Programming web application

 Page and Application Exception Handling
 Programming the Web.config File Settings
 Asynchronous Web Page Programming
 Creating a Custom HTTP Handler

 Using the asp.net intrinsic Objects
 Determining the Browser Type
 Accessing Web Page Headers

There are certain Web programming tasks that are
outside the bounds of the basic page request–response
scenario.

 Catch unhandled exceptions at the page or
application level.
 Read and modify settings in different configuration
files.
 Enable asynchronous communication inside Web
pages.
 Create a custom HTTP handler to respond to
requests for nonstandard file types.

 To catch errors at the page level, you create a
Page_Error event handler inside the code for each
page for which you wish to catch unhandled errors.

 Access the Server.GetLastError method to retrieve the
last error, and then call Server.ClearError to remove
the error from the queue.

private void Page_Error(object sender, EventArgs e)
{
Trace.Write("ERROR: " +
Server.GetLastError().Message);
Server.ClearError();
}

 You define an application-wide handler by adding
the Application_Error method to your application’s
Global.asax file.
 Here you typically pass the error handling onto
another page that might log the error and display
troubleshooting information to the user by using the
Server.Transfer method.

void Application_Error(object sender, EventArgs e)
{
//code that runs when an unhandled error occurs
Server.Transfer("HandleError.aspx");
}

 There are times when you might want to
programmatically edit configuration settings.

 ASP.NET provides the ASP.NET Configuration
application programming interface (API) for this purpose.

 You use a System.Configuration.Configuration class to read
the Web.config file and write any changes you might make.

 To create a Configuration object for the current
application, you can use the static class
WebConfigurationManager.

 With this class, you can read configuration sections by
calling the GetSection and GetSectionGroup methods.

 For example, the following code sample displays the
current authentication mode as defined in the
<system.web><authentication> section, and then displays it
in the Label1 control:

AuthenticationSection section =(AuthenticationSection)
WebConfigurationManager.GetSection("system.web/
authentication");
Label1.Text = section.Mode.ToString();

 Besides accessing the <system.web> section, you can
access custom application settings using the
WebConfigurationManager.AppSettings collection.

 The following code sample demonstrates how to display
the MyAppSetting custom application setting (which you
could add using the ASP.NET Web Site Configuration
tool) in a Label control:

 Label1.Text =
WebConfigurationManager.AppSettings["MyAppSetting"];

 Similarly, you can programmatically access connection
strings using the
WebConfigurationManager.ConnectionStrings collection:

 Label1.Text =
WebConfigurationManager.ConnectionStrings["Northwi
nd"].ConnectionString;

 If you want to make changes, however, you must choose
a specific configuration location.
 To do this, create an instance of a Configuration object.

 To create an instance of the root Web.config file that applies to
all applications, call the static
WebConfigurationManager.OpenWebConfiguration method
and pass a null parameter to create a Configuration object.

 Then, use the Configuration object to create objects for
individual sections.

 Edit values in those sections and save the changes by
calling Configuration.Save.

 Asynchronous programming is used to improve
the efficiency of long-running Web pages.

 During busy times when multiple pages are
requested simultaneously the thread pool
responding to user requests makes Web pages
more efficient.

1. Add the Async=”true” attribute to the @ Page directive.

2. Create events to start and end your asynchronous code
that implements
System.Web.IHttpAsyncHandler.BeingProcessRequest and
System.Web.IHttpAsyncHandler.EndProcessRequest

3. Call the AddOnPreRenderCompleteAsync method to declare
your event handlers

 An HTTP handler is code that executes when an
HTTP request for a specific resource is made to the
server.

 For example, when a user requests an .aspx page
from IIS, the ASP.NET page handler is executed.
When an .asmx file is accessed, the ASP.NET service
handler is called.

 To create a custom Hypertext Transfer Protocol
(HTTP) handler, you first create a class that
implements the IHttpHandler interface (to create a
synchronous handler) or the IHttpAsyncHandler (to
create an asynchronous handler).

 Both handler interfaces require you to implement
the IsReusable property and the ProcessRequest method.

 The IsReusable property specifies whether the
IHttpHandlerFactory object (the object that actually
calls the appropriate handler) can place your
handlers in a pool and reuse them to increase
performance or whether it must create new
instances every time the handler is needed.

 The ProcessRequest method is responsible for
actually processing the individual HTTP requests.

 Once it is created, you then register and configure
your HTTP handler with IIS.

public class ImageHandler : IHttpHandler
{
public ImageHandler()
{}
public bool IsReusable
{
get { return false; }
}
public void ProcessRequest(HttpContext context)
{
context.Response.ContentType = "image/jpeg";
}
}

 For performance reasons, IIS passes only requests
for specific file types to ASP.NET.

 For example, IIS passes requests for .aspx, .axd,
.ascx, and .asmx to the Aspnet_Isapi.dll file that
performs the ASP.NET processing.

 For all other file types, including .htm, .jpg, and
.gif, ASP.NET simply passes the file from the file
system directly to the client browser.

1.Open IIS Manager.
2. Expand the nodes until you get to your site or Default Web
Site. Select the node for your application.
3. Double-click the Handler Mappings icon in the center pane
of IIS Manager.

4. In the Actions pane (right side), select Add Managed
Handler.

5. In the Add Managed Handler dialog box, set the Request
path to the file name or extension you wish to map, in this
case, .jpg. The Type name is the class name of the HTTP
handler. If your HTTP handler is inside the App_Code
directory, it will appear in the drop-down list.

 Alternatively, if you are using IIS 7, you can simply
configure the handler for the file extension in your
Web.config file. You do not, then, need to use IIS
Manager.
 For each file extension or file name you want to
register, create an <add> element in the
<configuration><system.web><httpHandlers> section of
your Web.config file:

<configuration>
<system.web>
<httpHandlers>
<add verb="*" path="*.jpg" type="ImageHandler"/>
<add verb="*" path="*.gif" type="ImageHandler"/>
</httpHandlers>
</system.web>
</configuration>

 You can use the objects inside of ASP.NET to gain
access to a lot of useful information about your
application, the server hosting the application, and
the client requesting resources on the server.

 These objects are referred to as the ASP.NET intrinsic
objects. They are exposed through objects like Page,
Browser, Response, Request, Server, and Context.

 Together, these objects provide you a great deal of
useful information like the user’s Internet Protocol
(IP) address, the type of browser making the request,
errors generated during a response, the title of a
given page, and much more.

 To display different versions of Web pages for
different browsers, you will need to write code
that examines the HttpBrowserCapabilities object.

 This object is exposed through Request.Browser.
 Request .Browser has many members that you
can use to examine individual browser
capabilities.

 The header information of a rendered HTML
page contains important information that helps
describe the page.
 This includes the name of the style sheet, the title
of the page, and metadata used by search
engines.

 ASP.NET allows you to edit this information
programmatically using the
System.Web.UI.HtmlControls.HtmlHead control.

 This control is exposed via the Page.Header
property.

 For example, you might use this to set the title of a page
dynamically at run time based on the page’s content.

Page.Header.Title = "Current time: " + DateTime.Now;

 To set style information for the page (using the
<head><style> HTML tag), access Page.Header.StyleSheet.

Style bodyStyle = new Style();
bodyStyle.ForeColor = System.Drawing.Color.Blue;
bodyStyle.BackColor = System.Drawing.Color.LightGray;
Page.Header.StyleSheet.CreateStyleRule(bodyStyle, null, "
body");

Programming web application

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Programming web application