Try-Catch-Finally block issues with .NET4.5.1

Question

I have a simple test code that works as expected in .NET3.5, but the same code behaves completely different on a project created with .NET4.5.1.

class Program
{
         


        

Answer 1

string a = null;
var x = a.Length;

In RELEASE mode, the jitter (just in time compiler) is able to prove that x is never referenced, so is able to remove the assignment.

In DEBUG mode, the jitter does not perform that optimization.

To force the exception to be thrown, do something with x (e.g. as @Henk suggests in the comments, WriteLine(x)).

EDIT

Eric Lippert noted in the comments

...I am as surprised as anyone that the jitter would elide a string length instruction that could throw. That seems wrong to me...

The jitter optimization may be overly-aggressive.