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How can I create Custom Actions in Managed Languages, like C# ?

Natively, Windows Installer supports custom actions in form of a DLL, an executable, VBScript or JScript. DLL here means a "real" DLL that exports callable functions, typically written in C++. You cannot create such a DLL in Visual Basic.

Technically it is possible to use assemblies as custom actions (although not natively) but in the past this has been discouraged by many experts for the following reasons:

• Functionally they're not as rich. You can pass properties into them, but you can't create property values to return into the running installation.
• They often get used when not needed. For example, Visual Studio setups encourage Installer classes and associated code to install Services despite perfectly good support in Windows Installer with standard tables and actions.
• There are bear traps, such as calling a managed custom action that has a dependency on an assembly you've just installed in the GAC. Unfortunately assemblies aren't actually in the GAC until InstallFinalize, so your custom action fails.
• When they fail, they do so in mysterious ways, mainly because of the black box shim Dll between Windows Installer and your code (InstallUtilLib.Dll). How obvious would it be that "Unknown error in CorBindToRuntimeHost..." is caused by the config file string being in the wrong place? Installation errors can be pretty obscure already, and there's no need to add to that situation! The shim Dll is effectively hiding what's going on, and AFAIK it's undocumented.
• InstallUtil does not integrate properly with the Windows Installer and thus reference counting is not done properly.

Please also read the following article on Aaron Stebner's blog: Don't use managed code to write your custom actions! If you still want to write a custom action in C#, you can find some instructions in this blog enty: WiX: Managed Custom Actions

As of May 2008 there's a new development in the the long debate about custom actions. As part of the Windows Installer XML (WiX) project the Deployment Tools Foundation (DTF) have been published, a framework for .NET custom actions. It is still in beta and can also be used for MSI authoring tools other than WiX. You can find more information here:

• Deployment Tools Foundation joins the WiX toolset (Rob Mensching. leader of the WiX project)
• A brief history of the Deployment Tools Foundation project (Jason Ginchereau, developer of DTF)
• Deployment Tools Foundation (DTF) Custom Actions (Christopher Painter, Windows Installer expert)
• Data Driven Managed Custom Actions made easy with DTF (Christopher Painter)

Some Information in this article was taken from posts by Phil Wilson and Rob Mensching on the wix-users mailing list.

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