Controller Patterns for ASP.NET

This is the second in a series of three articles that looks at the built-in support for standard design patterns in ASP.NET, and how you can implement common patterns in your own applications.

In our previous article, we saw ASP.NET automatically implements the Model-View-Presenter pattern when you use the built-in "code-behind" approach. However, there are other patterns usually referred to as "controller patterns" that are suitable for use in ASP.NET, and this is the subject of this article. These patterns extend the capability for displaying a single view by allowing the application to choose which view to display at runtime depending (usually) on user interaction.

The Use Case Controller Pattern

The Use Case Controller pattern coordinates and sequences interaction between the system and its users in order to carry out a specific process. An example is the "Wizard" style of interface, where the user steps through a sequence of screens in a defined order. They may be able to go backwards as well as forwards, or jump to a specific step, but the overall approach suggests a defined "forward movement" through the process (see Figure 1).

The Use Case Controller Pattern
Figure 1 - The Use Case Controller Pattern

A single Controller interacts with the Model (the data) and displays the appropriate View, depending on the user's interaction or the application's requirements. It is possible to incorporate multiple Controllers in a hierarchy for complex processes, and use configuration files or other persisted data to define the steps to allow easy maintenance and extension to the overall process. However, the underlying principles are as shown in Figure 1.

The Page Controller and Front Controller Patterns

Two other design patterns related to Use Case Controller are the Page Controller and Front Controller patterns. These provide for implementation and extension of the principles of the Use Case Controller pattern to suit ASP.NET. They allow an application to either select the content to display in a page using different partial Views, or select which View (page) to display (see Figure 2).

The Page Controller and Front Controller Patterns
Figure 2 - The Page Controller and Front Controller Patterns

In the Page Controller pattern, the controller uses a single Presenter (part of the MVP pattern implemented by the ASP.NET code-behind technology), which interacts with the Model (the data for the page). When it receives a request, the Page Controller can determine which partial View to display within the page, and then interact with that View following the MVP pattern.

In the Front Controller pattern, a separate controller examines each request and determines which page to display. Each page is a complete MVP implementation, with its own View, and each Presenter interacts with the View and the Model (the data).

The Plug-in, Module, and Intercepting Filter Patterns

A series of related design patterns define how applications can use additional components or modules to extend their capabilities or perform specific functions. Typically, they allow the features or behavior of an application to change when it loads separate components that implement extra functionality

The Plug-in pattern usually relates to general-purpose software components such as ActiveX controls or Java applets. An example is the way that Web browsers can use ActiveX controls, Java applets, and the Macromedia plug-in to provide extra functionality or display Flash animations.

The Module pattern usually relates to the capability of an application to load and use custom assemblies (modules) that extend its functionality. An example is the Composite UI Application Block (CAB), which contains a feature that allows an application to load a complete interface component (such as a window with its associated Controller or Presenter and data), and integrate it seamlessly into a composite Windows Forms interface.

The Intercepting Filter pattern is slightly different in implementation in that it usually consists of a component that resides within an application pipeline, such as the HTTP pipeline that ASP.NET uses to process each request. The framework calls a method within the HTTP module at a specific point during pipeline processing, allowing the module to change the behavior of the application.

Implementing the Controller Patterns in ASP.NET

You can implement both the Page Controller and the Front Controller patterns in ASP.NET. In fact, ASP.NET makes it easy to combine them if required, as shown in Figure 3.

Combining Front Controller and Page Controller
Figure 3 - Combining Front Controller and Page Controller

In this schematic, the Front Controller specifies which of three Web pages will load depending on some feature of the request. Each Web page implements the MVP pattern, though one of them also uses the Page Controller pattern to determine which partial View to display within its page. This is, in fact, the overall structure of the sample application that you can download from here to experiment with on your own server.

All three of the patterns you will see implemented here, the Use Case Controller, Page Controller, and Front Controller, make use of the same set of three partial Views implemented as User Controls. Therefore, before looking at the pattern implementations in detail, the next section examines these three User Controls.

Implementation of the Three Partial View User Controls

In order to illustrate the various controller patterns, the example application uses three very simple User Controls that implement Views for several of the pages. Figure 4 shows the page from the Use Case Controller pattern example that uses the "CustomerList" User Control (CustomerList.ascx). Note that the page heading, buttons, and actual URL are part of the hosting page, and not part of the User Control.

The Customer List User Control displayed in the example application
Figure 4 - The Customer List User Control displayed in the example application

The View itself (the ASPX page) contains just a GridView control and a Label control (where any errors messages will appear). The code to populate the GridView is in the Page_Load event handler. It uses the CustomerModel class to get a DataSet containing the list of customers, and binds this DataSet to the GridView to show the results:

public partial class CustomerList : System.Web.UI.UserControl
{
  protected void Page_Load(object sender, EventArgs e)
  {
    try
    {
      // get DataSet of customers from CustomerModel class
      // and bind to GridView control in the page
      CustomerModel customerList = new CustomerModel();
      GridView1.DataSource = customerList.GetCustomerList();
      GridView1.DataBind();
    }
    catch (Exception ex)
    {
      Label1.Text += "USER CONTROL ERROR: " + ex.Message;
    }
    Label1.Text += "<p />";
  }
}

The second User Control, named "CustomerDetails" (CustomerDetails.ascx), displays details of a specific customer (see Figure 5). To simplify the example, this is hard-coded to show the details of the customer with ID value "ALFKI", but you could - of course - modify the page to allow users to enter a customer ID in the same way as in the Default.aspx page you saw earlier.

The Customer Details User Control displayed in the example application
Figure 5 - The Customer Details User Control displayed in the example application

The code in the Page_Load event handler of this User Control uses the GetCustomerDetails method of the CustomerModel class to get a DataRow containing details of the specified customer, converts this to an Object array, and then iterates through the array displaying the values:

public partial class CustomerDetails : System.Web.UI.UserControl
{
  protected void Page_Load(object sender, EventArgs e)
  {
    try
    {
      // use CustomerModel to get DataRow for specified customer  
      CustomerModel customers = new CustomerModel();
      DataRow[] details = customers.GetCustomerDetails("ALFKI");

      // convert row values into an array
      Object[] values = details[0].ItemArray;
      Label1.Text += "Customer Details from CustomerModel class: <br />";

      // iterate through the array displaying the values
      foreach (Object item in values)
      {
        Label1.Text += item.ToString() + ", ";
      }
    }
    catch (Exception ex)
    {
      Label1.Text += "USER CONTROL ERROR: " + ex.Message;
    }
    Label1.Text += "<p />";
  }
}

The third User Control, named "CityList" (CityList.ascx), displays a list of the cities where customers reside, together with a count of the number of customers in each city (see Figure 6).

The City List User Control displayed in the example application
Figure 6 - The City List User Control displayed in the example application

The code in the Page_Load event handler of the "CityList" User Control uses the GetCityList method of the CustomerModel class to get a SortedList instance (a class in the System.Collections namespace) containing the list of cities and the count of customers in each one. It can then iterate through the list displaying the key (the city name) and the value (the number of customers) for each item:

public partial class CityList : System.Web.UI.UserControl
{
  protected void Page_Load(object sender, EventArgs e)
  {
    try
    {
      // use CustomerModel to get SortedList of cities
      CustomerModel customers = new CustomerModel();
      SortedList cities = customers.GetCityList();
      // iterate through the SortedList displaying the values
      Label1.Text += "List of cities from CustomerModel class: <br />";
      foreach (String key in cities.Keys)
      {
        Label1.Text += key + " (" + cities[key].ToString() + ")<br />";
      }
    }
    catch (Exception ex)
    {
      Label1.Text += "USER CONTROL ERROR: " + ex.Message;
    }
    Label1.Text += "<p />";
  }
}

Implementation of the Use Case Controller Pattern

The example implementation of the Use Case Controller pattern uses a single ASPX page named TransferPage1.aspx, with a code-behind file that implements the Presenter. If this page load is a postback (caused by the user clicking one of the buttons in the page), code in the Page_Load event of the Presenter extracts the name of the partial View (User Control) to display from the viewstate that ASP.NET maintains in every page, and saves this in the local variable named viewName. If this page load is not a postback, the code just sets viewName to the default value "CustomerList" and calls the method LoadAndDisplayView within the Presenter:

public partial class TransferPage1 : System.Web.UI.Page
{

  String viewName = String.Empty;

  protected void Page_Load(object sender, EventArgs e)
  {
    if (Page.IsPostBack)
    {
      // get current view name from page viewstate
      viewName = (String)ViewState["ViewName"];
    }
    else
    {
      viewName = "CustomerList";
      LoadAndDisplayView();
    }

    // display actual URL of currently executing page
    lblActualPath.Text = Request.CurrentExecutionFilePath;
    String qs = Request.QueryString.ToString();
    if (qs != null && qs != String.Empty)
    {
      lblActualPath.Text += '?' + qs;
    }
  }

The remaining code in the Page_Load event displays the actual URL of the current page and any query string, so that you can see the effects of the Front Controller when it redirects requests to different pages. You will see how the Front Controller works in the next article in this series.

To load and display a User Control dynamically, code in the LoadAndDisplayView method creates a new instance of the control and then adds it to the Controls collection of an ASP.NET Placeholder control located in the main View (the ASPX page). After displaying the User Control, the code sets the Enabled properties of the "Back" and "Next" buttons, depending on the current view name, and displays the name of the view in the page header element (a <div> control with the runat="server" attribute). Finally, it saves the name of the view in the page viewstate ready for the next postback:

private void LoadAndDisplayView()
{
  // load and display the appropriate view
  if (viewName != null && viewName != String.Empty)
  {
    try
    {
      UserControl view = (UserControl)LoadControl(viewName + ".ascx");
      viewPlaceHolder.Controls.Add(view);
    }
    catch (Exception ex)
    {
      throw new Exception("Cannot load view '" + viewName + "'", ex);
    }
  }
  else
  {
    viewName = "No view specified";
  }
  // set state of buttons to match view
  btn_Back.Enabled = (viewName != "CustomerList");
  btn_Next.Enabled = (viewName != "CityList");
  // display name of current view
  pageHeaderElement.InnerText = "Current View is '" + viewName + "'";
  // save in page viewstate for use in postback
  ViewState["ViewName"] = viewName;
}

As an alternative, you could use the Server.Execute method to execute separate ASPX pages, each an MVP pattern implementation with its own Presenter (code-behind file) and View (ASPX page) that generates the appropriate output. This output will appear in the resulting page as a partial View, though you must remember not to include the , , and elements in the partial View implementation.

However, it is likely that you will have to include the ASP.NET

section in the partial View to be able to use ASP.NET Web Controls in that page, which means that you can only use one partial View per hosting page. User Controls are more likely to be easier to manage, and more efficient. Each can contain its own initialization code, and does not require a section. Neither do they, by default, contain the , , and elements.

The only other code required in the Presenter is that to handle clicks on the buttons. The event handlers for the "Back" for "Next" buttons change the value of the viewName local variable, and then call the LoadAndDisplayView method to display the current view. The event handler for the "Cancel" button just redirects the request back to the default page of the example application:

protected void btn_Back_Click(object sender, EventArgs e)
{
  switch (viewName)
  {
    case "CustomerDetails": 
      viewName = "CustomerList";
      break;
    case "CityList": 
      viewName = "CustomerDetails";
      break;
  }
  LoadAndDisplayView();
}

protected void btn_Next_Click(object sender, EventArgs e)
{
  switch (viewName)
  {
    case "CustomerList":
      viewName = "CustomerDetails";
      break;
    case "CustomerDetails":
      viewName = "CityList";
      break;
  }
  LoadAndDisplayView();
}

protected void btn_Cancel_Click(object sender, EventArgs e)
{
  Response.Redirect("Default.aspx");
}

To see the results, look back at Figures 4, 5, and 6. These show the three views that the Use Case Controller example displays as you click the "Back" and "Next" buttons.

You can open the Use Case Controller page by selecting either "Use Case Controller" or "TransferPage1.aspx" in the drop-down list at the bottom of the Default.aspx page of the sample application.

Implementation of the Page Controller Pattern

ASP.NET automatically provides a Presenter implementation for the Model-View-Presenter pattern through a code-behind file for each ASP.NET page. However, you can extend the implementation by adding your own Page Controller in the form of a base class that the code-behind files inherit.

By default, the code-behind class for an ASP.NET page inherits from the Page class in the System.Web.UI namespace. If you create a suitable base class that inherits from Page, you can use this as the base for your own concrete implementations of the code-behind class. This is useful if some or all of your pages require common functionality.

The base class can also perform some of the tasks of a Page Controller by handling the PageInit or PageLoad events. For example, code in the Page_Load event can select a View to display at runtime for a page, allowing a single page to change its content based on some external condition such as a value in the query string.

The example application uses a base class named PageControllerBase (in the App_code folder), which inherits from Page and exposes three values that the concrete classes that inherit from it can access. These values are a reference to the partial View (User Control) to load and display, the name of the View, and a String value to display when there is no View specified:

// base class for TransferPage2 and TransferPage3
// implements common tasks and can call method(s)
// in the concrete page class implementations 
public class PageControllerBase : Page
{
  // values that will be available in page class
  protected UserControl displayView = null;
  protected String displayViewName = String.Empty;
  protected String noViewSelectedText = "No View Selected";

The PageControllerBase class handles the PageLoad event to get the name of the View to display from the query string, and then attempts to create an instance of the appropriate User Control for the concrete page code to access. At the end of the PageLoad event handler, the code calls a method named PageLoadEvent, passing in the values of the sender and EventArgs originally received as parameters to this Page_Load event:

protected void Page_Load(Object sender, System.EventArgs e)
{
  // get name of view (UserControl) to show in the page 
  displayViewName = Context.Request.QueryString["view"];
  if (displayViewName != null && displayViewName != String.Empty)
  {
    try
    {
      // load view from ASCX file
      displayView = (UserControl)Page.LoadControl(displayViewName + ".ascx");
    }
    catch { }
  }
  // call concrete page class method
  PageLoadEvent(sender, e);
}

Every page that inherits from this base class must implement the PageLoadEvent. To ensure that this is the case, the PageControllerBase class declares a virtual method that each concrete page implementation must override:

// define method that concrete page classes must implement
// and will be called by this base class after Page_Load
virtual protected void PageLoadEvent(Object sender, System.EventArgs e)
{ 
  // overridden in concrete page implementation
}

The example application contains two pages, named TransferPage2.aspx and TransferPage3.aspx, which inherit from the PageControllerBase class. The code in the PageLoadEvent method that overrides the virtual base class method is the same in both - it is only the View (the ASPX UI) that differs. However, they could be completely different if required, although all inheriting page should make some use of the common functionality of the PageControllerBase class (or there is no point in inheriting from it!).

The next listing shows the PageLoadEvent method used in the Presenter code-behind file of both the TransferPage2.aspx and TransferPage3.aspx pages. It displays the name of the current View in the page title and heading, and adds the User Control instance created by the PageControllerBase class to the Controls collection of an ASP.NET Placeholder control declared in the ASPX file:

public partial class TransferPage2 : PageControllerBase
{
  // override virtual method in PageControllerBase class
  protected override void PageLoadEvent(Object sender, System.EventArgs e)
  {
    // use values that were set in base class Page_Load event
    if (displayView != null)
    {
      pageTitleElement.Text = "Displaying view '" + displayViewName + "'";
      pageHeadingElement.InnerHtml = "Displaying view '" + displayViewName + "'";
      PlaceHolder1.Controls.Add(displayView);
    }
    else
    {
      pageHeadingElement.InnerHtml = noViewSelectedText;
    }

    // display actual URL of currently executing page
    lblActualPath.Text = Request.CurrentExecutionFilePath;
    String qs = Request.QueryString.ToString();
    if (qs != null && qs != String.Empty)
    {
      lblActualPath.Text += '?' + qs;
    }
  }
}

The final section of the PageLoadEvent method displays the actual page URL so that you can see the effects of the Front Controller implementation (described in the next article in this series). Figure 7 shows the two pages described in this section displaying the "CityList" View.

The CityList View displayed by the two pages that implement the Front Controller pattern
Figure 7 - The CityList View displayed by the two pages that implement the Front Controller pattern

You can open the page TransferPage2.aspx with the appropriate View displayed by selecting the View name ("CustomerList", "CustomerDetails", or "CityList") in the drop-down list at the bottom of the Default.aspx page. To open the TransferPage3.aspx page with the appropriate View displayed, select "SpecialCustomerList", "SpecialCustomerDetails", or "SpecialCityList". To open the pages with no View displayed, select the page name ("TransferPage2.aspx" or "TransferPage3.aspx") in the drop-down list.

Implementation of the Front Controller Pattern

The most common approach for implementing the Front Controller pattern in ASP.NET is through an HTTP Module that handles one of the ASP.NET HTTP pipeline events, and executes a Server.Transfer action to load the appropriate target page. This is the technique implemented in the example application. A drop-down list at the bottom of the Default.aspx page displays a list of possible targets, from which you can select one (see Figure 8).

The list of targets for the Front Controller displayed in the default page
Figure 8 - The list of targets for the Front Controller displayed in the default page

You will recall from the discussion of the Singleton pattern in the previous article that the example application uses a Singleton class named TransferUrlList.cs to expose the contents of an XML file that contains the target "short names" (as displayed in the drop-down list in Figure 8) and the actual URL to load:

<?xml version="1.0" encoding="utf-8" ?>
<transferUrls>
  <item name="UseCaseController" url="TransferPage1.aspx" />
  <item name="CustomerList" url="TransferPage2.aspx?view=CustomerList" />
  <item name="CustomerDetails" url="TransferPage2.aspx?view=CustomerDetails" />
  <item name="CityList" url="TransferPage2.aspx?view=CityList" />
  <item name="SpecialCustomerList" url="TransferPage3.aspx?view=CustomerList" />
  <item name="SpecialCustomerDetails" 
        url="TransferPage3.aspx?view=CustomerDetails" />
  <item name="SpecialCityList" url="TransferPage3.aspx?view=CityList" />
  <item name="Publish-Subscribe" url="TransferPage4.aspx" />
  <item name="Command-Observer" url="TransferPage5.aspx" />
</transferUrls>

To demonstrate the Front Controller pattern, the application contains a class named FrontController.cs (in the App_Code folder) that implements an HTTP Module. The class declaration indicates that it implements the IHttpModule interface, which means that it must declare the public methods Init and Dispose.

In the Init method, the class subscribes to the PreRequestHandlerExecute event by registering an event handler named MyPreRequestHandler. As the class does not use any unmanaged resources, there is no action required for the Dispose method - so only an empty method declaration is required:

public class FrontController : IHttpModule
{
  public void Init(HttpApplication context)
  {
    // register a handler for the event you want to handle
    context.PreRequestHandlerExecute += MyPreRequestHandler;
  }

  public void Dispose()
  {
    // no clean-up required
  }

The ASP.NET HTTP pipeline raises the PreRequestHandlerExecute event just before it executes the request. The code in the event handler in the FrontController module can therefore make a decision about which page to load based on some external condition. This may be something like the browser language, country, browser type, a value in the user's ASP.NET Profile, or some other value from the environment.

The example module takes simpler approach in order to make it easier to specify the target you want when experimenting with the application. It examines the query string for a value that corresponds to one in the list of transfer URLs exposed from the XML file, and redirects to the appropriate page. The GetTransferUrl method of the TransferUrlList class returns the translated URL if found in the list, or the original URL if it is not in the list. The code then executes the Server.Transfer method to that URL:

  private void MyPreRequestHandler(Object sender, EventArgs e)
  {
    // use features of the request (user, browser, IP address, etc.)
    // to decide how to handle the request, and which page to show
    // this example looks for specific items in the query string
    // that indicate the required target (such as "CustomerList")
    // using a dictionary of values loaded from an XML disk file

    // get Singleton list of transfer URLs 
    TransferUrlList urlList = TransferUrlList.GetInstance();
    // get the current request query string
    String reqTarget = HttpContext.Current.Request.QueryString["target"];
    if (reqTarget!= null && reqTarget != String.Empty)
    {
      // see if target value matches a transfer URL
      // by querying the list of transfer URLs
      // method returns the original value if no match 
      String transferTo = urlList.GetTransferUrl(reqTarget);
      try
      {
        // transfer to the specified URL
        HttpContext.Current.Server.Transfer(transferTo, true);
      }
      catch { }
    }
  }
}

If you select one of the "short name" values in the drop-down list in the default page of the application, you will see the effects of the Front Controller module. For example, Figure 9 shows the result of selecting the "CustomerDetails" option. The code in the Front Controller HTTP module translates this into the URL "TransferPage2.aspx?view=CustomerDetails" - as you can see in the Label control that displays the actual URL of the current page.

Selecting "CustomerDetails" loads TransferPage2 with the CustomerDetails View displayed
Figure 9 - Selecting "CustomerDetails" loads TransferPage2 with the CustomerDetails View displayed

Notes on the Use of a Front Controller HTTP Module

You may be tempted to try using URLs for your Front Controller that include the "short names" as part of the path, rather than in the query string. This will work when you run your application within Visual Studio 2005 because all requests go to the built-in Visual Studio Web server. However, if you run your application under the IIS Web server you will get "Page Not Found" errors, because IIS only passes pages that actually exist to the ASP.NET runtime.

For example, if you use a URL such as "http://localhost/CityList", and there is no folder in your Web site named CityList, IIS will return a "404 Not Found" error. It will not initiate the ASP.NET HTTP pipeline, and so your module will not execute. Even if the CityList folder does exist, IIS will return either a "403 - Forbidden" error, or a file listing if Directory Browsing is enabled.

Of course, you could create a folder for each "short name" and put an empty ASPX page in each of these folders so that the Front Controller module redirects to the appropriate page as required. This is better than using pages that contain redirection code (such as Response.Redirect or elements) in each folder, as the Front Controller will intercept each request and will not actually load the empty ASP.NET page.

An alternative is to map all requests to the ASP.NET runtime DLL, rather than just the ASP.NET file extensions such as aspx and ascx. However, this is probably not a practical solution because it will result in an excessive processing overhead in ASP.NET.

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About the author

Alex Homer

Alex Homer United Kingdom

Alex spent most of his earlier working life as a technical salesman, and has had a love-hate relationship with computers that goes way back to the Sinclair Z80 and the Oric Atmos. In 1996 he ret...

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