VS2005 – Unable to add reference to Microsoft.Office.Interop.Word (Office 2010)

With the PIAs installed, you have to add a reference to the corresponding COM library - instead of generating an automatic interop, the PIA will be used instead.

To quote Microsoft's documentation

For Microsoft Office applications that do not have projects in Visual Studio Tools for Office, you must add a reference to the appropriate application or component to your project manually. Adding a reference to the component references the primary interop assembly, if the assembly is installed in the global assembly cache. Office applications and components are accessible from the COM tab of the Add Reference dialog box.

It seems like your code reference one version of Office, but is trying to use a different version. This is quite a common issue since a lot of different versions of Office are in use.

When I've had to use Office Interop, I avoid this issue by using Late Binding rather than Early Binding. This means that I don't add a reference to a specific version of Office and my code will work with any recent version as long as I'm not using a function or similar that only exist in certain versions.

This article includes basic tutorials to show you the difference between late and early binding: Binding for Office automation servers with Visual C# .NET

I'd also suggest looking at this article for more background information: Using early binding and late binding in Automation