Why are public fields faster than properties?

Question

I was poking around in XNA and saw that the Vector3 class in it was using public fields instead of properties. I tried a quick benchmark and found that, for a struct the difference is quite dramatic (adding two Vectors together a 100 million times took 2.0s with properties and 1.4s with fields). For a reference type, the difference doesn't seem to be that large but it is there.

So why is that? I know that a property is compiled into get_X and set_X methods, which would incur a method call overhead. However, don't these simple getters/setters always get in-lined by the JIT? I know you can't guarantee what the JIT decides to do, but surely this is fairly high on the list of probability? What else is there that separates a public field from a property at the machine level?

And one thing I've been wondering: how is an auto-implemented property (public int Foo { get; set; }) 'better' OO-design than a public field? Or better said: how are those two different? I know that making it a property is easier with reflection, but anything else? I bet the answer to both questions is the same thing.

BTW: I am using .NET 3.5 SP1 which I believe fixed issues where methods with structs (or methods of structs, I'm not sure) weren't in-lined, so that isn't it. I think I am using it at least, it's certainly installed but then again, I'm using Vista 64-bit with SP1 which should have DX10.1 except that I don't have DX10.1 ..

Also: yeah, I've been running a release build :)

EDIT: I appreciate the quick answers guys, but I indicated that I do know that a property access is a method call, but that I don't know why the, presumably, in-lined method is slower than a direct field access.

EDIT 2: So I created another struct that used explicit GetX() methods (o how I don't miss my Java days at all) and that performed the same whether I disabled in-lining on it (through [MethodImplAttribute(MethodImplOptions.NoInlining)]) or not, so conclusion: non-static methods are apparently never inlined, not even on structs.

I thought that there were exceptions, where the JIT could optmize the virtual method call away. Why can't this happen on structs which know no inheritance and thus a method call can only point to one possible method, right? Or is that because you can implement an interface on it?

This is kind of a shame, since it will really make me think about using properties on performance critical stuff, yet using fields makes me feel dirty and I might as well write what I'm doing in C.

EDIT 3: I found this posting about the exact same subject. His end conclusion is that the property call did get optimized away. I also could've sworn that I've read plenty of times that simple getter/setter properties will get in-lined, despite being callvirt in the IL. So am I going insane?

EDIT 4: Reed Copsey posted the answer in a comment below:

Re: Edit3 - see my updated comment: I believe this is x86 JIT vs x64 JIT issues. the JIT in x64 is not as mature. I'd expect MS to improve this quickly as more 64 bit systems are coming online every day. – Reed Copsey

And my response to his answer:

Thanks, this is the answer! I tried forcing a x86 build and all methods are equally fast, and much faster than the x64. This is very shocking to me actually, I had no idea I was living in the stone age on my 64-bit OS.. I'll include your comment in my answer so it stands out better. – JulianR

Thanks everyone!

Answer 1

• Public fields are direct assignments
• Properties are methods, then more code, insignificant but more.

Answer 2

Edit 2:

I had another potential thought here:

You mentioned that you are running on x64. I've tested this same issue on x86, and seen the same performance when using auto-properties vs. fields. However, if you look around on Connect and mailing list/forum posts, there are many references online to the fact that the x64 CLR's JIT is a different code base, and has very different performance characteristics to the x86 JIT. My guess is this is one place where x64 is still lagging behind.

Also, FYI, the struct/method/etc thing fixed in .net 3.5sp1 was on the x86 side, and was the fact that method calls that took structs as a parameter would never be inlined on x86 prior to .net3.5sp1. That's pretty much irrelevant to this discussion on your system.

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Edit 3:

Another thing: As to why XNA is using fields. I actually was at the Game Fest where they announced XNA. Rico Mariani gave a talk where he brought up many of the same points that are on his blog. It seems the XNA folks had similar ideas when they developed some of the core objects. See:

http://blogs.msdn.com/ricom/archive/2006/09/07/745085.aspx

Particularly, check out point #2.

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As for why automatic properties are better than public fields:

They allow you to change the implementation in v2 of your class, and add logic into the property get/set routines as needed, without changing your interface to your end users. This can have a profound effect on your ability to maintain your library and code over time.

---- From original post - but discovered this wasn't the issue--------

Were you running a release build outside of VS? That can be one explanation for why things aren't being optimized. Often, if you are running in VS, even an optimized release build, the VS host process disables many functions of the JIT. This can cause performance benchmarks to change.

Answer 3

You should read this article by Vance. It goes into detail about why methods are not always inlined by the JIT'er even if it looks completely obvious that they should be.

http://blogs.msdn.com/vancem/archive/2008/08/19/to-inline-or-not-to-inline-that-is-the-question.aspx

Answer 4

XNA has to target the XBox 360, and the JIT in the .NET Compact Framework isn't as sophisticated as its desktop counterpart. The .NET CF JIT'er won't inline property methods.

Answer 5

Accessing a field is just a memory reference whereas using a property is actually invoking a method and includes the function call overhead. The reason to use properties rather than fields is to insulate your code from changes and provide better granularity over access. By not exposing your field directly you have greater control over how the access is done. Using automatic fields allows you to get the typically getter/setter behavior but builds in the ability to change this without a subsequent need for the changes to propagate to other parts of the code.

For example, say that you want to change your code so that access to a field is controlled by the current user's role. If you had exposed the field publicly, you'd have to touch every part of the code that accessed it. Exposing it through a property allows you to modify the property code to add your new requirement but doesn't result in unnecessary changes to any code that accesses it.