An introduction to ADO.NET concluded
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This article, the second of two parts, explains how ADO.NET uses classes from the .NET Framework to provide access to the data in a database. It is excerpted from chapter two of the book
VB.NET Database Programming with ADO.NET, written by Anne Prince and Doug Lowe (Murach Publishing; ISBN: 1890774197).
The SqlDataReader class
A data reader provides an efficient way of reading the rows in a result set returned by a database query. In fact, when you use a data adapter to retrieve data, the data adapter uses a data reader to read through the rows in the result set and store them in a dataset.
A data reader is similar to other types of readers you may have encountered in the .NET Framework, such as a TextReader, a StreamReader, or an XMLReader. Like these other readers, a data reader lets you read rows but not modify them. In other words, a data reader is read-only. In addition, it only lets you read rows in a forward direction. Once you read the next row, the previous row is unavailable.
Figure 2-5 lists the most important properties and methods of the SqlDataReader class. You use the Read method to read the next row of data in the result set. In most cases, you’ll code the Read method in a loop that reads and processes rows until the end of the data reader is reached.
To access a column of data from the current row of a data reader, you use the Item property. To identify the column, you can use either its index value like this:
drVendors.Item(0)
or its name like this:
drVendors.Item("VendorName")
Since Item is the default property, you can also omit it like this:
drVendors("VendorName")
The SqlDataAdapter class
As you know, the job of a data adapter is to provide a link between a database and a dataset. The four properties of the SqlDataAdapter class listed in figure 2-5 identify the four SQL commands that the data adapter uses to transfer data from the database to the dataset and vice versa. The SelectCommand property identifies the command object that’s used to retrieve data from the database. And the DeleteCommand, InsertCommand, and UpdateCommand properties identify the commands that are used to update the database based on changes made to the data in the dataset.
To execute the command identified by the SelectCommand property and place the data that’s retrieved in a dataset, you use the Fill method. Then, the application can work with the data in the dataset without affecting the data in the database. If the application makes changes to the data in the dataset, it can use the data adapter’s Update method to execute the commands identified by the DeleteCommand, InsertCommand, and UpdateCommand properties and post the changes back to the database.
Figure 2-5. The SqlDataReader and SqlDataAdapter
classes
Common properties and methods of the SqlDataReader class
Common properties and methods of the SqlDataAdapter class
Description
- A data reader provides read-only, forward-only access to the data in a database. Because it doesn’t require the overhead of a dataset, it’s more efficient than using a data adapter. However, it can’t be used to update data.
- When the Fill method of a data adapter is used to retrieve data from a database, the data adapter uses a data reader to load the results into a dataset.
Next: ADO.NET datasets >>
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 This article is excerpted from chapter two of the book VB.NET Database: Database Programming with ADO.NET, written by Anne Prince and Doug Lowe (Murach Publishing; ISBN: 1890774197). Check it out today at your favorite bookstore. Buy this book now.
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