We have a Visual Studio solution with about 90 projects. Most of them are built to DLL files, some are written in C++, others in C#. The projects communicate with each other via COM. We use tlbexp to generate TLB files of some C# projects (the ones that are referenced in C++ projects). And we use tlbimp to generate interop DLLs of the C++ projects. I did not completely dig into the topic, but I think, the interop files just define the interfaces of the C++ classes, to make them usable from other projects is that right?
Now, the question is the following: In order to upgrade the entire solution to Visual Studio 2015 and let it compile against .NET 4.6.1, I inspected the resulting assemblies using dotPeek by JetBrains. I can see, that all the C# projects are correctly using .NET 4.6.1, the C++ DLLs themselves are native and do not reference any .NET. Now, what surprised me was the fact, that dotPeek told me, the interop DLLs (which resulted from the C++ projects), were referencing .NET 4.0.
After a "wonderful" day of trying to make them reference .NET 4.6.1 and lots of research, I finally did not find any way to make the interops reference .NET 4.6.1. Is that even possible? My current guess is, that all interop DLLs generated like this reference just the basic .NET 4.0, just because it uses the same CLR as .NET 4.6.1.
Is that right? It should be possible to execute the assemblies on any system that has .NET Framework 4.6.1 installed, shouldn't it?
You are getting bad info from dotPeek. It doesn't tell you how it figured out what .NET version is targeted. It can be specific on a .NET assembly that's generated by a compiler. Because it automatically inserts a [TargetFramework] attribute into the assembly, it states what version of .NET you selected when you built the project.
But an interop assembly does not specify a .NET version, primarily because it isn't generated by a compiler. Note how you never specified a version when you ran Tlbimp.exe. And can't. All that dotPeek can figure out is that it targets .NET 4, cued by the metadata format. No way it can be more specific.
And it does not matter, since the interop library does not use .NET Framework features. The only point of the library is that your program can use the COM component features. So the version number that dotPeek reports just doesn't matter; any .NET 4.x framework can use the library.
You don't have a real problem.