(I’m giving up with the numbering now, unless anyone particularly wants me to keep it up. What was originally going to be a limited series appears to be growing without end…)

As Chris Nahr pointed out in my previous post, my earlier idea about staged initialization was very half-baked. As he’s prompted me to think further about it, I’ve come up with another idea. It’s slightly more baked, although there are lots of different possibilities and open questions.

Let’s take a step back, and look at my motivation: I like immutable types. They’re handy when it comes to thread safety, and they make it a lot easier to reason about the world when you know that nothing can change a certain value after it’s been created. Now, the issues are:

• We really want to be able to fully construct the object in the constructor. That means we can mark all fields as initonly in the generated IL, potentially giving the CLR more scope for optimisation.
• When setting more than two or three values (while allowing some to be optional) constructor overloading ends up being a pain.
• Object initializers in C# 3 only apply to properties and fields, not method/constructor arguments – so we can’t get the clarity of naming.
• Ideally we want to support validation (or possibly other code) and automatic properties.
• The CLR won’t allow initonly fields being set anywhere other than in the constructor – so even if we made sure we didn’t call any setters other than in the constructor, we still couldn’t use them to set the fields.
• We want to allow simple construction of immutable types from code other than C#. In particular, I care about being able to use projects like Spring.NET and Castle/Windsor (potentially after changes to those projects) to easily create instances of immutable types without resorting to looking up the order of constructor parameters.

The core of the proposal is to be able to mark properties as initonly, and get the compiler to create an extra type which is thoroughly mutable, and contains those properties – as well as a constructor which accepts an instance of the extra type and uses it to populate the immutable instance of the main type before returning.

Extra syntax could then be used to call this constructor – or indeed, given that the properties are actually readonly, thus avoiding any ambiguity, normal object initializers could be used to create instances.

Just as an example, imagine this code:

That could be transformed into code a bit like this:

That’s the simple case, of course. Issues:

• Unlike other compiler-generated types (anonymous types, types for iterator blocks, types for anonymous functions) we do want this to be public, and have a name which can be used elsewhere. We need to find some way of making sure it doesn’t clash with other names. In the example above, I’ve used an attribute to indicate which type is used for initialization – I could imagine some way of doing this in the “pre-transform” code to say what the auto-generated type should be called.
• What happens if you put code in the setter, instead of making it automatically implemented? I suspect that code should be moved into the setter of the initialization class – but at that point it won’t have access to the rest of the state of the class (beyond the other properties in the initialization class). It’s somewhat messy.
• What if you want to add code to the generated constructor? (Possibly solution: allow constructors to be marked somehow in a way that means “add on the initialization class as a parameter at the end, and copy all the values as a first step.)
• How can you indicate that some parameters are mandatory, and some are optional? (The mandatory parameters could just be marked as readonly properties rather than initonly, and then the initialization class specified as an extra parameter for a constructor which takes all the mandatory ones. Doesn’t feel elegant though, and leaves you with two different types of initialization code being mixed in the client – some named, some positional.)
• How do you specify default values? (They probably end up being the default values of the automatically generated properties of the initialization class, but there needs to be some syntax to specify them.)

I suspect there are more issues too – but I think the benefits would be great. I know the C# team has been thinking about immutability, but I’ve no idea what kind of support they’re currently envisioning. Unlike my previous ideas, which were indeed unpalatable for various reasons, I think this one has real potential. Mind you, given that I’ve come up with it after only mulling this over in “spare” time, I highly doubt that it will be a new concept to the team…

Iterator blocks have an interesting property: they defer execution. When the method (or property) is called, none of your code is executed – it only starts running when MoveNext() is first called.

Deferred execution is a great thing in many ways, but it’s a pain when it comes to parameter checking. If you check parameters within an iterator block, you’ve effectively left a timebomb – the error will be potentially reported a long way from the original source of the problem. (Side-note: this is why I prefer using a cast to as when I know what type something really should be – I’d rather get an InvalidCastException immediately than a NullReferenceException later on.)

One solution to this is to check the parameters in the public method and then call a private method which is implemented with an iterator block. That’s a bit ugly though. It would be nice to be able to specify the parameter checking in an initial block which could only occur at the start of the method – something like this simple implementation of Repeat:

I’ve chosen yield do for two reasons: it fits in with the rest of the yield pattern, and by using an existing keyword I suspect it reduces/eliminates the chance of this being a breaking change. It’s not ideal from the point of self-description, but do is the closest keyword I could find when I looked at the spec. In a language with begin as a keyword, that would probably be a better choice – or initial. fixed appeals in that the code is fixed in the method rather than being shunted off into the compiler-generated type, but the normal use of fixed is far too different to make this an appealing choice. Any other suggestions are very welcome :)

Is this an important enough change to make it worth including in C# 4? I’m not sure. The negative side is that I suspect relatively few people use iterator blocks much, so it may not have a very wide benefit. The positive side is that it only actually makes any difference to those who do use iterator blocks, and I believe almost all of them would find it a welcome addition. It’s simple to understand, and it makes iterator blocks much easier to use in a “correct” manner when any form of initial checking is involved.

Note, mostly to myself and Marc Gravell: I haven’t checked, but I suspect most of Push LINQ is broken in this respect.

Further note to self – I really need to fix my code formatter to treat yield as a contextual keyword.

The final part of this little series is the one where I suggest my own ideas for C# 4, beyond those I’ve already indicated my approval for in earlier posts. Before I talk about individual features, however, I’d like to put forward a manifesto which could perhaps help the decision-making process. I hasten to add that I haven’t run all the previous parts through this manifesto to make sure that I’ve been consistent, but all of these thoughts have been running around in my head for a while so I hope I haven’t been wildly out.

Manifesto for C# 4

I would welcome the following goals:

Remember it’s C#

Many suggestions have been trying to turn C# into either Ruby, LISP, or other languages. I welcome diversity in languages, and I believe in using the right tool for the job – but that means languages should stick to their core principles, too. Now, I know that sounds like I might be bashing C# 3, given how much that has borrowed from elsewhere for lambda expressions and the like, and I don’t know exactly how I square that circle internally – but I don’t want C# to become a giant toolbox that every useful feature from every language in existence is dumped into.

There are useful ideas to think about from all kinds of areas – not just existing languages – but I’d be tempted to reject them if they just don’t fit into C# neatly without redefining the whole thing.

Consider how people will learn it

I’ve mentioned this before, but I am truly worried about people learning C# 3 from scratch. One of the reasons I didn’t attempt to write about C# from first principles, instead assuming knowledge of C# 1, is that I’m not sure people can sensibly learn it that way. Now, I don’t think I can sensibly get inside the head of someone who doesn’t know anything about C#, but I suspect that I’d want to cover query expressions right at the very end, preferably after quite a while of experience in C# without them.

That might not go for every new feature – it’s probably worth knowing about automatic properties right from the start, for instance, and introducing lambda expressions at the same time as anonymous methods (if C# 3 is the definite goal) but expression trees would be pretty late in my list of topics.

I learned C# 1 from a background of Java, and it didn’t take long to understand the basics of the syntax. Many of the subtleties took a lot longer of course (and it was a very long time before I really understood the differences between events and delegates, I’m sad to say) but it wasn’t a hard move. For a long time C# 2 just meant generics as far as I was concerned – with occasional nullable types, and some of the simpler features such as differing access for getters and setters. Anonymous methods and iterator blocks didn’t really hit me in terms of usefulness until much later – possibly due to using closures in Groovy and more iterators in LINQ. I suspect for many C# 2 developers this is still the case.

My method of learning C# 3 (from the spec, often in rather more detail than normal for the sake of accuracy in writing) is sufficiently far off the beaten track as to make it irrelevant for general purposes, but I wonder how people will learn it in reality. How will it be taught in universities (assuming there are any that teach C#)? How easily will developers moving to C# from Java cope? How about from other languages?

Interestingly, the move from VB 9 to C# 3 is now probably easier than the move from Java 6 to C# 3. Even with the differences in generics between Java and C#, that probably wasn’t true with C# 2.

To get back to C# 4, I’d like the improvements to be somehow blend in so that learning C# 4 from scratch isn’t a significantly different experience to learning C# 3 from scratch. It’s bound to be slightly longer as there will certainly be new features to learn – but if they can be learned alongside existing features, I think that’s a valuable asset. It’s also worth considering how much investment will be required to learn C# 4 from a position of understanding C# 3. Going from C# 2 to C# 3 is a significant task – but it’s one which involves a paradigm shift, and of course the payoffs are massive. I’d be very surprised (and disappointed) to see the same level of change in C# 4, or indeed the same level of payoff. Conservative though this is, I’m after “quick wins” for developers – even if in some cases such as covariance/contravariance the win is far from quick from the C# design/implementation team’s perspective.

Just to put things into perspective: think about how many new technologies developers have being asked to learn in the last few years – WCF, WPF, Workflow Foundation, Cardspace, Silverlight, AJAX, LINQ, ClickOnce and no doubt other things I’ve forgotten. I feel a bit like Joseph II complaining that Mozart had written “too many notes” – and if you asked me to start exorcising any of these technologies from history I’d be horrified at the prospect. That doesn’t actually make it any easier for us though. I doubt that the pace of change is likely to slow down any time soon in terms of technologies – let’s not make developers feel like complete foreigners in a language they were happy with.

Keep the language spec understandable

I know there aren’t many people who look at the language spec, but the C# spec has historically been very, very well written. It’s usually clear (even if it can be hard to find the right section at times) and leaves rigidly defined areas of doubt and uncertainty in appropriate places, while stamping out ambiguity in others. The new unified spec is a great improvement over the “C# 1 … and then the new bits in C# 2” approach from before. However, it’s growing at a somewhat alarming rate. As it grows, I expect it to become harder to read as a natural consequence – unless particular effort is put into countering that potential problem.

Stay technology-neutral…

Okay, this one is aimed fairly squarely at VB9. I know various people love XML literals, but I’m not a fan. It just feels wrong to have such close ties with a particular technology in the actual language, even one so widely used as XML. (Of course, there’s already a link between XML and C# in terms of documentation comments – but that doens’t feel quite as problematic.)

My first reaction to LINQ (before I understood it) was that C# was being invaded by SQL. Now that I’ve got a much better grasp of query expressions, I have no concern in that area. Perhaps it would be possible to introduce a new hierarchy construct which LINQ to XML understands with ease – or adapt the existing object/collection initializers slightly for this purpose. With some work, it may be possible to do this without restricting it to XML… I’m really just blue-skying though (and this isn’t a feature on my wishlist.)

… but bear concurrency in mind

While I don’t like the idea of tying C# to any particular technology, I think the general theme of concurrency is going to be increasingly important. That’s far from insightful – just about every technology commentator in the world is predicting a massively parallel computing landscape in the future. Developers won’t be able to get away with blissful ignorance of concurrency, even if not everyone will need to know the nuts and bolts.

Make it easier to do the right thing

This is effectively encouraging developers to “fall into the pit of success”. Often best practices are ignored as being inconvenient or impractical at times, and I’m certainly guilty of that myself. C# has a good history of enabling developers to do the right thing more easily as time progresses: automatic properties, iterator blocks and allowing different getter/setter access spring to mind as examples.

In some ways this is the biggest goal in this manifesto. It’s certainly guided me in terms of encouraging mixin and immutability support, ways of simplifying parameter/invariant checking, and potentially IDisposable implementation. I like features which don’t require me to learn whole new ways of approaching problems, but let me do what I already knew I should do, just more easily.

Wishlist of C# 4 features

With all that out of the way, what would I like to see in C# 4? Hopefully from the above you won’t be expecting anything earth-shattering – which is a good job, as all of these are reasonably minor modifications. Perhaps we could call it C# 3.5, shipping with .NET 4.0. That would really make life interesting, as people are already referring to C# 3 as C# 3.5 (and C# 2008)…

Readonly automatic properties

I’ve mentioned this before, but I’ll give more details here. I’d like to be able to specify readonly instead of protected/internal/private for the setter access, which would:

• Mark the autogenerated backing variable as initonly in IL
• Prevent code outside the constructor from setting the property

So, for example:

This would make it easier to write genuinely (and verifiably, as per Joe’s post) immutable classes, or just immutable parts of classes. As mentioned in previous comments, there could be interesting challenges around serialization and immutability, but frankly they really need to be addressed anyway – immutability is going to be one part of the toolkit for concurrency, whether it has language support or not. In the generated IL the property would only have a getter – calls to the setter in the constructor would be translated into direct variable sets.

This shouldn’t require a CLR change.

Property-scoped variables

I’ve been suggesting this (occasionally) for a long time, but it’s worth reiterating. Every so often, you really want to make sure that no code messes around with a variable other than through a property. This can be solved with discipline, of course – but historically we don’t have a good record on sticking to discipline. Why not get the compiler to enforce the discipline? I would consider code like this:

Just in case Eric’s reading this: yes, having Age as a property of a person is a generally bad idea. Specifying a date of birth and calculating the age is a better idea. Really, don’t use this code as a model for a Person type. However, treat it as a dumb example of a reasonable idea. I need to find myself a better type to use as my first port of call when finding an example…

The variable name would still have to be unique – it would still be the name generated in the IL, for instance. Multiple variables could be declared if required. The generated code could be exactly the same as that of existing code which happened to only use the property to access the variable.

A couple of potential options:

• The variables could be directly accessible during the constructor, potentially. This would help with things like serialization.
• Likewise, potentially an attribute could be applied to other members which needed access to the variables. Bear in mind that we’re only trying to save developers from themselves (and their colleagues). We’re not trying to cope with intruders in a security sense. An active “I know I’m violating my own rules” declaration should cause enough discomfort to avoid the accidental issues we’re trying to avoid.

This shouldn’t require a CLR change.

Extension properties

This has been broadly talked about, particularly in view of fluent interfaces. It feels to me that there are two very different reasons for extension properties:

1. Making fluent interfaces prettier, e.g. 19.June(1976) + 8.Hours + 20.Minutes instead of 19.June(1976) + 8.Hours() + 20.Minutes()
2. Genuine properties, which of course couldn’t add new state to the extended type, but could access it in a different way.

Point 1 feels like a bit of a hack, I must admit. It’s using properties not because the result is “property-like” but because we want to miss the brackets off. It’s been pointed out to me that VB already allows this, and that by brackets to be missed out for parameterless methods we could achieve the same effect – but that just feels wrong. Arguably fluent interfaces already mess around with the normal conventions of what methods do and how they’re named, so using properties like this probably isn’t too bad.

Point 2 is a more natural reason for extension properties. As an example, consider a type which exposes a Size property, but not Width or Height. Changing either dimension individually requires setting the Size to a new one with the same value for the other dimension – this is often much harder to read than judicious use of Height/Width. I suspect that extension properties would actually be used for this reason less often than for fluent interfaces, but there may be any number of interesting uses I haven’t thought of.

This shouldn’t require a CLR change, but framework changes may be required.

Extension method discovery improvements

I’ve made it clear before now that the way extension methods are discovered (i.e. with using directives which import all the extension methods of all the types within the specified namespace) leaves much to be desired. I don’t like trying to reverse bad decisions – it’s pretty hard to do it well – but I really feel strongly about this one. (Interestingly, although I’ve heard many people criticising this choice, I don’t actually remember hearing the C# team defending it. Given that reservations were raised back in 2005, when there was still plenty of time to change stuff, I suspect there are reasons no-one’s thought of. I’d love to hear them some time.)

The goal would be to change from discovering extensions at a namespace to discovering extensions at a type level. (By which I mean at a “type containing extension methods” level – e.g. System.Linq.Enumerable or System.Linq.Queryably. Admittedly discovery on a basis which explicitly specifies the type to extend would also be interesting.) I don’t mind exactly how the syntax works, but the usual ideas are ones such as:

That’s the easy part – the harder part would be working out the best way to phase out the “old” syntax. I would suggest a warning if extension methods are found and used without being explicitly mentioned by type. In C# 4 this could be a warning which was disabled by default (but could be enabled with pragmas or command line switches), then enabled by default in C# 5 (but with the same options available, this time to disable it). By C# 6 we could perhaps remove the ability to discover extension methods by namespace altogether, so the methods just wouldn’t be found any more.

The C# team could be more aggressive than this, perhaps skipping the first step and making it an enabled warning from C# 4 – but I’m happy to leave that kind of thing to them, without paying it much more attention. I know how seriously they take breaking changes.

No CLR changes required as far as I can see.

Implicit “select” at end of query expressions

I can’t say I’ve used VB9’s LINQ support, but I’ve heard about one aspect which has some appeal. In C# 3, every query expression ends with either “select” or “groupby”. The compiler is actually smart enough to ignore a redundant select clause (except for degenerate query expressions ) and indeed the language spec makes this clear. So why require it in the query expression in the first place? As a concrete example of before/after:

This isn’t a massive deal, but it would be quite neat. I worry slightly that there could be significant costs in terms of the specification complexity, however.

Internal members on internal interfaces

Interfaces currently only ever have public members, even if the interfaces themselves are internal. This means that implementing an internal interface in an internal class still means making the implementing method public, or using explicit interface implementation (which imposes other restrictions, particularly in terms of overriding). It would be nice to be able to make members internal when the interface itself is internal – either explicitly or implicitly. Implementing such members publicly would still be allowed, but you could choose to keep the implementation internal if desired.

This may require a CLR change – not sure.

“Namespace+assembly” access restriction

It’s not an uncommon request on the C# newsgroup for the equivalent of C++’s “friend” feature – where two classes have a special relationship. In many ways InternalsVisibleTo is an assembly-wide version of this feature, but I can certainly see how it would be nice to have a slightly finer grained version. Sometimes two classes are naturally tightly coupled, even though they have distinct responsibilities. Although loose coupling is generally accepted to be a good thing, it’s not always practical. At the same time, giving extra access to all the types within the same assembly can be a little bit much.

Instead of specifying particular types to share members with, I’d propose a new access level, which would make appropriately decorated members available to other types which are both within the same assembly and within the same namespace. This would be similar to Java’s “package level” access (the default, for some reason) except without the implicit availability to derived types. (Java’s access levels and defaults are odd to say the least.)

(Of course, this wouldn’t help in assemblies which consisted of types within a single namespace.)

This would almost certainly require a CLR change.

InternalsVisibleTo simplification for strongly named assemblies

This one’s just a little niggle. In order to use the InternalsVisibleToAttribute to refer to a strongly named assembly (which you almost always have to do if the declaring assembly is strongly named), you have to specify the public key. Not the public key token as the documentation claims, but the whole public key. Not only that, but you can’t have any whitespace in it – so you can’t use a verbatim string literal to easily put it in a block. Instead, you either have to have the whole thing on one line, or use compile-time string concatenation to make sure the key is still unbroken.

It’s not often you need to look at the assembly attributes, so it’s far from a major issue – but it’s a mild annoyance which could be fixed with very few downsides.

This may require a CLR change – not sure.

Is that all?

I suspect that soon after posting this, I’ll think of other ideas. Some may be daft, some may be more significant than these, but either way I’ll do a new post for new ideas, rather than adding to this one. I’ll update this one for typos, further explanations etc. I suspect if I don’t post this now I’ll keep tweaking it for hours – which is relatively pointless as I’m really trying to provoke discussion rather than presenting polished specification proposals.

Microsoft haven’t committed to anything in C# 4 yet. However, there have been hints about what they’ve been considering in Eric Lippert’s blog, and more than hints in Charlie Calvert’s blog. There’s not a lot to go on yet, but:

Immutability support

Most of Eric’s posts about immutability have so far been about immutable data structures. However, the first post in the series did mention that they’re playing around with immutability from the point of view of potential language support. Joe Duffy also wrote about immutability at roughly the same time.

What can we expect in terms of support? Possibilities:

• Compiler checking via attributes, as per Joe’s posts
• Immutable collections to make it easier to sensibly embed collections within immutable types
• Readonly automatic properties (as I mentioned in part 1 – I’ll expand on this in the next post)

I’m not sure what could be usefully added beyond those. One possibility, however: what about automatic equality and hashcode generation? I touched on this last time when talking about “instant data types” but I don’t see why it shouldn’t be applicable in general. After all:

• Immutable types are good candidates for dictionary keys – or to put it the other way round, using mutable types as dictionary keys is a risky idea
• If all immutable types automatically override GetHashCode and Equals, and immutable types can only compose other immutable types, everything should just work
• The obvious implementation of Equals and a “multiply, add, repeat” implementation of GetHashCode are pretty reasonable for many, many types. You could always manually override the methods if necessary.

One downside: Equality doesn’t really work well when there’s an inheritance hierarchy involved. Read Josh Bloch’s “Effective Java” for a detailed discussion. (Ooh, new edition coming out soon. Should make for great reading.)

Generic variance support

Once again, Eric has posted rather a lot on this, concentrating on two aspects: interfaces and delegates. I believe these are already supported in the CLR (I’m sure interfaces are, and I suspect delegates are too).

Just to be clear, I don’t think anyone should expect unsafe covariance/contravariance. The following code still won’t compile (at least unless things go very differently to how I expect):

This code, however, would be okay, due to the “readonly” nature of iteration:

That’s the interface side. For delegates, this would probably be possible:

All of this is conceptually good. It’s more for people to understand, of course, but it’s still a useful thing to have available.

Dynamic calling support

This one really surprised me – to the extent that I’ll now need to edit the last chapter in the book so as not to look stupid when it’s published. (It’s fine to be wrong when predicting the future, but making a prediction which has already been proven false before publication is embarrassing.) I expected C# to stay fully static forever. However, it looks like the C# team is at least strongly considering making dynamic calling support available. Read the blog for more details, but note what isn’t included: C# reacting dynamically. In other words, Ayende’s beloved IDynamicObject support isn’t being proposed (at the moment) – although I guess there’s always a possibility that the DLR support will somehow be available through “normal” C# without extra compiler support.

This will disappoint some people of course, but I’m happy enough. I can’t see myself using the new support particularly often, and I’m slightly worried at the extra complexity required in the language spec to explain what it will actually do, but I’m likely to treat it in roughly the same way as unsafe blocks – something to ignore most of the time.

Conclusion

As you can see, there’s not a lot really on the table yet. That in itself is quite interesting, however – as well as the nature of the changes. I believe that the changes from C# 3 to 4 will be much more like those from 1 to 2 than 2 to 3. Think about when VS 2005 was released – C# 3 extensions were already available in CTP form for the VS 2005 beta. Here we are with VS 2008 having RTM’d a while ago, and we only have a few ideas to mull over. Now, I’m sure that there are implementations of all of the above features and almost certainly more – but they’re not putting them out just yet. Hopefully we’ll learn more over the next few months.

The big difference with C# 3 was that there was a grand plan: LINQ. Almost every feature in C# 3 (basically not automatic properties or partial methods) supports LINQ in some way or other. I don’t see a big plan for C# 4. That’s a very good thing, in my view. We need time – quite a lot of time, I suspect – to digest LINQ. Awful as the phrase is, LINQ has the potential to be a paradigm shift in development. Those shouldn’t come along too often. Disparate changes can still be incredibly useful of course, so let’s not lack ambition for C# 4 – but I’m expecting idiomatic C# 4 to still be roughly similar to idiomatic C# 3, which certainly couldn’t be said of C# 3 to C# 2.

Only one more post in the immediate future: my own ideas for C# 4 (or at least those which I’ve come up with independently, even if others have mentioned them too).

There has been a fair amount of speculation online about what should be in C# 4. I’ve taken the list below from a few posts, primarily those by Ayende and Jeremy Miller. I’ve deliberately left out the ideas that Microsoft have mentioned that they’re at least considering – they’ll come in the next post.

Mixins

I suspect everyone has a different idea of what these mean, but I’ll say what I’d like. I want to be able to implement an interface by proxying all calls (other than those I’ve actually implemented) to a particular member variable, as an aid to favouring composition over inheritance. As an example, here’s a class which implements IList<string> but makes sure that only strings with length 5 or more can be added:

(Yes, you’d need to implement the setter as well to prevent other values being replaced. This is just sample code to give the rough flavour – and the syntax is pretty much made up as an example too. I’m not hung up on the syntax, I just want the functionality.

You could of course derive from System.ObjectModel.Collection<string> – but this prevents you from deriving from any other class, and fixes the inheritance forever. If you only really want to provide your clients with an IList<string> implementation, it’s nicer not to pin anything down. At a later date you could manually implement more of the interface members instead of proxying them, without changing any of the calling code.

Symbols

I don’t see the benefit over normal string interning here. That could just be because of a poor description of symbols in Ruby, admittedly… but I suspect any other benefit wouldn’t meet the “it’s got to be really useful in many situations” bar.

Hashes

I’ve only extensively used one language with hashes built in: Groovy. While I agree it’s nice occasionally, I don’t think it’s worth bending the language out of shape for as we’ve now got collection initializers anyway:

Automatic delegation

To be honest I don’t really know what Jeremy means here – although it’s possible that he means what I understand as mixins. Ah the joys of loose terminology.

Metaprogramming

I only have vague ideas of what metaprogramming is all about, and those are mostly through Ayende’s blog. I can see that it’s almost certainly very powerful, but I’m not sure I want it in C#. I don’t want C# to turn into a massive box with every nifty feature ever considered. It’s possible I could be turned on this one, if someone showed me it working really nicely.

Macros

Ick, no. I’ve seen what macros tend to be used for. I’m sure there are nice shiny reasons for them, but certainly in the C/C++ form I’d be heavily against them.

Update: Ian Griffiths mailed me drawing my attention to LISP macros and how different they are to C/C++ macros. The way Ian described it sounds similar to what I understand of the metaprogramming that Ayende wants to do. I can see why it’s a powerful tool… but personally I think I’d rather keep it away from a mainstream language like C#. I’ll be writing another blog post to explain my view on this, because it’s worthy of a much fuller discussion.

Everything virtual by default

Absolutely not! Yes, it would make mocking easier – but then making everything public by default would probably make things easier too. Inheritance is hard to control properly, and should only be done with very careful design. As I wrote in the previous post, I’d prefer classes to be sealed by default, i.e. a step in the opposite direction. Oh, there’s the performance implication too, which is one of the reason’s Java needs a much more complicated multi-pass JIT – to allow even virtual methods to be inlined until they’re actually overridden. The performance part is much, much less important than the “inheritance is powerful but easy to misuse” argument.

Not only should the default not change at this point, but it was the right default to start with.

Instant Data Type

This would basically be a way of using anonymous types at a higher level – returning them with a return type of var, for instance. I don’t support that proposal per se, but I can see a benefit in having “named anonymous types” – classes which have the same behaviour as anonymous types (in terms of immutability, equality, hash codes etc) but in a named manner. Something like this:

Again, the syntax isn’t terribly important to me – but the ability to define very simple immutable data objects is nice. It could also improve the readability of some LINQ code as you could make the meaning of the (currently anonymous) tuple clear in the name.

A few anticipated comebacks:

1. Clash with object initializers: yes, it looks like it’s setting properties rather than passing them in as constructor arguments. That’s unfortunate, and maybe parentheses would be better than braces here. That would require named parameters though. (I’ll come onto those in another post!)
2. Why not just refactor the anonymous type to a named type? ReSharper lets you do this! Indeed it does – but then you’ve got a complete class to maintain. Given a single line of code, I know the features of the Person class. I can add a new property (breaking existing uses, of course) without having to make sure I get the equality and hash code implementations right manually, etc. I prefer simplicity of language expression over just saving typing by using snippets etc – that’s why I like automatic properties.
3. It can’t use quite the same implementation as anonymous types. Indeed, anonymous types are quite interesting in terms of the number of types actually generated in the IL, due to sharing of generics. I don’t think it would be a great loss in this case though.
4. The use still isn’t as brief as with anonymous types, due to needing to specify the name. True, but unavoidable, I think.

MemberInfo (infoof)

I don’t think the C# team have actually stated that this is even potentially on the table, but one of the lovely things about having Eric Lippert as a tech reviewer for the book is I get to hear all kinds of anecdotes about what’s been considered before. Some of them will be on the book’s website in the notes section. In this case, I don’t think it’s a problem to reveal that the C# team have considered this before as an infoof operator (commonly pronounced “in-foof” of course).

I could go for this idea – it would certainly make reflection simpler in a number of cases.

Method Interception and IDynamicObject

I’ve lumped these two together as they’re similiar (in my view) – they’re leading down the road to a dynamic language. I can appreciate the benefits of dynamic languages, but that doesn’t mean I think every language ought to be dynamic. I’d pass on these two.

Static interfaces

I’m not entirely sure what Ayende means on this front, but I know I’ve seen a number of requests for the ability to declare that a type definitely has a given static method. Indeed, I’ve wanted it myself a few times. However, I’m not sure how I’d go about using it. Interfaces by their current nature are used when we’ve got an instance. We already know how to pass references etc around – but not types, other than as either type parameters or Type objects.

Now, having just written it I wonder whether that’s what Ayende means – if a type parameter is constrained to implement a particular interface, any static methods within that interface could be called using the type parameter. I can see the use in a few situations, but I’d need to be convinced that it was common enough to warrant a language change. The bar wouldn’t be too high for me on this one though, as I think we could use very natural syntax without having to make up anything significantly new.

Aspect-Oriented Programming

Ooh, tricky one. I’m definitely undecided on this. I can see benefits, but also drawbacks in terms of how obvious the flow of the code is, etc – all the normal objections.

I think I’d welcome additions to the framework and/or runtime to make AOP support simpler, but then leave it to IoC containers etc to actually implement, rather than embedding AOP directly in the language.

Design by Contract

There are parts of DbC that I’d really like to see in the language, or possibly as a language/framework mixture where the framework describes certain common attributes (invariants, non-null arguments etc) and then each compiler takes note of the same attributes. I would really, really like to get rid of having manually-written trivial argument checking in my code. I don’t think I’d immediately want to go as far as Spec# though, in terms of trying to deduce correctness. I wouldn’t like to say why, beyond unfamiliarity (which I know isn’t a good reason). Again, I could possibly be persuaded.

IDisposable implementation support

Good idea. It’s a pain to implement IDisposable properly – some help would be welcome. It would probably need to be flexible enough to allow the developer to say whether a finalizer was required or not, and possibly some other things – but in principle, I’m in favour.

Constructor inheritance

Aargh, no. Constructors effectively belong to the type rather than instances of the type, so they’re not inherited in the same way. They’re a bit like static members – and I know we can call static members as if they were inherited as normal (e.g. UnicodeEncoding.ASCII), but it’s generally a bad idea to do so in my view.

Also consider the lack of control. System.Object has a parameterless constructor – so should all types do so as well, given that they all inherit (directly or indirectly) from System.Object? What would new FileStream() really mean? I suppose one possibility would be to mark your type as intentionally inheriting constructors – which is all very well until the base class adds a new constructor you don’t want, and you don’t realise it until it’s too late. On this one the complexities and disadvantages outweigh the advantages for me.

“Const correctness”

I haven’t actually seen anyone asking for this specifically for C# 4, but it’s been a general feature request pretty much forever. Again, I can see the benefits but:

1. I suspect it’s the kind of thing you really need to get right in V1.0 for it to be genuinely useful.
2. I still haven’t seen an easy way to express “this is an immutable reference to a mutable list of immutable objects of a particular type”. Basically you need to express “constness” for every level down the composition hierarchy, which isn’t simple.

Conclusion

Just to wrap the above up, here are the above features in “yes, maybe, no” categorization (just for my own view, of course):

• Yes: Mixins, instant data types, IDisposable implementation, design by contract (partial), infoof
• Maybe: Automatic delegation, metaprogramming, static interfaces
• No: Symbols, hashes, everything virtual by default, macros, constructor inheritance, AOP, method interception and IDynamicObject

Next time (which may be tonight if I’m feeling energetic) I’ll look at what Microsoft has hinted at.

Everyone else is speculating about what’s going to be in C# 4 (and various possibilities are coming out of MS), so I thought it would be wise to start my own series of wishlist posts before I miss the boat completely.

In this first post, I’m not going to look at the future at all – I’m going to look at mistakes of the past. When I say “mistake” I of course mean “things I would have done differently had I been a language designer with 20/20 hindsight”. Of course, there’s a lot of room for argument :)

Mistakes in C# 1

• Lack of separate getter/setter access for properties. This came in C# 2, but it should have been obvious that it was highly desirable long before C# 1 came out.
• Lack of generics – ish. Don’t worry, I’m not going to claim that all the features of C# 2 and 3 should have been in C# 1, but if generics had been in at the start we could have avoided having the non-generic collections (and interfaces) completely. Mind you, I’m glad that the .NET team took their time instead of including the bodged (IMO) generics of Java 5.
• Classes not being sealed by default. I’ve believed for a long time that allowing inheritance incurs a design cost (and it’s not like I’m unique in that respect). C# fixed a mistake of Java by making methods non-virtual by default; the same should be true for classes in my view.
• Enums just being named numbers. Again, I’ve blogged about this before, but it’s worth mentioning again. It’s possible to work around the lack of this feature (as the blog post readers pointed out!) but framework and language support would have been very welcome.
• The “x” character escape sequence. Fortunately it’s rarely used, but it’s so error-prone. Quick, how different are “x8Good compiler”, “x8Bad compiler”? What’s the first character in each string? (This will appear soon on my brainteasers page).
• The switch statement. There are lots of ways in which this could have been better designed. VB addresses some of them (such as making it easier to express multiple matching values) but there are other ways in which this construct needed overhauling. Fallthrough is (rightly) prohibited, so why not just force braces round the code in the case block instead of requiring a break statement? Aside from anything else, that would fix the somewhat bizarre scoping rules.
• Wacky overload resolution. I entirely understand the point that introducing new methods in a base type shouldn’t change the behaviour in derived types – but if you’ve explicitly chosen to override that method, that should be more easily callable than it is. (See the first example of the brainteasers page to see what I’m talking about.)
• The “lock” keyword, and associated issues. Basically, the IDisposable pattern should have been used for locking, and not every object should have a monitor associated with it. Developers should keep a close eye on what’s being locked, and being able to lock on everything takes away from this. Likewise “lock” creates a keyword for little purpose (and one which would otherwise be useful as a variable name etc).

Mistakes in C# 2

• Lack of partial methods. I’m really only saying this because it broke the format of C# in Depth slightly. I’ve introduced partial methods along with partial types because they logically fit in with them, and they don’t fit in with any of the other features of C# 3 particularly. This is just a matter of not working out all of how partial types would be used – or at least not doing so early enough. (For all I know partial methods were on the table before C# 2 shipped – I wouldn’t be surprised.)
• Possibly the lack of generic variance. This is certainly a big issue of understanding which is often raised as a question. On the other hand, I suspect that if/when it becomes available, it will raise just as many questions in terms of the detail anyway…
• The System.Nullable class. It’s really only there as an historical accident, and I know it’s not a C# issue as such – but even so… (Note for extra clarity – I’m fine with nullable types and the System.Nullable<T> struct. It’s just the supporting class that I don’t like.)
• InternalsVisibleTo requiring the whole public key for strongly signed assemblies instead of the public key token (contrary to the documentation). Ick.

Mistakes in C# 3

• It’s a real shame that readonly automatic properties aren’t in C# 3. I suspect they’ll come in C# 4 (and they’ll be on my wishlist in future posts) but I think it’s reasonable to wonder why they weren’t included in C# 3. Immutability is a known pattern of goodness, and although C# 4 may well contain any number of more significant improvements towards making it easier, readonly automatic properties would have been a good start.
• The way that extension methods are found. This issue was raised time and time again before release, and I’ve never heard a good defence of finding them by whole namespace, instead of allowing developers to say “use the extension methods found in this class, please”. As it is, anyone writing their own extension methods is likely to end up with whole namespaces devoted to a single type. It’s very odd.

Of course that’s not to say there aren’t other things I’d like to see – but these are more “features which were slightly misdesigned” rather than “features which I really want”.

I’m not trying to take anything away from the language designers – C# is still easily my favourite language in terms of its design, particularly in C# 3, but nobody’s perfect :)

Next time I’ll start giving my opinions of features that other people are calling for.