What is the difference between [In, Out] and ref when using pinvoke in C#?

Question

Is there a difference between using [In, Out] and just using ref when passing parameters from C# to C++?

I've found a couple different SO posts, and some stuff from MSDN as well that comes close to my question but doesn't quite answer it. My guess is that I can safely use ref just like I would use [In, Out], and that the marshaller won't act any differently. My concern is that it will act differently, and that C++ won't be happy with my C# struct being passed. I've seen both things done in the code base I'm working in...

Here are the posts I've found and have been reading through:

Are P/Invoke [In, Out] attributes optional for marshaling arrays? Makes me think I should use [In, Out].

• MSDN: InAttribute
• MSDN: OutAttribute
• MSDN: Directional Attributes

These three posts make me think that I should use [In, Out], but that I can use ref instead and it will have the same machine code. That makes me think I'm wrong -- hence asking here.

Answer 1

The usage of ref or out is not arbitrary. If the native code requires pass-by-reference (a pointer) then you must use those keywords if the parameter type is a value type. So that the jitter knows to generate a pointer to the value. And you must omit them if the parameter type is a reference type (class), objects are already pointers under the hood.

The [In] and [Out] attributes are then necessary to resolve the ambiguity about pointers, they don't specify the data flow. [In] is always implied by the pinvoke marshaller so doesn't have to be stated explicitly. But you must use [Out] if you expect to see any changes made by the native code to a struct or class member back in your code. The pinvoke marshaller avoids copying back automatically to avoid the expense.

A further quirk then is that [Out] is not often necessary. Happens when the value is blittable, an expensive word that means that the managed value or object layout is identical to the native layout. The pinvoke marshaller can then take a shortcut, pinning the object and passing a pointer to managed object storage. You'll inevitably see changes then since the native code is directly modifying the managed object.

Something you in general strongly want to pursue, it is very efficient. You help by giving the type the [StructLayout(LayoutKind.Sequential)] attribute, it suppresses an optimization that the CLR uses to rearrange the fields to get the smallest object. And by using only fields of simple value types or fixed size buffers, albeit that you don't often have that choice. Never use a bool, use byte instead. There is no easy way to find out if a type is blittable, other than it not working correctly or by using the debugger and compare pointer values.

Just be explicit and always use [Out] when you need it. It doesn't cost anything if it turned out not to be necessary. And it is self-documenting. And you can feel good that it still will work if the architecture of the native code changes.