We have the unfortunate honour of supporting multiple versions of the .NET Framework across our entire software suite.

All of our client side software (i.e. the stuff that is installed on client machines, both desktop and server) tends to target .NET Framework 4.0. Historically, this was the last version available on Windows XP, and up until relatively recently, it was important that we continue to support that ancient and terrifying beast. You know how it works, old software, slow clients, we’ve all been there.

Meanwhile, all of our server side software tends to target .NET Framework 4.5. We really should be targeting the latest .NET Framework (as the deployment environments are fully under our control), but it takes effort to roll that change out (new environments mostly) and it just hasn’t been a priority. Realistically our next major move might be to just start using .NET Core inside Docker images instead, so it might never really happen.

One of the main challenges with supporting both .NET 4.0 and .NET 4.5 is trying to build libraries that work across both versions. Its entirely possible, but is somewhat complicated and unless you understand a bunch of fundamental things, can appear confusing and magical.

Manual Targeting Mode Engaged

The first time we had to support both .NET 4.0 and .NET 4.5 for a library was for Serilog.

Serilog by itself works perfectly fine for this sort of thing. As of version 1.5.9, it supported both of the .NET Framework versions that we needed in a relatively painless manner.

As is almost always the way, we quickly ran into a few situations where we needed some custom components (a new Sink, a custom JSON formatter, some custom destructuring logic), so we wrapped them up in a library for reuse. Targeting the lowest common denominator (.NET 4.0), we assumed that we would be able to easily incorporate the library wherever we needed.

The library itself compiled file, and all of its tests passed.

We incorporated it into one the pieces of software that targeted .NET 4.0 and it worked a treat. All of the custom components were available and worked exactly as advertised.

Then we installed it into a piece of software targeting .NET 4.5 and while it compiled perfectly fine, it crashed and burned at runtime.

What had actually happened was that Serilog changed a few of its public interfaces between 4.0 and 4.5, replacing some IDictionary<TKey, TValue> types with IReadOnlyDictionary<TKey, TValue>. When we created the library, and specifically targeted .NET 4.0, we thought that it would use that version of the framework all the way through, even if we installed it in something targeting .NET 4.5. Instead, when the Serilog Nuget package was installed into the destination via the dependency chain, it came through as .NET 4.5, while our poor library was still locked at 4.0.

At runtime the two disagreed and everything exploded.

Use The Death Dot

The easiest solution would have been to mandate the flip to .NET 4.5. We actually first encountered this issue quite a while ago, and couldn’t just drop support for .NET 4.0, especially as this was the first time we were putting together our common logging library. Building it such that it would never work on our older, still actively maintained components would have been a recipe for disaster.

The good news is that Nuget packages allow for the installation of different DLLs for different target frameworks. Unsurprisingly, this is exactly how Serilog does it, so it seemed like the sane path to engage on.

Targeting specific frameworks is pretty straightforward. All you have to do is make sure that your Nuget package is structured in a specific way:

  • lib
    • net40
      • {any and all required DLLs for .NET 4.0}
    • net45
      • {any and all required DLLs for .NET 4.5}

In the example above I’ve only included the options relevant for this blog post, but there are a bunch of others.

When the package is installed, it will pick the appropriate directory for the target framework and add references as necessary.  Its actually quite a neat little system.

For us, this meant we needed to add a few more projects to the common logging library, one for .NET 4.0 and another for .NET 4.5. The only thing inside these projects was the CustomJsonFormatter class, because it was the only thing we were interacting with that had a different signature depending on the target framework.

With the projects in place (and appropriate tests written), we constructed a custom nuspec file that would take the output from the core project (still targeting .NET 4.0), and the output from the ancillary projects and structure them in the way that Nuget wanted. We also had to add Nuget package dependencies into the nuspec file as well, because once you’re using a hand-rolled nuspec file, you don’t get any of the nice automatic things that you would if you were using the automatic package creation from a .csproj file.

Target Destroyed

With the new improved Nuget package in place, we could reference it from both .NET 4.0 and .NET 4.5 projects without any issue.

At least when the references were direct anyway. That is, when the Nuget package was referenced directly from a project and the classes therein only used in that context.

When the package was referenced as part of a dependency chain (i.e. project references package A, package A references logging package) we actually ran into the same problem again, where the disconnect between .NET 4.0 and .NET 4.5 caused irreconcilable differences in the intervening libraries.

It was kind of line async or generics, in that once you start doing it, you kind of have to keep doing it all the way down.

Interestingly enough, we had to use a separate approach to multi-targeting for one of the intermediary libraries, because it did not actually have any code that differed between .NET 4.0 and .NET 4.5.  We created a shell project targeting .NET 4.5 and used csproj trickery to reference all of the files from the .NET 4.0 project such that both projects would compile to basically the same DLL (except with different targets).

I don’t recommend this approach to be honest, as it looks kind of like magic unless you know exactly what is going on, so is a bit of a trap for the uninitiated.

Regardless, with the intermediary packages sorted everything worked fine.


The biggest pain throughout this whole process was the fact that none of the issues were picked up at compilation time. Even when the issues did occur, the error messages are frustratingly vague, talking about missing methods and whatnot, which can be incredibly hard to diagnose if you don’t immediately know what it actually means.

More importantly, it wasn’t until the actual code was executed that the problem surfaced, so not only was the issue only visible at runtime, it would happen during execution rather than right at the start.

We were lucky that we had good tests for most of our functionality so we realised before everything exploded in the wild, but it was still somewhat surprising.

We’re slowly moving away from .NET 4.0 now (thank god), so at some stage I will make a decision to just ditch the whole .NET 4.0/4.5 multi-targeting and mark the new package as a breaking change.

Sometimes you just have to move on.