Iterator blocks, missing methods, and .NET 4.5

We had some fun a while ago where we had a flurry of MissingMethodException occurring on some boxes, in particular, complaining that Environment.CurrentManagedThreadId was missing. And sure enough, this is a 4.5 method, and the affected servers were 4.0 – so it was kinda valid, but completely unexpected. At the time, our most pragmatic option was just: deploy 4.5, but we’ve finally had time to go back and understand it!

This happened after we upgraded our build server to 4.5, for some initial 4.5 usage – but all our core projects were still 4.0 at the time.

Fix first:

MAKE SURE YOUR BUILD SERVER HAS THE REFERENCE ASSEMBLIES FOR 4.0!

These are typically somewhere like:

C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0

Now, the full story:

fact 1: 4.5 is an in-place over-the-top install on top of 4.0, in the GAC; once you have installed 4.5, 4.0 runs with the 4.5 assemblies

fact 2: if your build server doesn’t have the reference assemblies, it looks in the GAC – so… once you’ve installed 4.5, it will get 4.5 even if you asked (in your project) for 4.0

fact 3: the c# compiler has a detail relating to iterator blocks (aka "yield return" and "yield break") that involves threading, i.e. "which thread am I?"

fact 4: the implementation for this changes between 4.0 and 4.5; in 4.0, it uses (in the generated iterator-type’s constructor) Thread.CurrentThread.ManagedThreadId – but in 4.5 it uses Environment.CurrentManagedThreadId; it makes this decision based on the framework version it finds it has loaded (taking into account targeting etc)

Put these pieces together, and you can get a scenario where a 4.0 project that uses 4.0-only methods, seems to build successfully, but builds in a way that will only work on 4.5; it will never work on 4.0.

Fun fun fun!