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Separating User Interface from Business Logic

In the ASP.NET pages created so far, you likely noted that the C# code is interspersed between the HTML code. Mixing both HTML and ASP.NET code makes the Web pages difficult to understand, maintain, and debug. HTML code represents the user interface (UI) of the application, whereas the ASP.NET code defines the business logic for that page. These are two diverse ideas that usually need to be handled differently.

ASP.NET provides a mechanism to separate the UI portion of a Web page from the business logic. This mechanism is known as code-behind . To implement code-behind, you write all the UI- related code in one file ( .aspx ) and the business logic in another file ( .cs for C#) and then link these two files using the Page directive in the ASPX page. The linking is provided in the ASPX page instead of the CS file because the user requests the ASPX page. When ASP.NET compiles the ASPX page, it examines the Page directive to locate the business logic associated with the UI in the ASPX page. You can use the Page directive in different ways to link the code to the UI.

Using Code-behind Without Precompilation

When you write both the UI and business logic in separate files, one way to link them is via the Src and Inherits attributes of the Page directive. The following example uses the code of the Example2_1.aspx file and converts it into the code-behind version. The UI is stored in Example2_6.aspx , and the code is stored in TemperatureCB.cs . Do the following:

Add a new text file to Web project 315C02 , and name it Example2_6.aspx . Switch to its HTML view and add the following code to the Web page:

<!--Example2_6.aspx -->
<%@ Page Language="C#"
     Inherits="Temperature.TemperatureCB"
     src="TemperatureCB.cs"%>
<html>
<body>
    <h2>Fahrenheit to Celsius Conversion Chart</h2>
    <table border="2">
        <tr>
            <th>&deg; Fahrenheit</th>
            <th>&deg; Celsius</th>
        </tr>
        <%CreateTemperatureTable();%>
    </table>
</body>
</html>

Right-click the project in the Solution Explorer, and then select Add, Add New Item. This opens the Add New Item dialog box. Select the Code File template and name the file TemperatureCB.cs .

Add the following code to the TemperatureCB.cs file:

//TemperatureCB.cs
namespace Temperature
{
    // Define TemperatureCB class
    public class TemperatureCB : System.Web.UI.Page
    {
        double ToCelsius(double f)
        {
            // Calculate the temperature in Celsius
            return  (5.0/9.0)*(f-32.0);
        }

        public void CreateTemperatureTable()
        {
            for (double f = 50.0; f<=100.0; f++)
                Response.Output.Write("<tr><td>{0}</td><td>" +
                  "{1:f}</td></tr>", f, ToCelsius(f));
        }
    }
}

Access the Properties Window for the TemperatureCB.cs file. Set the Build Action property to None ; this restricts Visual Studio .NET from automatically compiling this file.
Select Debug, Run to view the Example2_6.aspx file in your browser.

The Page directive in the Example2_6.aspx file provides the linking mechanism for the UI and code-behind. The Inherits attribute specifies that when the ASPX page is dynamically compiled, it inherits its functionality (such as methods and properties) from the TemperatureCB class of the Temperature namespace. In addition, the Src attribute specifies that the source code for the TemperatureCB class is stored in the TemperatureCB.cs file.

Using Code-behind with Visual Studio .NET Web Forms

Web forms are the preferred way of creating ASP.NET pages using Visual Studio .NET. They provide three distinct benefits over simple ASP.NET pages:

Support code-behind by separating UI from business logic
Provide an event-based programming model for designing Web pages
Support RAD tools, such as Visual Studio .NET

The following example demonstrates how to create a Web form using Visual Studio .NET for displaying a temperature-conversion chart:

Open the Solution Explorer window for project 315C02 ; right-click the project name and select Add, Add Web Form. In the Add New Item dialog box, name the file Example2_7.aspx .
The Web form is in Design view. In the background of the form, you see a message asking you to select between grid layout mode and flow layout mode. Open the Properties window by selecting View, Property Pages. Then, change the Page Layout property of Document to FlowLayout.
In the flow layout mode, type Fahrenheit to Celsius Conversion Chart . Then, select the text and change its block format to Heading 2 using the Block Format combo box on the formatting toolbar.
Select Table, Insert, Table. In the Insert Table dialog box, select options to make a table of size 1 row and 2 columns . Set the Width attribute to 200 pixels.

Switch to the HTML view of the Example2_7.aspx file, and modify the code in the <TABLE> tag as shown here:

<TABLE id="Table1" cellSpacing="1" cellPadding="1"
        width="200" border="1">
    <TR>
        <TD>&deg; Fahrenheit</TD>
        <TD>&deg; Celsius</TD>
    </TR>
    <%CreateTemperatureTable();%>
</TABLE>

Invoke the Solution Explorer, right-click Example2_7.aspx , and select View Code. This opens the code-behind file Example2_7.aspx.cs . Add the following code just above the Web Form Designergenerated code region:

double ToCelsius(double f)
{
    // Calculate and return the temperature in Celsius
    return  (5.0/9.0)*(f-32.0);
}

public void CreateTemperatureTable()
{
    for (double f = 50.0; f<=100.0; f++)
        Response.Output.Write("<tr><td>{0}</td><td>" +
            "{1:f}</td></tr>", f, ToCelsius(f));
}

In the Solution Explorer, right-click Example2_7.aspx and select Set As Start Page.
Compile the page by selecting Build, Build 315C02; then run the page by selecting Debug, Start Without Debugging. You should see that the temperature table is created just as in the case of earlier examples.

An ASP.NET Web form is made up of two distinct pieces:

The UI piece stored in the ASPX page
The business logic piece stored in a CS file

Analyzing the User Interface Piece of a Web Form

In the Example2_7.aspx file, the Page directive is written as follows :

<%@ Page language="c#"
         Codebehind="Example2_7.aspx.cs"
         AutoEventWireup="false"
         Inherits="_315C02.Example2_7" %>

This usage of the Page directive has two new attributes AutoEventWireup and Codebehind . AutoEventWireup is used for event handling. Visual Studio .NET, by default, sets the AutoEventWireup attribute to false . The Codebehind attribute, on the other hand, is used by Visual Studio .NET to track the location of the source code of the code-behind file. ASP.NET does not understand the Codebehind attribute and therefore ignores it. This attribute does not play any role in the execution of the ASP.NET page.

In the Page directive, when the name of the source code file is specified using the Src attribute, the source code file is dynamically compiled at runtime. However, the same does not hold true for the Codebehind attribute, which is necessary only for design-time support in Visual Studio .NET. The Codebehind attribute plays no role at runtime because it is not recognized by ASP.NET.

If you are handling events in your program by attaching delegates, set the AutoEventWireup attribute of the Page directive to false . If you keep it true , as the page runs, some of the event handlers might be called twice.

To understand why, assume that you want to handle the Load event for the Page object, so you create a method named Page_Load() and attach it to the Load event of the Page using a delegate object. Now, because the AutoEventWireup attribute is true , based on the name of the method, the ASP.NET page framework will automatically attach Page_Load() as an event handler for the Load event of the Page . This results in the registration of the same event handler twicehence, the event handler will also be invoked twice when the event is raised.

It is for this reason that Visual Studio .NET by default always sets AutoEventWireup to false .

Analyzing the Business Logic Piece of a Web Form

When you analyze the file storing the business logic piece, Example2_7.aspx.cs .

The class definition for the Example2_7 class looks like this:

public class Example2_7 : System.Web.UI.Page
{
   ...
}

In the Example2_7.aspx.cs file, you can also write this definition as

public class Example2_7 : Page
{
   ...
}

How does the compiler locate a class if its full namespace-qualified name is not provided?

Inclusion of these using directives directs the C# compiler to search for each class used in the code in the namespaces specified by the using directives. The compiler looks up each namespace one by one, and when it finds the given class in one of the namespaces, it internally replaces the reference of the class with NamespaceName.ClassName in the code.

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Separating User Interface from Business Logic