Using Visual Studio 2010 to build a setup project that installs a Windows Forms application .Net 4.0 C#. It has worked fine for ages but now when I'm trying to install the finished setup file, I'm getting this error message:
Error 1001. Unable to get installed types in the "Path" assembly. -->
Unable to load one or more of the requested types. Retrieve the
LoaderExceptions property for more information.
Based on the error message in your second comment, it appears that your SysDir.exe assembly has been added as a Custom Action with the InstallerClass property set to true, but either no installer classes could be found in the exe or the exe could not be loaded due to missing dependencies.
You can see the list of Custom Actions by right-clicking on the installer project, selecting View and then Custom Actions.
If your exe does not can an installer class, then you can remove it from the list of custom actions.
If it does contain an installer class, then the issue is going to be missing dependencies. If fuslogvw doesn't work for you (it has always helped resolve this kind of issue for us), you can carefully review the list of references in the exe's project and compare them to what is listed in the installer project.
The other trick that we use is to examine the install directory while the error message is displayed on the screen. We can often see that DLLs are missing by doing this, usually because the path was entered incorrectly in the DLL entry within the installer project or because a condition was set incorrectly.