The IllegalPrepareConstrainedRegion managed debugging assistant (MDA) is activated when a System.Runtime.CompilerServices.RuntimeHelpers.PrepareConstrainedRegions method call does not immediately precede the try statement of the exception handler. This restriction is at the MSIL level, so it is permissible to have non-code-generating source between the call and the try, such as comments.
A constrained execution region (CER) that is never treated as such, but as a simple exception handling block (finally or catch). As a consequence, the region does not run in the event of an out-of-memory condition or a thread abort.
The preparation pattern for a CER is not followed correctly. This is an error event. The PrepareConstrainedRegions method call used to mark exception handlers as introducing a CER in their catch/finally/fault/filter blocks must be used immediately before the try statement.