Shoud I use LayoutKind.Auto for my structs if they don't perform in COM Interop?

Question

By default structs in C# are implemented with [StructLayout( LayoutKind.Sequential )] for reasons basically stating that these type of objects are commonly used for COM Interop and their fields must stay in the order they were defined. Classes have LayoutKind.Auto defined.

My question is should I explicitly state my structs as [StructLayout( LayoutKind.Auto )] and would this give me any benefits over the default? I mean that if structs are initialized on stack, will it make any difference - i.e. the GC doesn't have to move them around? Also will it help when structs are initialized on the heap - i.e. are part of some class?

Answer 1

The only possible benefit I can think of is your struct taking up less memory. But if you have such a large struct in the first place you should probably refactor it into a class.

A potential problem is it you want to Marshall your struct into a byte[] using Marshal.PtrToStructure, how can you guarantee the order of the bytes will be as you expect?

Doing this just seems like you're introducing more possible problems than those you're solving... That being said if the order of the fields in never important to you, then do it, but bear in mind that the next person who comes alone might not be expecting it.

Answer 2

It may give you benefits, even though I don't suppose that it will do much. I usually stick to the defaults.

Basically, with an auto layout, the CLR can choose how to align data, therefore maybe making some space tradeoffs for speed (this will depend on the platform also, keeping things aligned can be more important in some than in others). However, since structs are also often used on the stack or as composite helper structures (think of KeyValuePair), sequential does usually make sense as default.