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CHAPTER 11 PROGRAMMING ADO.NET
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Finally, the code assigns the connection string to the cnn1 SqlConnection object. Dim cnn1 As New SqlConnection Dim cmd1 As New SqlCommand 'Compute top-level project folder and use it as a prefix for 'the primary data file in the connection string for cnn1 Dim pdbfph As String = _ My.Computer.FileSystem.CombinePath( _ Server.MapPath("/WebCh11/App_Data"), "Northwnd.mdf") Dim cst As String = "Data Source=.\sqlexpress;" & _ "Integrated Security=SSPI;" & _ "AttachDBFileName=" & pdbfph cnn1.ConnectionString = cst
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Instead of returning a single value from a SQL string wrapped in an ADO.NET command object, it is more common to want to retrieve one or more rows of data. If there is no need to update the retrieved data, the fastest way to accomplish the job in ADO.NET is with a DataReader object. Because DataReader objects are provider specific like the Connection and Command ADO.NET objects, you must use a different DataReader object type for each ADO.NET provider. For example, you can use the SqlDataReader object for SQL Server databases and the OleDbDataReader object for databases connected via the .NET OLE DB provider. DataReader objects do not have a constructor that you invoke with the New keyword. Instead, you invoke the ExecuteReader method for an ADO.NET Command object to instantiate a new DataReader. Before you can invoke the ExecuteReader method for an ADO.NET Command object, the Command object must have an open connection. You cannot typically use a Connection object for any other purpose while a DataReader is using it.
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A new, advanced Connection object feature in ADO.NET 2.0 allows for the use of multiple active result sets (MARS) with a single Connection object. By using MARS, you can have more than one DataReader object use the same Connection in an interleaved fashion. If you require true parallel processing of multiple DataReader objects, Microsoft recommends using a separate Connection object for each DataReader. By using a separate Connection object for each DataReader, you can achieve faster performance.
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The ExecuteReader method is the same across all providers, but the Command and DataReader types are unique for each provider. While the general process for creating a DataReader object is the same for all databases, you must use the proper ADO.NET provider for the database that you are using. The following two cases describe the use of the ExecuteReader method for SQL Server Express and Access databases: For a SQL Server Express database, create a SqlDataReader object by invoking the ExecuteReader method for a SqlCommand object. For an Access database, create an OleDbDataReader object by invoking the ExecuteReader method for an OleDbCommand object. After you create a DataReader object, you can pass through successive rows in a forward-only manner by repeatedly invoking the Read method for a DataReader object. If the Command object for a DataReader wraps a single SELECT statement, your DataReader object returns a single result set. In
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CHAPTER 11 PROGRAMMING ADO.NET
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this case, the DataReader points at the first row of its result set after you initially invoke the Read method. For all rows until the last one, each successive invocation of the Read method returns a value of True and moves to the next row in the result set. Invoking the Read method from the last row of a result set returns a value of False. When you are finished reading data with a DataReader object, invoke the Close method. You can also invoke the Close method for the Connection object associated with a DataReader to release the connection resources associated with the DataReader object. You can retrieve column values for the current row of a DataReader object with a variety of GetXXX methods, such as GetInt32 or GetSqlInt32. Using the GetInt32 method returns a column value as a 32-bit signed .NET Framework Integer data type. Using the GetSqlInt32 method returns a column value as a 32-bit signed Integer value from within SQL Server 2005. Other methods for returning column values in popular SQL Server database formats include GetSqlMoney, GetSqlDouble, and GetSqlDateTime. For columns with a varchar or nvarchar data type, you can retrieve values with the GetSqlString method. SQL Server data types do not necessarily correspond to .NET Framework data types without conversion. For example, to process a value retrieved with the GetSqlString method, you must convert the value to a .NET Framework String value with the ToString method. You can denote columns by a zero-based ordinal index value for the GetXXX methods, such as GetSqlInt32. If rdr1 represents a DataReader object, then rdr1.GetSqlString(0).ToString can retrieve an nvarchar column value into ADO.NET as a SqlString value from the first column of the current row, and convert the value to a .NET Framework String value. Invoke the GetDataTypeName method with an ordinal column index value to retrieve the database data type name for values from a column. Invoke the GetOrdinal method to return the ordinal index value of a column that corresponds to the name you supply.
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Use the GetOrdinal method to return the ordinal index value nested within a GetXXX method that returns Tip a column value so that your code can represent a column value by its column name instead of its column index value. For example, use rdr1.GetSqlString(rdr1.GetOrdinal("column_name")).ToString to return the .NET Framework converted String value for a column named column_name from the current row of the rdr1 DataReader object.
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You can use a single DataReader object to successively return multiple result sets. Use two nested loops; in the inner loop, read the rows of the current result set, and in the outer loop, navigate from one result set to the next. Here s the process: Start by specifying a SQL string with multiple query statements for the CommandText property of a Command object; delimit each successive query statement after the first one by a semicolon. Next, invoke the ExecuteReader method to create a DataReader object for the Command object. This locates the DataReader just before the first result set. Then, use the Read method to iteratively pass through successive rows of the result set until there are no more rows in the current result set. After reading the last row in the current result set, invoke the NextResult method. This advances the DataReader to the next result set (if there is one). Return to reading the rows of the current result set in the inner loop. When there are no remaining result sets, exit the outer loop instead of returning to the inner loop.
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