The order simply isn’t guaranteed; whatever happens…. Happens.
Obvious cases where it could change:
- anything that implements ICustomTypeDescriptor
- anything with a TypeDescriptionProvider
But a more subtle case: partial classes. If a class is split over multiple files, the order of their usage is not defined at all. See Is the “textual order” across partial classes formally defined?
Of course, it isn’t defined even for a single (non-partial) definition ;p
But imagine
File 1
partial class Foo {
public int A {get;set;}
}
File 2
partial class Foo {
public int B {get;set:}
}
There is no formal declaration order here between A and B. See the linked post to see how it tends to happen, though.
Re your edit; the best approach there is to specify the marshal info separately; a common approach would be to use a custom attribute that takes a numeric order, and decorate the members with that. You can then order based on this number. protobuf-net does something very similar, and frankly I’d suggest using an existing serialization library here:
[ProtoMember(n)]
public int Foo {get;set;}
Where “n” is an integer. In the case of protobuf-net specifically, there is also an API to specify these numbers separately, which is useful when the type is not under your direct control.