Home > Articles > Programming > Windows Programming

This chapter is from the book

This chapter is from the book

Creating Multitiered Web Applications

You can use components to divide your Web application into multiple layers, thereby creating a multitiered application. For example, in a two-tiered application, you might have a user interface layer built from ASP.NET pages and a data layer built from components. In a three-tiered application, you might introduce a third layer, representing your application's business logic, between the user interface and data layers.

Why would you want to create multiple layers? The advantage of dividing an application into multiple layers is that you can more easily manage and change the application.

In the preceding section, for example, you created a component that enables you to save form information to a file. In essence, you created a single component for the data layer. Now suppose that your needs change, and you decide to save the data to a database table instead of a file. In that case, you need to modify only the component. You don't have to touch a line of code in the ASP.NET page itself. In other words, you can isolate changes in the data layer from changes in the user interface layer.

An additional benefit of dividing an application into multiple layers is that doing so makes it easier for multiple teams of programmers to work on the same application at the same time. If all the application logic is contained in a single page, you face the problem of multiple programmers stumbling over each other while performing modifications.

To illustrate these points, you can construct a simple three-tiered Web application consisting of user interface, business, and data layers to create a simple order entry application.

The application consists of the following three objects:

  • OrderForm.aspx—This ASP.NET page represents the user interface layer.

  • BizObject—This component represents the business layer.

  • DataObject—This component represents the data layer.

First, build the user interface layer. The ASP.NET page for the user interface is contained in Listing 8. The page requests five pieces of information: the customer name, product, unit price of the product, quantity, and customer's state of residence (see Figure 3).

Figure 3 A simple order form.

Listing 8—OrderForm.aspx

<%@ Import Namespace="myComponents" %>

<Script Runat="Server">

Sub Button_Click( s As Object, e As EventArgs )
 Dim myBizObject As New BizObject

 If isValid Then
  myBizObject.CheckOrder( _
   txtCustomer.Text, _
   dropProduct.SelectedItem.Text, _
   txtUnitPrice.Text, _
   txtQuantity.Text, _
   dropState.SelectedItem.Text )
 Catch expException As Exception
  lblError.Text = expException.Message
 End Try
 End If
End Sub



<form Runat="Server">

<h2>Enter an Order:</h2>

 Runat="Server" />

Customer Name:
 Runat="Server" />

 Text="You must enter a customer name!"
 Runat="Server" />

 <asp:ListItem Text="Hair Dryer" />
 <asp:ListItem Text="Shaving Cream" />
 <asp:ListItem Text="Electric Comb" />

 Text="You must select a product!"
 Runat="Server" />

Unit Price:
 Runat="Server" />

 Text="You must enter a unit price!"
 Runat="Server" />

 Runat="Server" />

 Text="You must enter a quantity!"
 Runat="Server" />

 Text="Quantity must be a number!"
 Runat="Server" />

Customer State:
 <asp:ListItem Text="California" />
 <asp:ListItem Text="Nevada" />
 <asp:ListItem Text="Washington" />

 Text="Place Order"
 Runat="Server" />



When all the information is entered into the form, and the Place Order button is clicked, the Button_Click subroutine executes. This subroutine creates an instance of the BizObject component and passes all the form information to the component's CheckOrder() method.

The business component is contained in Listing 9.

Listing 9—BizObject.vb

Imports System

Namespace myComponents

Public Class BizObject

 Sub CheckOrder( _
  Customer As String, _
  Product As String, _
  UnitPrice As Double, _
  Quantity As Integer, _
  State As String )

  If Quantity <= 0 Or quantity > 100 Then
   Throw New ArgumentException( "Invalid Quantity!" )
  End If

  If State = "California" And Product="Hair Dryer" Then
   Throw New ArgumentException( "Californians cannot own Hair Dryers!" )
  End IF

  If State = "Washington" Then
   unitPrice += unitPrice * .06
  End If

  Dim myDataObject As New DataObject
  myDataObject.SaveOrder( customer, product, unitPrice, quantity, state )

 End Sub

End Class

End Namespace

The business component contains all the business rules for the application. The component encapsulates the following rules:

  1. The product quantity must be greater than 0 and less than 100.

  2. People who live in California cannot buy hair dryers.

  3. People who live in Washington must pay an additional 6% sales tax.

If an order fails either of the first two requirements, an exception is raised. The error is caught in the OrderEntry.aspx page and displayed in a Label control. For example, Figure 4 displays what happens when you try to order more than 100 products.

Figure 4 Raising an exception in the order form.

If an order satisfies all the business rules, it is passed to the SaveOrder() method of the DataObject component, which saves the order to disk. This component is contained in Listing 10.

Listing 10—DataObject.vb

Imports System
Imports System.IO

Namespace myComponents

Public Class DataObject

 Sub SaveOrder( _
  Customer As String, _
  Product As String, _
  UnitPrice As Double, _
  Quantity As Integer, _
  State As String )

  Dim strPath As String = "c:\orders.txt"
  Dim strmFile As StreamWriter

  strmFile = File.AppendText( strPath )
  strmFile.Write( "customer: " & customer & Environment.NewLine )
  strmFile.Write( "product: " & product & Environment.NewLine )
  strmFile.Write( "unit price: " & unitPrice.ToString() & Environment.NewLine )
  strmFile.Write( "quantity: " & quantity.ToString() & Environment.NewLine )
  strmFile.Write( "state: " & state & Environment.NewLine )
  strmFile.Write( "=============" & Environment.NewLine )
 End Sub

End Class

End Namespace

Remember that you must compile both the BizObject and DataObject components and copy them to your application's /BIN directory before you can use them. The order of compilation is important. Because the BizObject component refers to the DataObject component, you must create the DataObject component first. You can compile this component by using the following statement (executed from a DOS prompt):

vbc /t:library DataObject.vb

After you copy the DataObject.dll file to your application's /BIN directory, you can compile the BizObject component by using the following statement:

vbc /t:library /r:DataObject.dll bizObject.vb

Notice the /r option used when compiling the BizObject component; it references the DataObject.dll file. If you don't include this reference, you receive the error User-defined type not defined: DataObject.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020