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Windows Installer
Help screen of Windows Installer 5.0 running onWindows 11
Developer	Microsoft
Initial release	31 August 1999; 26 years ago (1999-08-31)
Final release	5.0 / 22 July 2009; 16 years ago (2009-07-22)[1]
Operating system	Microsoft Windows
Platform	IA-32,x86-64,ARM32,ARM64,Itanium
Included with	Windows 2000 or later
Type	Installer
License	Freeware
Website	learn.microsoft.com/en-us/windows/win32/msi/windows-installer-portal

Windows Installer (msiexec.exe, previously known asMicrosoft Installer,^[3]codenameDarwin)^[4][5] is asoftware component andapplication programming interface (API) ofMicrosoft Windows used for theinstallation, maintenance, and removal of software. The installation information, and optionally the files themselves, are packaged ininstallation packages, looselyrelational databases structured asCOM Structured Storages and commonly known as "MSI files", from their defaultfilename extensions. The packages with the file extensionsmst contain Windows Installer "Transformation Scripts", those with themsm extensions contain "Merge Modules" and the file extensionpcp is used for "Patch Creation Properties".^[6] Windows Installer contains significant changes from its predecessor, Setup API. New features include aGUIframework and automatic generation of theuninstallation sequence. Windows Installer is positioned as an alternative to stand-alone executable installer frameworks such as older versions ofInstallShield andNSIS.

Before the introduction ofMicrosoft Store (then named Windows Store), Microsoft encouraged third parties to use Windows Installer as the basis for installation frameworks, so that they synchronize correctly with other installers and keep the internal database of installed products consistent. Important features such asrollback andversioning depend on a consistent internal database for reliable operation. Furthermore, Windows Installer facilitates theprinciple of least privilege by performing software installations by proxy for unprivileged users.

A package describes the installation of one or more fullproducts and is universally identified by aGUID. A product is made up ofcomponents, grouped intofeatures. Windows Installer does not handle dependencies between products.

A single, installed, working program (or set of programs) is aproduct. A product is identified by a unique GUID (the ProductCode property) providing an authoritative identity throughout the world. The GUID, in combination with the version number (ProductVersion property), allows for release management of the product's files and registry keys.

Apackage includes the package logic and othermetadata that relates to how the package executes when running. For example, changing anEXE file in the product may require the ProductCode or ProductVersion to be changed for the release management. However, merely changing or adding a launch condition (with the product remaining exactly the same as the previous version) would still require the PackageCode to change for release management of the MSI file itself.

Afeature is a hierarchical group of components. A feature may contain any number of components and other sub-features. Smaller packages can consist of a single feature. More complex installers may display a "custom setup" dialog box, from which the user can select which features to install or remove.

The package author defines the product features. Aword processor, for example, might place the program's core file into one feature, and the program's help files, optional spelling checker and stationery modules into additional features.

Acomponent is the basic unit of a product. Each component is treated by Windows Installer as a unit. The installer cannot install just part of a component.^[7] Components can contain programfiles,folders,COM components,registry keys, andshortcuts. The user does not directly interact with components.

Components are identified globally by GUIDs; thus the same component can be shared among several features of the same package or multiple packages, ideally through the use ofMerge Modules.

Akey path is a specific file, registry key, orODBC data source that the package author specifies as critical for a given component. Because a file is the most common type of key path, the termkey file is commonly used. A component can contain at most one key path; if a component has no explicit key path, the component's destination folder is taken to be the key path. When an MSI-based program is launched, Windows Installer checks the existence of key paths. If there is a mismatch between the current system state and the value specified in the MSI package (e.g., a key file is missing), the related feature is re-installed. This process is known asself-healing orself-repair. No two components should use the same key path.

Creating an installer package for a new application is not trivial. It is necessary to specify which files must be installed, to where and with what registry keys. Any non-standard operations can be done using Custom Actions, which are typically developed inDLLs. There are a number of commercial and freeware products to assist in creating MSI packages, includingVisual Studio (natively up to VS 2010,^[8] with an extension on newer VS versions^[9]),InstallShield,WiX,InnoSetup, InstallSimple and Advanced Installer. To varying degrees, the user interface and behavior may be configured for use in less common situations such as unattended installation. Once prepared, an installer package is "compiled" by reading the instructions and files from the developer's local machine, and creating the .msi file.

Windows Installer may be slower than native code installation technologies, such asInstallAware,^[10] due to the overhead of component registration and rollback support, which often involves generating tens of thousands of registry keys and temporary files.

The user interface (dialog boxes) presented at the start of installation can be changed or configured by the setup engineer developing a new installer. There is a limited language of buttons, text fields and labels which can be arranged in a sequence of dialogue boxes. An installer package should be capable of running without any UI, for what is called "unattended installation".

Microsoft provides a set of Internal Consistency Evaluators (ICE) that can be used to detect potential problems with an MSI database.^[11] The ICE rules are combined into CUB files, which are stripped-down MSI files containing custom actions that test the target MSI database's contents for validation warnings and errors. ICE validation can be performed with the Platform SDK tools Orca and msival2, or with validation tools that ship with the various authoring environments.

For example, some of the ICE rules are:

• ICE09: Validates that any component destined for the System folder is marked as being permanent.
• ICE24: Validates that the product code, product version, and product language have appropriate formats.
• ICE33: Validates that the Registry table is not used for data better suited for another table (Class, Extension, Verb, and so on).

Addressing ICE validation warnings and errors is an important step in the release process.

• APPX – Software package format used on Microsoft's Windows Store
• App-V – Software package format used for virtualization and streaming
• .exe
• List of installation software
• Package management system
• Windows Package Manager
• ZAP file – a way to perform an application installation when no MSI file exists

• Official website

• Windows commands
• Installation software
• Microsoft application programming interfaces
• Windows administration
• Windows components
• Windows-only software

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