.Net Foundation On .NET, 13 Mar 2018

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From YouTube: Lesser known features of C# 7

Description

Did you know that there have been multiple minor releases of C# since 7.0? Many of these features make it much easier for developers to write more performant code by minimizing value type copying.
Our good friend and C# language designer, Mads Torgersen (@MadsTorgersen), joins this episode of On .NET to talk to us about some of these lesser known features that are available for developers to try out in their C# 7.x projects.
01:20 - Motivations for the minor releases of C#
02:31 - Who do the minor releases target?
03:48 - ref locals and returns (C# 7.0)
10:26 - ref readonly and in parameters (C# 7.2)
13:46 - How can these features be used with immutable types?
18:42 - How to opt into C# 7.2 features?

Learn more about some of the other new features of C# 7 through the links below
What's new in C# 7.2: https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-7-2?WT.mc_id=docs-channel9-cephilli
What's new in C# 7.1: https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-7-1?WT.mc_id=docs-channel9-cephilli
What's new in C# 7: https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-7?WT.mc_id=docs-channel9-cephilli

A

Okay. welcome to another episode of the on dotnet show., and here today,. We are lucky to have mads torgersen., yes, you., oh., thanks. and yes., and we're going to talk about lesser-known features in recent c, sharp versions.

A

So thanks for coming to talk with us. thanks for having me. yes. so, you're the language designer for c sharp., that's right? isn't it?, I'm one of them. yeah. awesome.! So you know this stuff..

A

I know a bit. yeah..

A

So today we're going to talk about some things that people might not know in recent c sharp versions.

B

Right. yeah., we had a major release.

B

Last year, c sharp, 7.0.

B

and then,. Since then, we've actually had a couple of point releases,, which is unusual for the language,, where we added smaller but interesting features.

B

And, across both the major version and the minor ones,.

B

We added things that many people might not be aware: of, and they're sort of not the blockbusters that tuples or pattern matching are..

B

Those are the ones that you will focus on when you say,. Why do you c sharp seven. yeah.? So those are the ones we won't talk. About. okay. now remind me,, so you said,.

A

There's been 7.0,, there's definitely been 7.1,. Was there a 7.2?.

B

There was a 7.2 as well.. I actually did not realize that. right., it's going so fast now that even richard lander can not keep track..

B

So sorry about that., it's okay., the 7.2 release was motivated by aligning with.

B

Some work that was going on in the framework in runtime around span of t. and in general,.

B

Around making low-level programming.

B

Safer and more efficient., or rather making efficient programming, safer, or safe programming more efficient.. Whatever you want to say. so,, there was sort of like a cross stack effort,.

B

That also bled into the language sum.

B

and,. That was the main motivator, for even putting out 7.2. and then,, because we haven't done point releases, forever-.

B

Never., we kind of warm ourselves up to that,.

B

And get the gears rolling, on what is a point release even. we had a small one,, which was 7.1,, which was really just a few convenient features, to see. If we could do point, release. makes sense.

B

so a bunch of what I'll show here. Today,.

B

Is stuff that relates to that value? Proposition of the safe,.

B

Efficient, low-level like code,.

B

That we decided to focus on.

A

Right. and just before we do that,, who would you say this particular feature? Is for?.

A

Is there like a category of folks?.

B

Yeah. normally,: we try to do features that are for everyone. yes., and these are for everyone,, but they're more for people who built libraries and frameworks,, probably than they are directly for people doing app. Development.- in that, they typically have to do with like,.

B

You'll use them when you're doing buffer, management, or parsing of payloads,, or something like that.

B

That's where they really shine. but, in people in those categories,.

B

Using those to make the frameworks and libraries safer and more efficient,, it benefits everyone sort of at the next level.

B

right.. If you're a consumer of those apis, you're, now a happier consumer. right. now is another way of thinking of it. Is,.

A

If I'm the software developer, who only uses collections and dictionaries,, then this is less for me?.

A

But if I use and copy around segments of arrays,.

A

Then maybe I should pay attention?.

B

Absolutely., that's it. yes. or, if you're a game developer and you work with structs a lot,.

B

Because you don't want to allocate,, these features are also for you. okay., so let's begin., so, let's see here.

B

to start out with, in c sharp 7.0,.

B

I already put this in my code. Here,, we added ref returns.

B

so,. This is definitely aimed at people who want to work with big structs to make.

B

It easier for you to write functionality. say I have a big struct array of objects in a space game,.

B

And I want to get a particular asteroid. now I can have a method that returns me a ref to the struct that sits in the array, rather than copying that struct out and wasting resources.

B

Right. so, one thing when this feature first came out,.

A

That I didn't quite get is,.

A

That you can explain is,.

A

Because it's a struct,.

A

I always thought of like I return something, most of the time, I'm using reference types, and I always thought like well, it's a reference type,, I'm returning that by reference,.

A

The caller should get everything that I passed.

A

Not as copied by reference, is this specifically oriented around you returning a struct, instead of a reference type?.

B

Or can you use it with reference types?? You can use it with reference types,, but it's most interesting with struct. because, for a struct,.

B

If you retain that increase in c sharp 7.0,, you have no choice but to copy the struct., and if it's a big struct, like a big asteroid,, you don't want to copy it..

B

You want to just return a ref to the place in memory where it is.

A

Yeah. now, does it have value, for reference types as well?.

B

It has a little less value.

B

what it does,. It retains your memory, location., so essentially, a place where there's a variable, or an array, element, or so on, that you can then assign to.. So if you want to give people a memory, location,.

B

To stick a string in, rather than just return. The string. like here is a place where a string sits,.

B

And now you can modify it if you want to., it has some utility like that., but you have to be a little more creative to come up with the scenarios. I see. okay.. It makes sense.

B

yeah.. So that's c sharp 7.0, that came out last year,.

B

And that was the first in a series of ref related features., and you see what I get here,.

B

I have a silly little method, here, that takes two refs, two ints., so two storage locations that have ints in them, and it modifies both of those ints, and then it returns a ref to one of them..

B

So returning a ref is the new thing it can do.

B

right., so clearly, the ref that exists in.

A

The actual right-hand side of the method signature,, those are not new. right., the ref that is on the left-hand side,.

A

I assume the semantics are exactly the same as the ones on the right.

A

B

Okay, good. just going the other way, you go down the stack instead of up.

A

Whichever way you view it.

B

Yes. perfect., so what you see here, is, I create two variables,.

B

I pass them by ref to my method,.

B

I get one of them back, I don't know which, and I can store that in a ref local.

B

So now I have a variable z, that is pointing to the same memory location as x,.

B

Or y,, and that means when I modify z,.

B

When I write 100 into that,, then I get that 100 into either of these two variables.

B

okay.. So if we run it,, you can write which writes amount,.

B

You can see that I have an error.

B

I'm guessing that wasn't part of the demo., oh no.. That's because I must have done some.

B

Oh, yes.: let's just delete this stuff.

B

B

so,, let's run it again.. That was a part of my demo for later that I stuck in there and I didn't check this file.

B

so,. You can see x changed to 100,, even though I make the assignment to z., because z is a ref to the same,, to the variable x.

B

I see. so,, that's the main point of all that.

B

You now get to trade in storage, locations.

A

Got it., I have question which,.

A

This may be like a language designer kind of question.

A

So,, I see the ref that is on line 16,, that's just before or maybe..

A

I understand my all. The other refs are there.

A

right.. Why is that one there?? That one is there.

B

To signify that this is a ref assignment.

B

Right? so,, this assigns the ref, not the value.

B

And so, think of it as I'm assigning this, I mean,.

B

I'm assigning the storage location into something.

B

I'm not assigning the contents of the storage location.. This might be a bad analogy,.

A

But it feels a little bit like when you put a cast to the left-hand side of something, you're, basically saying,. This is the nature that I wanted to be..

A

It feels somewhat similar.

B

Yeah. yeah. yeah, you're right., that's a bad analogy.

B

it's,. I mean we're., it's sort of opposite to what you do. If you're in a language or an unsafe code, where you use pointers and dereference.

B

Right? there you are trading in the pointers, and then you have to do something explicit to dereference them. yeah., but in c sharp, we're stuck with this syntax, where you're dereferencing by default.

B

yes., when you assign from a variable,, you are assigning the contents, not the variable.

A

And so, we have to put syntax.

B

To stop it from dereferencing.

B

yes. and that's takes a little getting used: to. yeah. so,! That's the essence of that feature.

B

And then, in c sharp 7.2, when we wanted to do more for this scenario,.

B

We added various little refinements around refs.

B

okay. so,, the first one is, that something that, by the way vb was always able to do,.

B

Is that a ref parameter can now,.

B

You can now use that for an extension, method. so,. I can put extension methods on structs that work by ref.

B

Which is important because although, so,, let me do it.

B

so,, I'm making this an extension method.

B

On ints, passed by ref.

B

Which means that here, I can choose to call x.ormaybe and then delete x as the first argument. Here,.

B

And then x gets passed by ref to the extension.

B

I see. and that's something people have been clamoring for for a long time. Because, up until now, in c sharp, we couldn't put an extension method on a struct that would mutate the struct. because,. This parameter has to be a value parameter, which means that,. It was copied.

B

you copied the struct,, then you mutated the copy and the extension method,., which is bizarre. and,. Then you throw it away when you return. right?.

A

So not super useful.

B

So,, this finally makes extension method useful on structs.

B

I see. interesting.

A

So,, that's one little thing: we added.

B

Another is that, a reason why people are sometimes hesitant to use. Refs is that,.

B

Because of passing a storage location., somebody else might mutate that storage location..

B

You might want to pass it by ref for efficiency,.

B

Like you, don't want to copy the whole thing,, but you don't want them to muck with it. so,. We also wanted to add a way that you can pass things by ref, but prevent them from being mucked with.

B

okay. okay. so,. If we do it on the returns first,, you can declare the ref return here as readonly.

A

Okay., that's what I was expecting. yes.

A

so,, I'm returning ref readonly,.

B

And now you see immediately,, I get a complaint down here, that I'm assigning a ref readonly to something that I can mutate through,, that I can make the local variable, ref readonly as well,.

B

And then that bug goes away. but now,. I can't mutate it,, you see, the assignment,. The z is no longer allowed..

B

So this returning it, ref readonly prevented the color from mutating. What I sent to them.. I gave them visibility without giving them mutation, rights., wow., so,, that's pretty cool. and,. You can do that the other way as well for refs coming in.

B

but, there. After much deliberation,, we chose a different syntax.. I mean, you could make, you say: ref readonly here,. We could allow it.

B

well, especially-.. If I could spell it. yes., but it doesn't really matter.. It's still not allowed.

B

what we did. Instead was, we said,. We are passing something ref readonly., that's really the opposite of an out parameter.! It's an in parameter., okay., because,, an out parameter,! You pass a variable and you expect it to be assigned to., right?, yes., yes.

A

B

And,, an in parameter,, you pass it by ref and you expect it not to be assigned to. yes. and for it to have a value already.

A

B

And for it to have a value already. and,, so we call these in parameters. and I can put in on these as well. and that means, now that my method body is prevented from mutating them. right?.

A

Yeah. that keyword didn't exist in that location. Before,, that's correct., that's correct., okay., just checking., because all we had was out and ref.

B

Out and ref., and now we added in., which was a keyword already, but not there.

B

yeah., that's what I said.

A

Yeah. we're like reusing keywords wherever we can.

B

So, now these assignments are disallowed.

B

But,, you can also see that I'm not allowed to pass things by ref to it,, just like with an out parameter. You have to say out, with an in parameter. The keyword you have to use is in. okay?, yeah. so,. Now I'm passing y by in.

B

right. as an in parameter, and now I know, and it's evident in my source code- that,, it's not going to go there to be mutated.

B

y is going to remain the same after I made, just call it protected.

B

right. now, in this example,.

A

We're dealing entirely with structs, slash, value types, with these, in and out parameters, like with in,, can use that with reference types as well or is it? yeah. okay.

B

So,, just like ref,, just like out, and just like, we talked about before.

B

All of this works with storage locations, regardless.

B

okay., it's just that,, it's more useful when applied,.

B

More useful, when you're working with structs. in particular,, when you're working with big structs,, which are not here, and then trying to prevent them from being copied.

A

Yeah. now, immutability is an important concept.

A

We have various forms of immutable collections.

A

And other types of immutable data types in the product now., if you were building one of those from scratch,, would any of these features be relevant for that as part of your implementation?.

B

They definitely could be,, even as part of my public interface actually.

B

so,. Let's say that I'm providing an indexer into my collection, type,.

B

And that say it's generic,, but it can't be instantiated with structs. again, an index so that you write today or pre c sharp 7.2,.

B

You would write,.

B

It would return a copy of the value. That's in there.

B

Which would protect from mutation,.

B

But it really creates a copy. right., but you could instead, return a ref readonly to actually the variable.

B

Inside of the array that you have in your implementation of the data structure, or whatever you use to implement it. exactly., there's a storage location in there somewhere. That holds the value, and you could return that out of your indexer as a ref, readonly.

B

yeah.. And then people get to observe the actual value, in its actual place, inside of the data structure,. But they can't muck with it.

A

Yeah., because that's one of the things I've always thought was odd about some of these immutable structures,, especially if you had an immutable array,.

A

It's only immutable kind of in one way.

A

As soon as you go grabs like imagine, it's an immutable array of some reference. Type,.

A

As soon as you grab that reference type,, you can party on that object..

A

That is true., but that's still true, with this.

B

Let's not change there., that's sort of like shallow immutability that says that,.

B

The collection itself can't change, but the objects inside can.

B

right., and so this doesn't really change. That.

B

okay. okay. makes sense. so, that's most of what there is to say,.

B

There's a nifty little thing, which is that, passing,.

B

Semantically passing something by in, and passing it by value,. Just copying it.

B

isn't all that different? right. you're passing something in and it doesn't change. okay?, that's true. so,. It seems like it's just a more efficient way.

B

Of doing it, when you have large values., and so for that reason,, we actually let you not give the in keyword.

B

it's not,, because the reason we have these ref and out keywords there is, to warn the caller that something might happen to the thing they're passing in, and something might not happen. Here. so,. Why require the in keyword.

B

and,, not only that,, but you can actually put something here. That's not a variable, here or here as a receiver for an in parameter.

B

Um,, you can put any old expression and what the compiler will do is it'll new up, it'll create a new storage location., put that value in that storage location, and set that by in up here.

B

and,. So you can see that,, there's actually a third..

B

What gets returned to you now is z,.

B

Is actually a third storage location that was created for it. for this parameter. you can see if we run it,. You can see that the that there are now three different variables with three different values.

B

make sense. so, anyway.

A

Just one question. is, you said you don't actually have to specify in in that case, is. The reason that it continues to be in is the fact.

A

That the method signature for or maybe dictates, that.

A

That dictates that.

B

Yeah. okay. good, good.

A

So,., have you seen any evidence that people are actually taking advantage of this feature?.

B

It's early days yet,, but we certainly know that the people who are working on span and the set of types around that.

B

They make a lot of use of this.

A

This is integral of two those types.

B

And so it's not the accidental that we do these things now., it's not just to help out people with this,, but it's also to create the foundation for span..

B

I see. now, the span of people,.

A

Span people uses this for performance reasons or more on the safety side?.

B

It's that combination.

B

okay., it's being able to return you something in a performance but safe way for instance.

B

Okay. there's something called a readonly span.. It gives you access to a to a storage, location or.

B

A set of storage, location somewhere, but it gives you readonly access.

B

And then,, if you index into it,, then you get exactly ref.

B

Readonly back from that indexing. so,, we actually do use that.

A

Okay. makes sense. now,. Can you remind us? I think there are some special thing that you have to do to opt-in, or first question, was stuff. You just showed us as 7.2 or 7.1?

A

7.2., seven two. and then,.

A

Can you remind us how to opt in to 7.2?.

A

B

We'll do something special. well, the easiest way to do. It is to,.

B

Well, first all,: you need to have visual studio, 15.5 or greater. okay.. If you have that, and you start using one of the features, and you get an error,, because it's defaulting to c sharp 7.0,.

B

There will actually be a code fix.

B

That allows you to upgrade to the latest version of c sharp. okay. so,. There is a menu in there somewhere,. I can't even remember how to find it. it's not that hard,, but in the setting somewhere, for the project., but the easiest way to just use a feature and say yes to being upgrade., I see. yeah.. I can't remember.

A

What the first 7.1 feature was that I used, I did use something.. It was async main or something else?

A

and, I went to other route..

A

I think it's in project properties.

B

Yeah., I think so. I mean, it's not super buried. no., but more and more, I find myself,. That's just plugging ide experience. Here.

B

I find myself not going to add references in much..

B

I don't go to the project settings dialog much,, because more and more often, you do something and it wasn't right: yet,, that's a code fix that offers you, or let me go reference that and put a using up here, or let me just,. Let me go twitch this for you, so it works.

B

so,, more and more. You can just be in the code and get those things done., which is great. okay. well,. This was awesome.

A

I definitely learned some things I completely didn't know. cool. and that you might not have any need,.

B

But then it certainly. yeah.

A

Well, one thing is, I'll, sometimes be on github. I'll, see this code. File, and readability is super important.

A

So,, I may not have ever written this code,, but it's super helpful for me to be able to read it,.

A

To understand what it's doing for main literate basically.

B

And,, if you become a consumer of span,, which I think a lot of people will over time,, then you will start to see these things in the public interface of those types.

B

And,, it's good that you understand what they are.

B

yeah. absolutely. okay. well,, thanks for coming by,.

A

And showing us some new and kind of slightly off the beaten track, features.

A

Awesome. awesome. okay. there was another episode of the.

A

On.Net show- and we learned a little bit more about c sharp. thanks for watching.