Performance of static methods vs instance methods

In theory, a static method should perform slightly better than an instance method, all other things being equal, because of the extra hidden this parameter.

In practice, this makes so little difference that it'll be hidden in the noise of various compiler decisions. (Hence two people could "prove" one better than the other with disagreeing results). Not least since the this is normally passed in a register and is often in that register to begin with.

This last point means that in theory, we should expect a static method that takes an object as a parameter and does something with it to be slightly less good than the equivalent as an instance on that same object. Again though, the difference is so slight that if you tried to measure it you'd probably end up measuring some other compiler decision. (Especially since the likelihood if that reference being in a register the whole time is quite high too).

The real performance differences will come down to whether you've artificially got objects in memory to do something that should naturally be static, or you're tangling up chains of object-passing in complicated ways to do what should naturally be instance.

Hence for number 1. When keeping state isn't a concern, it's always better to be static, because that's what static is for. It's not a performance concern, though there is an overall rule of playing nicely with compiler optimisations - it's more likely that someone went to the effort of optimising cases that come up with normal use than those which come up with strange use.

Number 2. Makes no difference. There's a certain amount of per-class cost for each member it terms of both how much metadata there is, how much code there is in the actual DLL or EXE file, and how much jitted code there'll be. This is the same whether it's instance or static.

With item 3, this is as this does. However note:

1. The this parameter is passed in a particular register. When calling an instance method within the same class, it'll likely be in that register already (unless it was stashed and the register used for some reason) and hence there is no action required to set the this to what it needs to be set to. This applies to a certain extent to e.g. the first two parameters to the method being the first two parameters of a call it makes.

2. Since it'll be clear that this isn't null, this may be used to optimise calls in some cases.

3. Since it'll be clear that this isn't null, this may make inlined method calls more efficient again, as the code produced to fake the method call can omit some null-checks it might need anyway.

4. That said, null checks are cheap!

It is worth noting that generic static methods acting on an object, rather than instance methods, can reduce some of the costs discussed at http://joeduffyblog.com/2011/10/23/on-generics-and-some-of-the-associated-overheads/ in the case where that given static isn't called for a given type. As he puts it "As an aside, it turns out that extension methods are a great way to make generic abstractions more pay-for-play."

However, note that this relates only to the instantiation of other types used by the method, that don't otherwise exist. As such, it really doesn't apply to a lot of cases (some other instance method used that type, some other code somewhere else used that type).

Summary:

1. Mostly the performance costs of instance vs static are below negligible.
2. What costs there are will generally come where you abuse static for instance or vice-versa. If you don't make it part of your decision between static and instance, you are more likely to get the correct result.
3. There are rare cases where static generic methods in another type result in fewer types being created, than instance generic methods, that can make it sometimes have a small benefit to turn rarely used (and "rarely" refers to which types it's used with in the lifetime of the application, not how often it's called). Once you get what he's talking about in that article you'll see that it's 100% irrelevant to most static-vs-instance decisions anyway. Edit: And it mostly only has that cost with ngen, not with jitted code.

Edit: A note on just how cheap null-checks are (which I claimed above). Most null-checks in .NET don't check for null at all, rather they continue what they were going to do with the assumption that it'll work, and if a access exception happens it gets turned into a NullReferenceException. As such, mostly when conceptually the C# code involves a null-check because it's accessing an instance member, the cost if it succeeds is actually zero. An exception would be some inlined calls, (because they want to behave as if they called an instance member) and they just hit a field to trigger the same behaviour, so they are also very cheap, and they can still often be left out anyway (e.g. if the first step in the method involved accessing a field as it was).

When keeping state is not a concern (no fields or properties are required), is it always better to use a static class?

I would say, yes. As declaring something static you declare an intent of stateless execution (it's not mandatory, but an intent of something one would expect)

Where there is a considerable number of these static classes (say 100 for instance, with a number of static methods each) will this affect execution performance or memory consumption negatively as compared with the same number of instance classes?

Don't think so, unless you're sure that static classes are really stateless, cause if not it's easy mess up memory allocations and get memory leaks.

When the [this] keyword is used to call another method within the same instance class, does the instance resolution still occur?

Not sure, about this point (this is a purely implementation detail of CLR), but think yes.