Build Your Own ASP.NET 2.0 Web Site Using C# & VB, Second Edition

locally, and you need to be aware of all the possibilities in order to be able to

make an informed decision about which mode to use.

Working with Data Sets and Data Tables

We’ve been working with databases for a while now. You first learned about the

SqlDataReader, and used it to populate your controls. Then, in the first half of

this chapter, we gained first-hand experience with the data source controls, which

can automate many features for you. Let’s learn about one more technique you

can use to get data to your visitors.

I know that all these options can be confusing at first—you need to play around

with all of them before you can become an experienced ASP.NET developer!

The DataSet object is at the center of Microsoft’s model for presenting discon-

nected data. Disconnected data (data that resides in memory and is completely

independent of the data source) gives us a raft of new opportunities of developing

desktop and web apps.

In Chapter 9, you learned about the role that data readers play in relation to

applications and database data. Whenever we need to access data from a database,

we create a connection object and a command object. These two objects are used

together to create a data reader object, which we can use within our application

by binding it to a data control for presentation purposes. Figure 12.18 illustrates

this process.

So, what’s the problem? Well, while being the fastest way to retrieve data from

a database, data readers can’t be used to carry out any significant work—you

can’t use them to sort, filter, or page through the data. As the arrows in Fig-

ure 12.18 show, data readers present a forward-only stream of data to the applic-

ation: you can’t go back to a previous record, or reuse that data reader somewhere

else. Moreover, once a data reader has been created, it remains tied to the data-

base. This means that you can’t make multiple requests to a database using the

same data reader.

Data sets, on the other hand, are much more flexible. Imagine a virtual database

that you’re free to use in code whenever and however you wish. That’s a data

set. As we’ll see in the next section, data sets have all the bells and whistles that

databases offer, including tables, columns, rows, relationships, and even queries!

A data set is a memory-resident copy of database data, so, once a data set has

been created, it can be stored in memory and its ties to the database can be

broken. Then, when you need to work with the data, you don’t need to re-query

494

Chapter 12: Advanced Data Access

Figure 12.18. Retrieving data using a data reader

Figure 12.19. Breaking the data set’s ties to the data source once

it has been created

the database—you simply retrieve the data from the data set again and again.

Figure 12.19 illustrates this point.

495

Working with Data Sets and Data Tables

Figure 12.20. Multiple pages making multiple requests from the

same data set

An even greater advantage is that data sets can be shared among multiple requests,

as illustrated in Figure 12.20.

What this means is that you simply need to create the data set once per request.

Once the data set has been created, it can be used by many different pages, all

of which may make multiple—and even different—requests to that data set.

However, data sets require much more memory and resources than do data

readers. A data reader simply keeps track of its position within the results of a

query, whereas a data set can contain a local copy of an entire database. The

larger the amount of data kept in the data set, the more memory the data set

uses.

When deciding whether to use data readers or data sets, you need to consider

the purpose for which you need the data. This decision is important, because it

affects:

❑

resources consumed on the database server

❑

resources consumed on the application server

❑

the overall application architecture

If you’re only reading the data, using a data reader can make sense. If you also

need to update, insert, or delete data, or you need to process the data within

your code, data sets might be of more help.

This section will teach you everything you need to know to begin working with

data sets.

496

Chapter 12: Advanced Data Access