Hosts of the common language runtime (CLR), such as Microsoft SQL Server 2005, can establish different policies for failures in critical and non-critical regions of code. A critical region is one in which the effects of a thread abort or an unhandled exception might not be limited to the current task. By contrast, an abort or failure in a non-critical region of code affects only the task in which the error occurs.

For example, consider a task that attempts to allocate memory while holding a lock. If the memory allocation fails, aborting the current task is not sufficient to ensure stability of the AppDomain, because there can be other tasks in the domain waiting for the same lock. If the current task is terminated, other tasks could be deadlocked.

When a failure occurs in a critical region, the host might decide to unload the entire AppDomain rather than take the risk of continuing execution in a potentially unstable state. To inform the host that your code is entering a critical region, call Thread.BeginCriticalRegion. Call Thread.EndCriticalRegion when execution returns to a non-critical region of code.

Using this method in code that runs under SQL Server 2005 requires the code to be run at the highest host protection level.