1.
C# Light
A proposal for a new C# syntax
Inspired by a post by PhilTrelford
UPGRADE
YOUR MONITOR
Read these slides
and you might get a free
triple height monitor*
*or visual equivalent

2.
Why change something that works?

3.
Why change something that works?
Because...
Why not compete with scripting languages?
Lightweight, but keep the power of static typing.

4.
Why change something that works?
Because...
Why not compete with scripting languages?
Lightweight, but keep the power of static typing.
Because...
Times are changing. Defaults should change too.
Make immutability the new default.

5.
Why change something that works?
Because...
Why not compete with scripting languages?
Lightweight, but keep the power of static typing.
Because...
Times are changing. Defaults should change too.
Make immutability the new default.
Because...
Many common patterns are too hard.
Make good practices easy (but allow deviations).

6.
! HEALTH WARNING !
These slides are for consumption
by open-minded programmers
ONLY.
When taken with pre-existing
closed-mindedness, can cause
shock, high blood pressure,
anxiety, nausea, confusion and
panic.

7.
You must be at least this
open minded to read
these slides
How open minded are you?
Slime mould
Bony fishes
People who
don't use LINQ
People who don't
use generics
Opposable
thumbs
People who
want C# v6
People who
use lambdas

8.
Vision for C# Light
Clean and lightweight code.
Immutability by default.
Common scenarios are easy.
We make mutability a special case,
rather than the other way around.

9.
public class Person
{
public Person(string name, DateTime birthday)
{
_name = name;
_birthday = birthday;
}
private readonly string _name;
private readonly DateTime _birthday;
/// <summary>
/// Full name
/// </summary>
public string Name
{
get { return _name; }
}
/// <summary>
/// Birthday
/// </summary>
public DateTime Birthday
{
get { return _birthday; }
}
}
Here is a typical
immutable class in C#
which we'll use as an
example throughout.

10.
Here is a typical
immutable class in C#
which we'll use as an
example throughout.
27 lines of code.
Can we do better?
public class Person
{
public Person(string name, DateTime birthday)
{
_name = name;
_birthday = birthday;
}
private readonly string _name;
private readonly DateTime _birthday;
/// <summary>
/// Full name
/// </summary>
public string Name
{
get { return _name; }
}
/// <summary>
/// Birthday
/// </summary>
public DateTime Birthday
{
get { return _birthday; }
}
}

11.
Here is a typical
immutable class in C#
which we'll use as an
example throughout.
27 lines of code.
Can we do better?
Why not remove the lines
that don't contain useful
information?
public class Person
{
public Person(string name, DateTime birthday)
{
_name = name;
_birthday = birthday;
}
private readonly string _name;
private readonly DateTime _birthday;
/// <summary>
/// Full name
/// </summary>
public string Name
{
get { return _name; }
}
/// <summary>
/// Birthday
/// </summary>
public DateTime Birthday
{
get { return _birthday; }
}
}

12.
In a typical C# project
less than 50% of the
lines contain useful
information!
Surely we can do better?

13.
Proposal 1:
Simplify doc strings

14.
We can start by
simplifying the doc
strings.
Let's make "summary"
the default.
public class Person
{
public Person(string name, DateTime birthday)
{
_name = name;
_birthday = birthday;
}
private readonly string _name;
private readonly DateTime _birthday;
/// <summary>
/// Full name
/// </summary>
public string Name
{
get { return _name; }
}
/// <summary>
/// Birthday
/// </summary>
public DateTime Birthday
{
get { return _birthday; }
}
}

15.
We can start by
simplifying the doc
strings.
Let's make "summary"
the default.
public class Person
{
public Person(string name, DateTime birthday)
{
_name = name;
_birthday = birthday;
}
private readonly string _name;
private readonly DateTime _birthday;
/// <summary>
/// Full name
/// </summary>
public string Name
{
get { return _name; }
}
/// <summary>
/// Birthday
/// </summary>
public DateTime Birthday
{
get { return _birthday; }
}
}

16.
public class Person
{
public Person(string name, DateTime birthday)
{
_name = name;
_birthday = birthday;
}
private readonly string _name;
private readonly DateTime _birthday;
/// Full name
public string Name
{
get { return _name; }
}
/// Birthday
public DateTime Birthday
{
get { return _birthday; }
}
}
We can start by
simplifying the doc
strings.
Let's make "summary"
the default.
4 lines saved!

17.
Proposal 2:
Automatically create
backing fields from
constructor parameters
Yes, I know C# 6 has something
similar. Keep reading...

18.
public class Person
{
public Person(string name, DateTime birthday)
{
_name = name;
_birthday = birthday;
}
private readonly string _name;
private readonly DateTime _birthday;
/// Full name
public string Name
{
get { return _name; }
}
/// Birthday
public DateTime Birthday
{
get { return _birthday; }
}
}
Because immutability is
the default, we can
simplify further.

19.
public class Person
{
public Person(string name, DateTime birthday)
{
_name = name;
_birthday = birthday;
}
private readonly string _name;
private readonly DateTime _birthday;
/// Full name
public string Name
{
get { return _name; }
}
/// Birthday
public DateTime Birthday
{
get { return _birthday; }
}
}
Why not automatically
define and initialize read-
only backing fields from
the constructor
parameters?

20.
public class Person
{
public Person(string name, DateTime birthday)
{
}
/// Full name
public string Name
{
get { return name; }
}
/// Birthday
public DateTime Birthday
{
get { return birthday; }
}
}
Of course, you can still define mutable
private fields in the usual way.
Unlike C# 6, you don't need to in the
immutable case.
4 lines saved!
Why not automatically
define and initialize
read-only backing fields
from the constructor
parameters?

21.
Proposal 3:
Merge the primary constructor
with the class definition
C# 6 has this too.

22.
public class Person
{
public Person(string name, DateTime birthday)
{
}
/// Full name
public string Name
{
get { return name; }
}
/// Birthday
public DateTime Birthday
{
get { return birthday; }
}
}
How often do you have
more than one
constructor?

23.
public class Person
{
public Person(string name, DateTime birthday)
{
}
/// Full name
public string Name
{
get { return name; }
}
/// Birthday
public DateTime Birthday
{
get { return birthday; }
}
}
How often do you have
more than one
constructor?
Why not merge the
constructor with the class
definition?

24.
public class Person(string name, DateTime birthday)
{
/// Full name
public string Name
{
get { return name; }
}
/// Birthday
public DateTime Birthday
{
get { return birthday; }
}
}
How often do you have
more than one
constructor?
4 more lines saved!
Why not merge the
constructor with the class
definition?
You can still define secondary
constructors separately if you
need to.

25.
Proposal 4:
(this one is controversial)

26.
Proposal 4:
Use indentation instead
of curly braces

27.
public class Person(string name, DateTime birthday)
{
/// Full name
public string Name
{
get { return name; }
}
/// Birthday
public DateTime Birthday
{
get { return birthday; }
}
}
Do we really need the
braces?
The indentation already
gives us all the clues we
need.

28.
public class Person(string name, DateTime birthday) =
/// Full name
public string Name =
get { return name; }
/// Birthday
public DateTime Birthday =
get { return birthday; }
Do we really need the
braces?
The indentation already
gives us all the clues we
need.
6 more lines saved!
You can still have explicit
begin/end markers for blocks if
you need to.

29.
public class Person(string name, DateTime birthday) =
/// Full name
public string Name =
get { return name; }
/// Birthday
public DateTime Birthday =
get { return birthday; }
Add "equals" as an indicator to
start a new block.

30.
People who
complain about
using a language
with syntactic
whitespace
People who
have spent time
using a language
with syntactic
whitespace
"Oddly enough, Python's use of whitespace stopped feeling unnatural
after about twenty minutes. I just indented code, pretty much as I
would have done in a C program anyway, and it worked."
- Eric Raymond
Helpful Venn Diagram
Overlap

31.
Observation
With these 4 proposals:
27 lines of code has shrunk
to 9 lines!

32.
C#
Because
"C# Light"
means you see
3x more code
on your screen!
Why use "C# light"?
Reason 1

33.
public class Person
{
public Person(string name, DateTime birthday)
{
_name = name;
_birthday = birthday;
}
private readonly string _name;
private readonly DateTime _birthday;
/// <summary>
/// Full name
/// </summary>
public string Name
{
get { return _name; }
}
/// <summary>
/// Birthday
/// </summary>
public DateTime Birthday
{
get { return _birthday; }
}
}
public class Person(string name, DateTime birthday) =
/// Full name
public string Name =
get { return name; }
/// Birthday
public DateTime Birthday =
get { return birthday; }
Why use "C# light"?
C#
C# Light
Reason 2 Because
"C# Light"
means you write
1/3 as much
code.

34.
Proposal 5:
Eliminate syntax noise
There is a lot of syntax "noise"
that is not needed, IMO.

35.
Proposal 5a:
Eliminate "get" keyword for
immutable properties

36.
public class Person(string name, DateTime birthday) =
/// Full name
public string Name =
get { return name; }
/// Birthday
public DateTime Birthday =
get { return birthday; }
The class is immutable.
Every property is "get"
only.

37.
public class Person(string name, DateTime birthday) =
/// Full name
public string Name =
get { return name; }
/// Birthday
public DateTime Birthday =
get { return birthday; }
The class is immutable.
Every property is "get"
only.
So why bother with the
"get" keyword?

38.
public class Person(string name, DateTime birthday) =
/// Full name
public string Name =
return name;
/// Birthday
public DateTime Birthday =
return birthday;
The class is immutable.
Every property is "get"
only.
So why bother with the
"get" keyword?
A bit cleaner, IMO.

39.
Proposal 5b:
Eliminate "return"

40.
public class Person(string name, DateTime birthday) =
/// Full name
public string Name =
return name;
/// Birthday
public DateTime Birthday =
return birthday;
Why not take a leaf out of
other languages and
make the return implicit
for the last line in a
block?
This is the
"expression-based"
approach.

41.
public class Person(string name, DateTime birthday) =
/// Full name
public string Name =
name;
/// Birthday
public DateTime Birthday =
birthday;
Why not take a leaf out of
other languages and
make the return implicit
for the last line in a
block?

42.
Proposal 5c:
Make immutable properties
public by default

43.
public class Person(string name, DateTime birthday) =
/// Full name
public string Name =
name;
/// Birthday
public DateTime Birthday =
birthday;
The properties are
immutable.

44.
public class Person(string name, DateTime birthday) =
/// Full name
public string Name =
name;
/// Birthday
public DateTime Birthday =
birthday;
The properties are
immutable.
Why not make them
public by default. There's
no way a client of the
object can corrupt it!

45.
class Person(string name, DateTime birthday) =
/// Full name
string Name =
name;
/// Birthday
DateTime Birthday =
birthday;
The properties are
immutable.
Why not make them
public by default. There's
no way a client of the
object can corrupt it!
You can still use the "private"
keyword if you need to.
Only the default has changed.

46.
Proposal 5d:
Make semicolons optional

47.
class Person(string name, DateTime birthday) =
/// Full name
string Name =
name;
/// Birthday
DateTime Birthday =
birthday;
How often do you have
more than one semicolon
on a line?
Excepting for loops, of course.

48.
class Person(string name, DateTime birthday) =
/// Full name
string Name =
name
/// Birthday
DateTime Birthday =
birthday
How often do you have
more than one semicolon
on a line?
So why bother?

49.
Observation:
Removing syntax noise opens up
more opportunities to save space.

50.
class Person(string name, DateTime birthday) =
/// Full name
string Name =
name
/// Birthday
DateTime Birthday =
birthday
Why not move short code
fragments on to the same
line?

51.
class Person(string name, DateTime birthday) =
/// Full name
string Name = name
/// Birthday
DateTime Birthday = birthday
Why not move short code
fragments on to the same
line?
C# 6 plans to have "expression-
bodied members" too.

52.
Proposal 6:
Use type inference for properties

53.
class Person(string name, DateTime birthday) =
/// Full name
string Name = name
/// Birthday
DateTime Birthday = birthday
Why do we have to
repeat the type?
Can't the compiler
figure it out for us?

54.
class Person(string name, DateTime birthday) =
/// Full name
Name = name
/// Birthday
Birthday = birthday
Why do we have to
repeat the type?
Can't the compiler
figure it out for us?
Cleaner, but now we can't tell
that it's a property!

55.
class Person(string name, DateTime birthday) =
/// Full name
member Name = name
/// Birthday
member Birthday = birthday
We need some way to indicate properties.
Let's use the word "member".

56.
class Person(string name, DateTime birthday) =
/// Full name
member Name = name
/// Birthday
member Birthday = birthday
/// Age
member Age() =
DateTime.Today.Subtract(birthday).Days / 365
We can define methods the same way.

57.
class Person(string name, DateTime birthday)
{
/// Full name
public string Name { get; } = name;
/// Birthday
public DateTime Birthday { get; } = birthday;
/// Age
public int Age() =>
DateTime.Today.Subtract(birthday).Days / 365;
}
Here is the C# 6
equivalent with
auto-properties and
expression-bodied
members
This is nice and compact too.
Still need explicit types and
"public" keyword, though.

58.
Review

59.
Let's review the proposals:
1) Make doc strings more compact
2) Move backing fields into constructor (because immutable)
3) Move constructor into class definition
4) Remove curly braces
5) Remove syntax noise
- Remove "get" keyword from immutable properties
- Return last value in a block automatically
- Make immutable properties public by default
- Make semicolons optional
6) Allow types of properties to be inferred from constructor

60.
class Person(string name, DateTime birthday) =
/// Full name
member Name = name
/// Birthday
member Birthday = birthday
public class Person
{
public Person(string name, DateTime birthday)
{
_name = name;
_birthday = birthday;
}
private readonly string _name;
private readonly DateTime _birthday;
/// <summary>
/// Full name
/// </summary>
public string Name
{
get { return _name; }
}
/// <summary>
/// Birthday
/// </summary>
public DateTime Birthday
{
get { return _birthday; }
}
}
Normal C#
Before and After
C# Light
27 lines before.
7 lines after.

61.
What about the goal
"make common scenarios easy"?

62.
The next set of proposals aims to do
exactly this.

63.
Proposal 7:
Automatically generate
code for equality

64.
class Person(string name, DateTime birthday) : IEquatable<Person> =
/// Full name
member Name = name
/// Birthday
member Birthday = birthday
override Equals(Person obj) =
if (obj == null)
return false
Person p = obj as Person
if ((Person)p == null)
{
return false
}
// Return true if the fields match:
(name == p.Name) && (birthday == p.Birthday)
override Equals(Person p) =
if ((object)p == null)
return false
// Return true if the fields match:
(name == p.Name) && (birthday == p.Birthday)
override GetHashCode() =
name.GetHashCode ^ birthday.GetHashCode
How often do you have to
write all this equality
code?

65.
[StructuralEquality]
class Person(string name, DateTime birthday) =
/// Full name
member Name = name
/// Birthday
member Birthday = birthday
Why not let the compiler
write it for you!
You just need to use a
special attribute.
How often do you have to
write all this equality
code?

66.
Proposal 8:
Compact syntax for DTOs

67.
[StructuralEquality]
class Person(string name, DateTime birthday) =
/// Full name
member Name = name
/// Birthday
member Birthday = birthday
"Dumb" objects with no
methods (aka DTOs) are
very common.
Since they have no
methods, can we make
the syntax even simpler?

68.
[StructuralEquality]
class Person(string name, DateTime birthday) =
/// Full name
member Name = name
/// Birthday
member Birthday = birthday
"Dumb" objects with no
methods (aka DTOs) are
very common.
Since they have no
methods, can we make
the syntax even simpler?
class Person = {string name, DateTime birthday}
var person = {name="Alice", birthday=Today}
Yes, we can. Just define a
named class in the same
way as an anonymous
type.

69.
Proposal 9:
Create non-nullable reference types

70.
class Person = {string name, DateTime birthday}
// ok
var person = {name="Alice", birthday=Today}
// error
person = null
This one is a no-brainer.
If a class is defined with:
• the [StructuralEquality] attribute
• the anonymous type syntax
then it is automatically non-nullable
No more null testing
needed!

71.
Proposal 10:
Allow anonymous types
to implement interfaces

72.
public class TempDisposable: IDisposable
{
public void Dispose()
{
Console.Write("Disposed");
}
}
var tempDisposable = new TempDisposable();
Sometimes you don't
want to have to create a
whole class just to
implement an interface
temporarily.

73.
var tempDisposable =
{new IDisposable with
member this.Dispose() = Console.Write("Disposed") }
Sometimes you don't
want to have to create a
whole class just to
implement an interface
temporarily.
Why not use the
"anonymous type" syntax
to create an object
without having to create
a class explicitly?

74.
Proposal 11:
Allow subclasses to be merged into a
single "case" class

75.
Requirement: We accept three forms of payment:
Cash. Check or Card.
For Cash we don't need any extra information
For Checks we need a check number
For Cards we need a card type and card number
This one needs a bit of background.
Say that you have a requirement for taking
payments, as shown below:
How would you implement this?

76.
interface IPaymentMethod
{..}
class Cash() : IPaymentMethod
{..}
class Check(int checkNo): IPaymentMethod
{..}
class Card(string cardType, string cardNo) : IPaymentMethod
{..}
You would probably implement it as an
interface and a set of subclasses, like this:

77.
interface IPaymentMethod
{..}
class Cash() : IPaymentMethod
{..}
class Check(int checkNo): IPaymentMethod
{..}
class Card(string cardType, string cardNo) : IPaymentMethod
{..}
But that is a lot of code for such a common scenario. Also...
(a) The implementation and data is
probably scattered around
four separate files.
(b) The requirements are hard to
reconstruct from the classes.
(c) The subclasses are not "closed".
Any class that implemented the interface would work.
This might not be what you want!

78.
class PaymentMethod =
| Cash
| Check(int checkNo)
| Card(string cardType, string cardNo)
Answer: Create a "case" or "choice" class that encapsulates all
three payment methods in one class, in one place.
There are three different constructors, each
with different data.

79.
class PaymentMethod =
| Cash
| Check(int checkNo)
| Card(string cardType, string cardNo)
Answer: Create a "case" or "choice" class that encapsulates all
three payment methods in one class, in one place.
There are three different constructors, each
with different data.
PaymentMethod cash = Cash();
PaymentMethod check = Check(123);
PaymentMethod card = Card("Visa", "4012888888881881");
The compiler keeps track of which constructor
was used to create the instance.

80.
Answer: Create a "case" or "choice" class that encapsulates all
three payment methods in one class, in one place.
void PrintPayment(payment) =
switch (payment)
{
case Cash : // print cash
case Check(checkNo) : // print check info
case Card(cardType,cardNo) // print card info
}
A case statement is then used to match the
subclass that was created, and at the same
time extract the relevant data.
class PaymentMethod =
| Cash
| Check(int checkNo)
| Card(string cardType, string cardNo)
PaymentMethod cash = Cash()
PaymentMethod check = Check(123);
PaymentMethod card = Card("Visa", "4012888888881881");
There are three different constructors, each
with different data.
The compiler keeps track of which constructor
was used to create the instance.

81.
Proposal 12:
Require all properties to be
initialized in the constructor

82.
Avoid initialization errors.
Require all properties to
be initialized in the
constructor.
No more properties
left as null by mistake!
public class Person(string name, DateTime birthday) =
/// Full name
public string Name =
get { return name; }
/// Birthday
public DateTime Birthday =
get { return birthday; }

83.
For example: Compiler error.
"birthday" is missing.
public class Person(string name) =
/// Full name
public string Name =
get { return name; }
/// Birthday
public DateTime Birthday =
get { return birthday; }
Avoid initialization errors.
Require all properties to
be initialized in the
constructor.
No more properties
left as null by mistake!

84.
If they don't have to be initialized,
you must make them optional.
public class Person(string name, DateTime? birthday) =
/// Full name
public string Name =
get { return name; }
/// Birthday
public DateTime? Birthday =
get { return birthday; }
Avoid initialization errors.
Require all properties to
be initialized in the
constructor.
No more properties
left as null by mistake!

85.
Excited by C# Light?
Want it now?

86.
But C# Light is available for
download right now*
C# v6 is not yet available
*syntax not identical to that shown in this proposal.

87.
Will C# Light work with legacy C# code?
C# Light will be integrated with Visual Studio.
Syntax highlighting, debugging support, and more.
What tooling is available for C# Light?
Of course.
As with any .NET language, assemblies written in C# Light can be
mixed with normal C# assemblies, and you can make calls
between them.

88.
How can I find out more
about C# Light?
Full details of C# light and how to download it, go to:
bit.ly/csharp-light