Being quite a perfectionist, there was one issue in older versions of my MZ-Tools 8.0 add-in for VBA that was not right for me: it required .NET Framework 2.0, which is installed by default on Windows 7, but not on Windows 8, Windows 8.1 or Windows 10, it’s an optional feature:

So, installing MZ-Tools 8.0 for VBA on Windows 8.0 or higher likely you would get this dialog:

And then the user should download and install .NET Framework 2.0. Needless to say, this is mostly inconvenient and can hurt sales, so I thought it would be nice if MZ-Tools 8.0 for VBA could use whatever higher .NET Framework was installed on Windows (such as .NET Framework 4.0 on Windows 8 or higher).

Before digging on how this can be done, some basics: a .NET Framework is composed of a Common Language Runtime (CLR), which contains the virtual machine for .NET, with the JIT compiler, garbage collector, etc. and a set of libraries. So:

.NET Framework = CLR + libraries

There are three kinds of .NET Frameworks:

1. .NET Frameworks that provide their own, new, CLR. This has been the case for:
   • .NET Framework 1.0 (provides CLR 1.0)
   • .NET Framework 1.1 (provides CLR 1.1)
   • .NET Framework 2.0 (provides CLR 2.0)
   • .NET Framework 4.0 (provides CLR 4.0)
2. .NET Frameworks that don’t provide a new CLR, they just provide new libraries. This has been the case for:
   • .NET Framework 3.0 (uses CLR 2.0 of .NET Framework 2.0)
   • .NET Framework 3.5 (uses CLR 2.0 of .NET Framework 2.0)
3. .NET Frameworks that provide an update of an existing CLR (backwards compatible). This has been the case for:
   • .NET Framework 4.5, 4.5.1, 4.5.2, 4.6, etc. (all of them update the CLR 4.0).

A native process (such as an Office app) must load a CLR in order to load a managed (.NET) add-in. CLRs 1.0, 1.1 and 2.0 cannot coexist in the same process, that is, once of those CLRs is loaded, no other CLR version can be loaded in the same process. However, CLR 4.0 can coexist with CLR 2.0 in the same process.

Note: from this point, I will talk only about CLR 2.0 / CLR 4.0 (.NET Framework 2.0 or higher).

So, there are two aspects that an add-in for VBA must consider:

• The lowest .NET Framework that it requires, which depends on the libraries used by the add-in. MZ-Tools 8.0 uses WinForms (not WPF), so it can work on .NET Framework 2.0 (it doesn’t require libraries from .NET Framework 3.x or 4.x).
• The lowest CLR that it requires, which depends on the lowest .NET Framework that it requires. Since MZ-Tools 8.0 requires .NET Framework 2.0, the lowest CLR is 2.0.

Now the question is: given a native process that must load a .NET-based add-in, which CLR version does it use?.

An .exe process can have an .exe.config file that specifies the supported/required runtimes of the .NET Framework. Visual Studio uses such .config files but Office apps don’t, so they don’t care about CLR versions (they can use CLR 2.0, CLR 4.0, or even both at the same time). By no means an add-in should install an .exe.config file to modify the behavior of an Office application regarding supported/required CLRs, because it can break other add-ins.

However, there are still two approaches that an add-in written against .NET 2.0 (or .NET 3.x, which also uses CLR 2.0) can use to work with CLR 2.0 on systems that have it installed (such as Windows 7) and with CLR 4.0 on systems that don’t have CLR 2.0 installed but have CLR 4.0 installed (such as Windows 8, Windows 8.1 or Windows 10):

The first approach is to make the setup to detect which CLR versions are installed on the system:

• If CLR 2.0 is detected, register the .NET assembly for COM Interop setting the Runtimeversion value of the InprocServer32 key to v2.0.50727, as it would be the usual since it’s an assembly compiled against .NET 2.0.
• If CLR 2.0 is not detected and CLR 4.0 is detected, set the Runtimeversion value to  v4.0.30319. This forces the assembly to use .NET 4.0 even if it was compiled against .NET 2.0:

Note: to register the assembly for COM Interop, the setup cannot use regasm.exe, not even the version of .NET 4.0 (C:\Windows\Microsoft.NET\Framework\v4.0.30319\regasm.exe) because regasm.exe sets always the Runtimeversion value to the value of the CLR that the assembly was compiled against. Instead, the setup must create the registry entries for COM Interop directly (see the required entries here).

The second approach is to use a COM Shim, which I explained in Isolating .NET-based add-ins for the VBA editor with COM Shims. Since a COM Shim is a native C++ dll that loads the CLR to create a custom AppDomain and to load the assembly, it can decide which CLR version to load, depending on which CLRs are installed on the machine: if CLR 2.0 is installed, it loads it, and if CLR 2.0 is not installed but CLR 4.0 is installed, it loads CLR 4.0.

Office 2010 introduced the 64-bit version, which supports only 64-bit COM add-ins. This meant that all add-ins created with VB6 for its VBA editor stopped to work, because they are 32-bit add-ins. The only possibilities to create 64-bit add-ins are using C++ 64-bit, or using .NET x64 and COM Interop. So, it is not a surprise that quite a few add-ins for the VBA editor are created now using .NET. I wrote the article  HOWTO: Create an add-in for the VBA editor (32-bit or 64-bit) of Office with Visual Studio .NET back in 2012 along with a few others referenced at the end of that article. One thing that many developers of add-ins for the VBA editor may not know is that by default all .NET-based add-ins are loaded in the same AppDomain (a .NET concept for isolation of assemblies), and therefore are not properly isolated. A couple of bad things that can happen are the following:

1. “System.InvalidCastException: Unable to cast object of type ‘<Namespace of add-in 1>.VBProjectClass’ to type ‘<Namespace of add-in 2>._VBProject'”. This exception happens because two add-ins use different Interop Assemblies (IAs) for the “Microsoft Visual Basic for Applications Extensibility 5.3” COM reference. For example, my MZ-Tools add-in uses its own local Interop Assemblies, while others use their own Interop Assemblies or Primary Interop Assemblies (PIAs) provided by Microsoft. I have good reasons to use my own IAs, but I have tested that even two add-ins using PIAs can suffer the problem depending on the version of the PIAs or their locations. This problem happens because both add-ins are loaded in the same AppDomain, otherwise it wouldn’t happen.
2. Since Office is a COM application, .NET-based add-ins use Runtime Callable Wrappers (RCWs). It is important that an add-in releases its count on RCWs when no longer used, calling Marshal.ReleaseComObject. For example, if a .NET add-in calls VBE.SelectedVBComponent.CodeModule.CodePane, it is leaking RCWs for VBComponent and CodeModule. How bad is this depends on the host, but suffice to say that for VB6 you can freeze the code editor. The article Office application does not quit after automation from Visual Studio .NET client explains the issue when automating Office but it applies also to using the extensibility model for VBA. The problem is that RCWs are created by AppDomain, so when an add-in needs a RCW, if another add-in in the same AppDomain already created one for the same COM object, the same RCW is reused by the second add-in. Apart from an add-in failing to decrease its usage count on a RCW, it can also happen than an add-in decreases usage counts not belonging to it, for example calling Marshal.FinalReleaseComObject.

So, how are both problems solved? The solution is well-known for developers of add-ins for Office applications using .NET since 2002 or 2003, through the use of a COM Shim. A COM Shim is an ActiveX dll, written in C++, that acts as a native add-in, and loads the Common Language Runtime (CLR) of .NET, creates an AppDomain, and loads the .NET-based add-in. Calls to the native C++ add-in (OnConnection, OnDisconnection, etc.) are passed to the managed (.NET) add-in.

If you are a developer of .NET-based add-ins for the VBA editor, the following are the MSDN articles that you need to read, understand and apply for your add-in to be a good citizen, running isolated from other add-ins in its own AppDomain. When reading the articles, replace “add-in for Office” or “Office extension” by “add-in for VBA editor”:

Isolating Office Extensions with the COM Shim Wizard
This article is relevant because it explains all about isolation and COM Shims.

Isolating Microsoft Office Extensions with the COM Shim Wizard Version 2.0 (VS 2005)
This article is relevant because it doesn’t use the Managed Aggregator introduced in next version 2.3. Add-ins don’t require “aggregation”, just “containment” is required.

Isolating Microsoft Office Extensions with the COM Shim Wizard Version 2.3 (VS 2008)
This article is relevant because it uses “aggregation” instead of “containment”, and you need to realize that add-ins don’t require “aggregation” and you can remove with some adjustments the ManagedAggregator project and its output assembly.

COM Shim Wizards for VS 2010 (2.3.2.0)
This article is relevant because it’s the latest available COM Shim wizard with bug fixes, yet using “aggregation”. I used this and then removed “aggregation” to use “containment” of version 2.0.

Taking COM Shim Wizards to 64-bit
This article is relevant because it explains how to port the COM Shim to 64-bit.

There are two ways of knowing which AppDomain an add-in is loaded into:

Using Process Explorer, you can use the .NET Assemblies tab of the properties window of a process. For example, this image shows that MZ-Tools is loaded in its own MZToolsAppDomain using CLR 4.0:

For some reason that I don’t know, Process Explorer doesn’t show AppDomains for CLR 2.0:

The other way is calling from the own add-in System.AppDomain.CurrentDomain and showing its FriendlyName property. This image shows the MZ-Tools uses its own MZToolsAppDomain for CLR 2.0 (that Process Explorer didn’t show):

If not using the COM Shim, the AppDomain is the DefaultDomain: