Allow field layout to be determined for more types

[jack-pappas]

Use the FormatterServices.GetUninitializedObject() method to get an instance of a specified type without having to invoke one of the ctors for the type; this allows the field layout to be determined for a wider set of types, e.g., types whose ctors check-then-throw for null arguments. An additional benefit of this change is that because the ctors for those types are no longer run, they no longer raise exceptions which cause the VS debugger to break (pause execution).

I built/ran the full test suite after making the change, and all of the tests passed except for one (an ISO8601-related test for TimeSpan) which doesn't seem related.

[Wazner]

If this pull request is approved, it will fix issue #207 . The failing unit test is no issue, I've found that some of the tests fail randomly because of a precision error.

As explained in the issue, GetUnitializedObject has a performance penalty of around 11%. I see you're always using it when it's available. It might benefit performance to only call GetUnitializedObject when no parameterless constructor is available.

[Timovzl]

As explained in the issue, GetUnitializedObject has a performance penalty of around 11%. I see you're always using it when it's available. It might benefit performance to only call GetUnitializedObject when no parameterless constructor is available.

I strongly urge against this. In the spirit of correctness over performance, I suggest sticking to GetUninitializedObject(). Calling the constructor sometimes is more obscure, and could surprise the developer. More importantly, the behavior in question seems to be used even when serializing for the first time, if I remember correctly. When breakpoints are active, or there are other expectations on code [not] being hit, it is very disconcerting if a constructor is unexpectedly hit when serializing. When you serialize to string, you don't expect new instances of the classes in question to be constructed. And who knows - good practice or not, some constructors have side-effects.