ASP.NET.in.a.Nutshell.Second.Edition [Electronic resources] نسخه متنی

14.2 Properties Reference

HttpApplicationState = Context.Application

Returns the current Application state object.

Parameter

HttpApplicationState

An HttpApplicationState object that will receive the value of the
property.

Example

The example sets an application value using the Application instance
exposed by the Page class and gets the instance
from Context. Finally, it displays the newly set value by using the
HttpApplicationState instance retrieved from the Context object
(proving they are the same object):

Sub Page_Load( )
Dim App as HttpApplicationState
Page.Application("Test")="Value"
App = Context.Application
Message.Text = App("Test")
End Sub

Notes

Often, you will use the copy of Application from the
Page class. However, for objects not derived from
Page (for instance, HttpHandlers and HttpModules)
this is a convenient way to access the Application state.

Do not confuse this property, which returns an instance of type
HttpApplicationState, with the ApplicationInstance property, which
returns an instance of type HttpApplication.

Context.ApplicationInstance = HttpApplication
HttpApplication = Context.ApplicationInstance

Returns or sets the current HttpApplication object.

Parameter

HttpApplication

An HttpApplication object that will receive or set the value of the
ApplicationInstance property.

Notes

Generally, you do not need to use the HttpApplication object, since
the properties it exposes are usually available through other
objects. One exception is when accessing all the methods that allow
you to add event handlers for this request.

HttpCache = Context.Cachea

Returns an instance of the Cache class.

Parameter

HttpCache

An Object variable of type Cache.

Example

The example retrieves an instance of the Cache
class into a local variable and then adds a value to the cache:

Sub Application_BeginRequest( )
Dim myCache As Cache
myCache = Context.Cache
myCache.Add("Test", "Test", Nothing, _ 
System.DateTime.Now.AddHours(1), Nothing, _ 
CacheItemPriority.High, Nothing)
End Sub

Notes

Note that rather than using the Page_Load event, the example above
shows the Application_BeginRequest event handler in

global.asax ; a common use of the Cache property
is to access the cache at points during request processing when the
Cache property of the Page object is not available, such as before
the Page object is instantiated.

HttpContext = HttpContext.Current

Retrieves the current HttpContext instance.

Parameter

HttpContext

The current context.

Example

This example uses the IsDebuggingEnabled property, described later in
this chapter, to show whether debugging is enabled:

Sub Page_Load( )
Message.Text = HttpContext.Current.IsDebuggingEnabled.ToString( )
End Sub

Notes

Current is a shared (static) property, indicating that you can access
it without creating an instance of the HttpContext
class.

Exception = Context.Error

Returns the first error, if any, associated with
the current request.

Parameter

Exception

An Exception variable to receive the value of the property.

Example

The example checks to see if the Error property on the current
context object is null (Nothing in VB.NET). If it
is not null, it displays the error; otherwise, it displays a message
indicating there is no error.

Sub Page_Load( )
If Not HttpContext.Current.Error Is Nothing then
Message.Text = HttpContext.Current.Error.ToString( )
Else
Message.Text = "No error detected"
End If 
End Sub

IHttpHandler = Context.Handler
Context.Handler = IHttpHandler

Sets or returns an instance of the IHttpHandler for the current
request.

Parameter

IHttpHandler

An Object variable of a type that implements the IHttpHandler
interface.

Notes

The Handler property can be used to specify special handling for this
request. To understand this idea, you need to understand how requests
are processed in IIS. When a request comes in, unless the item
requested is a simple HTML file, the request is generally handled by
an Internet Server API (ISAPI) extension. For instance, classic ASP
and ASP.NET pages are directed to ISAPI applications that process the
request. ISAPI applications are reasonably easy to write in
principal, but are very difficult to create in practice. The
IHttpHandler interface is the .NET way of allowing the developer to
write code to for requests to be handled in a way similar to ISAPI
applications in IIS, without all of the problems of ISAPI.

Boolean = Context.IsCustomErrorEnabled

Returns a Boolean
value specifying whether custom errors are enabled for the current
request.

Parameter

Boolean

A Boolean variable to receive the value of this property.

Example

The following example displays True if custom
errors are enabled; otherwise, it displays False.

Sub Page_Load( )
Message.Text = "Custom Error Enabled?" & _
Context.IsCustomErrorEnabled
End Sub

Notes

This flag is controlled by the customErrors
section in

web.config . If the
customErrors element's
mode attribute is set to On,
IsCustomErrorEnabled returns True. If the
customErrors element's
mode attribute is set to False
or RemoteOnly, this flag is
False.

Boolean = Context.IsDebuggingEnabled

Returns a Boolean value
specifying whether debugging is enabled for the current request.

Parameter

Boolean

A Boolean variable to receive value of this flag.

Example

The example displays True if debugging is enabled;
otherwise, it displays False:

Sub Page_Load( )
Message.Text = "Debugging Enabled?" & _
Context.IsDebuggingEnabled
End Sub

Notes

This flag is controlled by the compilation section
in

web.config . If the
compilation section's
debug attribute is set to True,
IsDebuggingEnabled returns True. If the
debug attribute in the compilation section is set
to False, this property is
False.

HttpRequest = Context.Request

Returns the HttpRequest object for the current
request.

Parameter

HttpRequest

An HttpRequest variable to receive the current HttpRequest object.

Notes

This property is provided for applications other than ASP.NET pages
(where the Page.Request property is normally used to retrieve the
HttpRequest object). Code that does not have access to the properties
of the Page class includes HttpHandlers and
HttpModules, as well as event handlers in

global.asax .

HttpResponse = Context.Response

Returns the HttpResponse object for the current
request.

Parameter

HttpResponse

An HttpResponse variable to receive the current HttpResponse object.

Notes

This property is provided for applications other than ASP.NET pages
(where the Page.Response property is normally used to retrieve the
HttpResponse object). Code that does not have access to the
properties of the Page class includes HttpHandlers
and HttpModules, as well as

global.asax . One
common reason for using Context.Response is to write cookies in an
HttpModule.

HttpSessionState = Context.Session

Returns the HttpSession object for the current
request.

Parameters

HttpSessionState

An HttpSessionState object to receive the current session.

Notes

This property is provided for applications other than ASP.NET pages
(where the Page.Session property is normally used to retrieve the
HttpSessionState object). Code that does not have access to the
properties of the Page class includes HttpHandlers
and HttpModules, as well as

global.asax . When
using Context.Session in the Application event handlers of

global.asax , Session is not available in the
Application BeginRequest event, but can be used in, for instance, the
Application_PreRequestHandlerExecute event.

Boolean = Context.SkipAuthorization
Context.SkipAuthentication = Boolean

Sets or returns a flag
indicating whether the URLAuthorization module will skip the
authorization check. The default is False.

Parameter

Boolean

A Boolean variable returning or setting the flag regarding
authorization checks.

Example

The following example retrieves the status of the SkipAuthorization
property and displays it in the Message label control:

Sub Page_Load( )
Message.Text = "SkipAuthorization? " _ 
& Context.SkipAuthorization
End Sub

Notes

To set this value, the ControlPrincipal Flag must be set in the Flags
property of the SecurityPermission object. This property is used
internally by the Forms and Passport authentication modules.

timestamp = Context.Timestamp

Returns a DateTime object
containing the date and time of the request on the server.

Parameter

timestamp

An Object variable of type DateTime.

Example

The following example retrieves the date and time of the request and
displays it in the Message label control:

Sub Page_Load( )
Message.Text = "Date/Time of Request: " _ 
& Context.Timestamp
End Sub

TraceContext = Context.Trace

Returns the TraceContext for the current
request. The members of the TraceContext class are
listed in the entry for the Trace property in Chapter 12.

Parameter

TraceContext

A TraceContext variable to receive the TraceContext object for the
current request.

Example

The example retrieves the date and time of the request and displays
it in the Message label control.

Sub Page_Load( )
Message.Text = "Trace Enabled? " _ 
& Context.Trace.IsEnabled
End Sub

Notes

Trace can be enabled by setting the Trace
attribute of the @ Page
directive to True.

IPrincipal = Context.User
Context.User = IPrincipal

Returns or sets the IPrincipal object for the
current request.

Parameter

IPrincipal

An object that implements the IPrincipal interface. The IPrincipal
interface is implemented by the GenericPrincipal
and the WindowsPrincipal classes. IPrincipal
defines one property and one method:

Identity

A property that returns a class that implements the IIdentity
interface.

IsInRole(Role as String)

A method that returns a Boolean indicating whether the current
principal belongs to the specified Role.

The IIdentity interface provides several useful properties:

AuthenticationType

A property that returns the type of authentication used, if the
request is authenticated.

IsAuthenticated

A property that returns a Boolean indicating if the user has been
authenticated. This property should be checked (to ensure it is
True) before checking for AuthenticationType or
Name. IsAuthenticated returns False if anonymous
authentication is enabled in Internet Information Server and neither
Forms nor another authentication method is in use by ASP.NET.

Name

A property that returns the Name of the current user. The user name
format depends upon the type of authentication used. For Windows
authentication, the name is in the format of
DOMAIN\UserName.

Example

The example checks to see if the user is authenticated and if so,
returns the name of the user:

Sub Page_Load( )
Message.Text = "User.Identity.IsAuthenticated?  " _
& Context.User.Identity.IsAuthenticated & "<br/>"
If Context.User.Identity.IsAuthenticated Then
Message.Text = Message.Text & "User.Identity.Name?  " _
& Context.User.Identity.Name & "<br/>"
End If
End Sub

Notes

Understanding the values you receive back when checking the User
property requires an understanding of ASP.NET and Internet
Information Server, since the returned values depend on settings in
both.