Can C# 'is' operator suffer under release mode optimization on .NET 4?

Question

Below is a simple test fixture. It succeeds in Debug builds and fails in Release builds (VS2010, .NET4 solution, x64):

[TestFixture]
public sealed class Test
{
          


        

Answer 1

The store and load is essentially a nop as far as flow-control goes, but probably massage some CPU caches in some way. The actual flow just loads the argument on the stack, checks if it's an instance (which return null or the instance), pushes null onto the stack, and compares (greater-than) which results in a boolean being left on the stack.

Now what the JITter does with it is another story altogether (and would depend on what platform you are using. The JITter will do all sorts of crazy things in the name of performance (our team recently got hit because tail-call optimization changed to optimize across domain boundaries which broke GetCallingAssembly()). It's possible the JITter is inlining IsDateTime, noticing that there's not way it can't not be a DateTime and just pushing true onto the stack.

It's also possible your release version is targetting a slightly different Framework so DateTime in the test assembly is not DateTime in the tested assembly.

I realize that doesn't answer why your code is breaking.