ClickOnce is a Microsoft technology that enables the user to install and run a Windows application by clicking a link in a web page. (Such applications are known as Smart clients.) ClickOnce is a component of Microsoft .NET Framework 2.0 and later, supports deploying applications made with Windows Forms or Windows Presentation Foundation. It is similar to Java Web Start for the Java Platform or Zero Install for Linux.
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The core principle of ClickOnce is to bring the ease of deployment of web applications to the Windows user. In addition, ClickOnce aims to solve three other problems with conventional deployment models: the difficulty in updating a deployed application, the impact of an application to the user's computer, and the need for administrator permissions to install applications.
ClickOnce-deployed applications are considered 'low impact', in that they are installed per-user, not per-machine. No administrator privileges are required to install one of these applications. Each ClickOnce application is isolated from the other. This means one ClickOnce application is not able to 'break' another.
ClickOnce employs Code Access Security (CAS) to ensure that system functions cannot be called by a ClickOnce application from the web, ensuring the security of data and the client system in general.
The ClickOnce model supports both installed applications (akin to conventional Windows applications with start menu integration) and online applications (browser hosted applications that are not installed, only run and cached). ClickOnce applications can be deployed to a computer from a Web location, a network share, or even from a file location such as a CD.
The ClickOnce deployment technology has been integrated into the Visual Studio 2005 and later, and is also natively supported by MSBuild, Microsoft's build management technology.
A ClickOnce deployment is controlled through the use of two XML manifest files: a deployment manifest and an application manifest. The manifests are in the same XML format as the Side-by-Side Assembly implementation. The deployment manifest (*.application file) describes the deployment model: the current version, update behavior, publisher identity along with digital signature; this manifest is intended to be authored by administrators who handle deployment. The application manifest (*.exe.manifest file) describes the application assemblies, dependent libraries and lists permissions required by the application. This file is intended to be authored by the application developer. In order to launch a ClickOnce application, a user clicks on its deployment manifest file.
ClickOnce applications can be self-updating; they can check for newer versions as they become available and automatically replace any updated files. For its installed application flavor, ClickOnce presents several update options. Applications can be configured to check for updates on startup or after startup. ClickOnce also exposes programmatic APIs to customize update behavior. There is also support for required or mandatory updates for managing ongoing updates and ensuring that the entire user-base can be moved up to a new version in a timely manner.
Prior to .NET Framework 3.5 with Service Pack 1, ClickOnce worked only with Internet Explorer, although the FFClickOnce add-on[1] allowed Firefox to support it.
Microsoft .NET Framework 3.5 with Service Pack 1 includes a Firefox extension called .NET Framework Assistant that enables ClickOnce support in Firefox 3 and later. [2] The first release of this extension however, had a problem that prevented users from uninstalling the add-on in the same manner that other add-ons are uninstalled: the corresponding Uninstall button in the Add-ons dialog box was disabled.[3]On 6 May 2009, Microsoft released an update that addressed this problem and also published a support article that helped users manually remove this component. In addition, the later versions of Microsoft .NET Framework Assistant included with Windows 7 and .NET Framework 4 did not have this issue.[4]
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