ( String columnName ) (
DataRowVersion version )
Object [ Get ]
public Item ( Int32 columnIndex ) Object [ Get , Set ]
public Item ( String columnName ) Object [ Get , Set ]
public ItemArray Object [ Get , Set ]
public RowError String [ Get , Set ]
public RowState DataRowState [ Get ]
public Table DataTable [ Get ]
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Methods
Visibility Name Parameters Return Type
public AcceptChanges ( ) Void
public BeginEdit ( ) Void
public CancelEdit ( ) Void
public ClearErrors ( ) Void
public Delete ( ) Void
public EndEdit ( ) Void
public GetChildRows ( String
relationName )
( String
relationName )
public GetChildRows ( String
relationName ,
DataRowVersion
version )
DataRow
DataRow GetChildRows ( DataRelation
relation ,
DataRowVersion
version )
DataRow
public GetChildRows ( DataRelation
relation )
DataRow
public GetColumnError ( Int32 columnIndex
)
String
public GetColumnError ( String
columnName )
String
public GetColumnError ( DataColumn
column )
String
public GetColumnsInError ( ) DataColumn
public GetParentRow ( String
relationName ,
DataRowVersion
version )
DataRow
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public GetParentRow ( String
relationName )
DataRow
public GetParentRow ( DataRelation
relation ,
DataRowVersion
version )
DataRow
public GetParentRow ( DataRelation
relation )
DataRow
public GetParentRows ( DataRelation
relation ,
DataRowVersion
version )
DataRow
public GetParentRows ( DataRelation
relation )
DataRow
public GetParentRows ( String
relationName ,
DataRowVersion
version )
DataRow
public GetParentRows ( String
relationName )
DataRow
public HasVersion ( DataRowVersion
version )
Boolean
public IsNull ( String
columnName )
Boolean
public IsNull ( DataColumn
column ,
DataRowVersion
version )
Boolean
public IsNull ( DataColumn
column )
Boolean
public IsNull ( Int32 columnIndex
)
Boolean
public RejectChanges ( ) Void
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public SetColumnError ( Int32 columnIndex
, String error )
Void
public SetColumnError ( DataColumn
column , String error
)
Void
public SetColumnError ( String
columnName , String
error )
Void
public SetParentRow ( DataRow
parentRow ,
DataRelation relation
)
Void
public SetParentRow ( DataRow
parentRow )
The DataSet is a much more powerful and complicated object than the DataReader. The
DataReader is a read-only forward-only object wherein once a record is read it is gone,
unless it is saved into an array. The DataSet and DataTable objects are virtualls datasets
and tables which are used in client-side operations. Though there is no ‘current record’
concept, the user is able to navigate between the rows using loops, either using the Rows
collection of the DataTable object, or the DataRow object.
For i=1 to 3
tblEmployee.Rows(i)(“FirstName”)=”Daffy”
or
dim dr as DataRow=tblEmployee.Rows(i)
dr(“LastName”)=”Duck”
Next
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Don’t forget that the DataSet is disconnected from the original server-side source of the
data. So any changes in the DataSet will not be reflected in the source unless and until
the DataSet and the Source are reconciled. You can find the status of any record by
querying the RowState property of the DataRow object. The result will be one of the
following: Detached, Added, Modified, Deleted, or Unchanged. Though the RowState
property is read only, the DataRow has two properties which can change it: the
AcceptChanges, and RejectChanges Properties.
Of further note are the DataTable.ColumnChanging, the DataTable.ColumnChanged, the
DataTable.RowChanging, and the DataTable.RowChanged events which allow you to trap
and code for these events within your code.
The DataView Object
The function of the DataView object is to facilitate data binding of data to Windows forms
and web pages. Additionally, it can be used along with the Select method od the
DataTable Object to present subsets of records from tables. Use the RowFilter property to
select which records are visible, and use the Sort property to order their presentation.
DataView Class Members
Constructors
Visibility Constructor Parameters
public DataView ( )
public DataView ( DataTable table )
public DataView ( DataTable table ,
String RowFilter ,
String Sort ,
DataViewRowState
RowState )
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Properties
Visibility Name Value Type Accessibility
public AllowDelete Boolean [ Get , Set ]
public AllowNew Boolean [ Get , Set ]
public ApplyDefaultSort Boolean [ Get , Set ]
public Container IContainer [ Get ]
public Count Int32 [ Get ]
public DataViewManager DataViewManager [ Get ]
public DesignMode Boolean [ Get ]
public Item ( Int32
recordIndex )
DataRowView [ Get ]
public RowFilter String [ Get , Set ]
public RowStateFilter DataViewRowState [ Get , Set ]
public Site ISite [ Get , Set ]
public Sort String [ Get , Set ]
public Table DataTable [ Get , Set ]
Methods
Visibility Name Parameters Return Type
public AddNew ( ) DataRowView
public BeginInit ( ) Void
public CopyTo ( Array array ,
Int32 index )
Void
public Delete ( Int32 index ) Void
public EndInit ( ) Void
public Find ( Object key ) Int32
public Find ( Object key ) Int32
public FindRows ( Object key ) DataRowView
public FindRows ( Object key ) DataRowView
public GetEnumerator ( ) IEnumerator
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Events
Multicast Name Type
multicast Disposed EventHandler
multicast ListChanged ListChangedEventHandler