Why does LayoutKind.Sequential work differently if a struct contains a DateTime field?

Question

Why does LayoutKind.Sequential work differently if a struct contains a DateTime field?

Consider the following code (a console app which must be compiled with \"unsaf

Answer 1

Go read the specification for layout rules more carefully. Layout rules only govern the layout when the object is exposed in unmanaged memory. This means that the compiler is free to place the fields however it wants until the object is actually exported. Somewhat to my surprise, this is even true for FixedLayout!

Ian Ringrose is right about compiler efficiency issues, and that does account for the final layout that is being selected here, but it has nothing to do with why the compiler is ignoring your layout specification.

A couple of people have pointed out that DateTime has Auto layout. That is the ultimate source of your surprise, but the reason is a bit obscure. The documentation for Auto layout says that "objects defined with [Auto] layout cannot be exposed outside of managed code. Attempting to do so generates an exception." Also note that DateTime is a value type. By incorporating a value type having Auto layout into your structure, you inadvertently promised that you would never expose the containing structure to unmanaged code (because doing so would expose the DateTime, and that would generate an exception). Since the layout rules only govern objects in unmanaged memory, and your object can never be exposed to unmanaged memory, the compiler is not constrained in its choice of layout and is free to do whatever it wants. In this case it is reverting to the Auto layout policy in order to achieve better structure packing and alignment.

There! Wasn't that obvious!

All of this, by the way, is recognizable at static compile time. In fact, the compiler is recognizing it in order to decide that it can ignore your layout directive. Having recognized it, a warning here from the compiler would seem to be in order. You haven't actually done anything wrong, but it's helpful to be told when you've written something that has no effect.

The various comments here recommending Fixed layout are generally good advice, but in this case that wouldn't necessarily have any effect, because including the DateTime field exempted the compiler from honoring layout at all. Worse: the compiler isn't required to honor layout, but it is free to honor layout. Which means that successive versions of CLR are free to behave differently on this.

The treatment of layout, in my view, is a design flaw in CLI. When the user specifies a layout, the compiler shouldn't go lawyering around them. Better to keep things simple and have the compiler do what it is told. Especially so where layout is concerned. "Clever", as we all know, is a four letter word.