C# 4 and .NET 4

Clickonce ❘ 421
application. The Prerequisite button displays a list of common prerequisites that can be installed along
with the application. You have the choice of installing the prerequisites from the same location that the
application is being published to or optionally having the prerequisites installed from the vendor’s web site.
The Update button displays a dialog that has the information about how the application should be updated.
As new versions of an application are made available, ClickOnce can be used to update the application.
Options include to check for updates every time the application starts or to check in the background. If
the background option is selected, a specified period of time between checks can be entered. Options for
allowing the user to be able to decline or accept the update are available. This can be used to force an
update in the background so that the user is never aware that the update is occurring. The next time the
application is run, the new version will be used instead of the older version. A separate location for the
update files can be used as well. This way the original installation package can be located in one location
and installed for new users, and all the updates can be staged in another location.
The application can be set up so that it will run in either online or offline mode. In offline mode the
application can be run from the Start menu and acts as if it were installed using the Windows Installer.
Online mode means that the application will run only if the installation folder is available.
application Cache for Clickonce files
Applications distributed with ClickOnce are not installed in the Program Files folder. Instead, they are
placed in an application cache that resides in the Local Settings folder under the current user’s Documents
and Settings folder. Controlling this aspect of the deployment means that multiple versions of an application
can reside on the client PC at the same time. If the application is set to run online, every version that the
user has accessed is retained. For applications that are set to run locally, the current and previous versions
are retained.
Because of this, it is a very simple process to roll back a ClickOnce application to its previous version. If the
user goes to the Add/Remove Programs Control Panel applet, the dialog presented will contain the choice of
removing the ClickOnce application or rolling back to the previous version. An Administrator can change
the manifest file to point to the previous version. If the administrator does this, the next time the user
runs that application, a check will be made for an update. Instead of finding new assemblies to deploy, the
application will restore the previous version without any interaction from the user.
application security
Applications deployed over the Internet or an intranet have a lower security or trust setting than
applications that have been installed to the local drive. For example, by default, if an application is launched
or deployed from the Internet it is in the Internet Security Zone. This means that it cannot access the file
system, among other things. If the application is installed from a file share, it will run in the Intranet Zone.
If your ClickOnce application requires a higher level of trust than the default, the user will be prompted
to grant the permissions required for the application to run. These permissions are set in the trustInfo
element of the application manifest. Only the permissions asked for in this setting will be granted. So, if
an application asks for file access permissions, Full Trust will not be granted, only the specific permissions
requested.
Another option is to use Trusted Application Deployment. Trusted Application Deployment is a way to
grant permissions on an enterprise-wide basis without having to prompt the user. A trust license issuer is
identified to each client machine. This is done with public key cryptography. Typically, an organization will
have only one issuer. It is important to keep the private key for the issuer in a safe, secure location.
A trust license is requested from the issuer. The level of trust that is being requested is part of the trust
license configuration. A public key used to sign the application must also be supplied to the license
issuer. The license created contains the public key used to sign the application and the public key of the
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