What does "Cannot evaluate expression because the code of the current method is optimized." mean?

Question

I wrote some code with a lot of recursion, that takes quite a bit of time to complete. Whenever I "pause" the run to look at what's going on I get:

Cannot evaluate expression because the code of the current method is optimized.

I think I understand what that means. However, what puzzles me is that after I hit step, the code is not "optimized" anymore, and I can look at my variables. How does this happen? How can the code flip back and forth between optimized and non-optimzed code?

Answer 1

While the Debug.Break() line is on top of the callstack you can't eval expressions. That's because that line is optimized. Press F10 to move to the next line - a valid line of code - and the watch will work.

Answer 2

The Debugger uses FuncEval to allow you to "look at" variables. FuncEval requires threads to be stopped in managed code at a GarbageCollector safe point. Manually "pausing" the run in the IDE causes all threads to stop as soon as possible. Your highly recursive code will tend to stop at an unsafe point. Hence, the debugger is unable to evaluate expressions.

Pressing F10 will move to the next Funceval Safe point and will enable function evaluation.

For further information review the rules of FuncEval.