| IBackendHelper | Interface provided to help backend extensions. |
| IBundleValidator | Interface provided to help with bundle validation. |
| IBurnBackendHelper | Interface provided to help Burn backend extensions. |
| ICommandLine | Command-line parsing mechanism. |
| ICommandLineParser | Provides the command-line arguments. |
| IExtensionManager | Loads extensions and uses the extensions' factories to provide services. |
| IFileResolver | Interface to resolve file paths using extensions and bind paths. |
| IFileSystem | Abstracts basic file system operations. |
| ILayoutServices | Interface provided to track files for use by layout later. |
| IMessaging | Interface for handling messages (error/warning/verbose). |
| IParseHelper | Interface provided to help compiler and optimizer extensions parse. |
| IPathResolver | Support for processing paths. |
| IPreprocessHelper | Interface provided to help preprocessor extensions. |
| IVariableResolution | Result when resolving a variable. |
| IVariableResolver | |
| IWindowsInstallerBackendHelper | Interface provided to help Windows Installer backend extensions. |
| IWindowsInstallerDecompilerHelper | Interface provided to help Windows Installer decompiler extensions. |
| IWixBranding | WiX branding interface. |
| IWixToolsetCoreServiceProvider | The core of the service provider used to add services to the service provider. |