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

A few factors

• doubles are a lot faster if they are aligned
• CPU caches may work better if there are no “holes” in the struck

So the C# compiler has a few undocumented rules it uses to try to get the “best” layout of structs, these rules may take into account the total size of a struct, and/or if it contains another struct etc. If you need to know the layout of a struct then you should specify it yourself rather than letting the compiler decide.

However the LayoutKind.Sequential does stop the compiler changing the order of the fields.