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1 .Herding Nulls
and other C# stories from the future
Mads Torgersen, C# lead designer
Microsoft
2 . Chapter One
Herding nulls
3 .Every reference type allows null!
Code must be defensive about it
C# shares this with most object oriented languages
4 .Not all languages have this problem!
Differentiate between nullable and non-nullable
Use pattern matching to conditionally “unpack” nullables
5 .1. Expression of intent
2. Enforcement of intent
Within an existing language!
6 .public class Person
{
public string FirstName { get; set; }
public string MiddleName { get; set; }
public string LastName { get; set; }
}
7 .public class Person
{
public string! FirstName { get; set; }
}
public string? MiddleName { get; set; }
public string! LastName { get; set; } L
8 .public class Person
{
public string! FirstName { get; set; }
}
public string MiddleName { get; set; }
public string! LastName { get; set; } L
9 .public class Person
{
public string FirstName { get; set; }
}
public string? MiddleName { get; set; }
public string LastName { get; set; } J
10 .1. Protect non-null types from nulls
2. Protect nulls from dereference
11 .Must be optional
Turn it on when you’re ready to know
Can’t affect semantics - warnings, not errors
12 .Must do a good job with existing code
Can’t force you to rewrite good code
Recognize existing ways of ensuring null safety (checks and assignments)
13 .Can’t be exhaustive
Tradeoff between completeness and convenience
14 . Color illustration
Nullable reference types
15 . Chapter Two
Async streams
16 . Chapter Three
Extension everything
17 .extension Student extends Person
{
// static field
static Dictionary<Person, Professor> enrollees = new Dictionary<Person, Professor>();
// instance method
public void Enroll(Professor supervisor) { enrollees[this] = supervisor; }
// instance property
public Professor? Supervisor => enrollees.TryGetValue(this, out var supervisor) ? supervisor : null;
// static property
public static ICollection<Person> Students => enrollees.Keys;
// instance constructor
public Person(string name, Professor supervisor) : this(name) { this.Enroll(supervisor); }
}
18 .extension Student extends Person
{
// static field
static Dictionary<Person, Professor> enrollees = new Dictionary<Person, Professor>();
// instance method
public void Enroll(Professor supervisor) { enrollees[this] = supervisor; }
// instance property
public Professor? Supervisor => enrollees.TryGetValue(this, out var supervisor) ? supervisor : null;
// static property
public static ICollection<Person> Students => enrollees.Keys;
// instance constructor
public Person(string name, Professor supervisor) : this(name) { this.Enroll(supervisor); }
}
Person mads = new Person("Mads Torgersen", tonyHoare);
19 .extension Student extends Person
{
// static field
static Dictionary<Person, Professor> enrollees = new Dictionary<Person, Professor>();
// instance method
public void Enroll(Professor supervisor) { enrollees[this] = supervisor; }
// instance property
public Professor? Supervisor => enrollees.TryGetValue(this, out var supervisor) ? supervisor : null;
// static property
public static ICollection<Person> Students => enrollees.Keys;
// instance constructor
public Person(string name, Professor supervisor) : this(name) { this.Enroll(supervisor); }
}
Person mads = new Person("Mads Torgersen", tonyHoare);
WriteLine(mads.Supervisor);
20 .extension Student extends Person
{
// static field
static Dictionary<Person, Professor> enrollees = new Dictionary<Person, Professor>();
// instance method
public void Enroll(Professor supervisor) { enrollees[this] = supervisor; }
// instance property
public Professor? Supervisor => enrollees.TryGetValue(this, out var supervisor) ? supervisor : null;
// static property
public static ICollection<Person> Students => enrollees.Keys;
// instance constructor
public Person(string name, Professor supervisor) : this(name) { this.Enroll(supervisor); }
}
Person mads = new Person("Mads Torgersen", tonyHoare);
WriteLine(mads.Supervisor);
var tonysStudents =
from s in Person.Students
where s.Supervisor == tonyHoare
select s.Name;
21 .extension Student extends Person
{
// static field
static Dictionary<Person, Professor> enrollees = new Dictionary<Person, Professor>();
// instance method
public void Enroll(Professor supervisor) { enrollees[this] = supervisor; }
// instance property
public Professor? Supervisor => enrollees.TryGetValue(this, out var supervisor) ? supervisor : null;
// static property
public static ICollection<Person> Students => enrollees.Keys;
// instance constructor
public Person(string name, Professor supervisor) : this(name) { this.Enroll(supervisor); }
}
class Professor { … }
interface IStudent
{
string Name { get; }
Professor? Supervisor { get; }
void Enroll(Professor supervisor);
}
22 .extension Student extends Person : IStudent
{
// static field
static Dictionary<Person, Professor> enrollees = new Dictionary<Person, Professor>();
// instance method
public void Enroll(Professor supervisor) { enrollees[this] = supervisor; }
// instance property
public Professor? Supervisor => enrollees.TryGetValue(this, out var supervisor) ? supervisor : null;
// static property
public static ICollection<Person> Students => enrollees.Keys;
// instance constructor
public Person(string name, Professor supervisor) : this(name) { this.Enroll(supervisor); }
}
class Professor { … }
interface IStudent
{
string Name { get; }
Professor? Supervisor { get; }
void Enroll(Professor supervisor);
}
23 .extension Student extends Person : IStudent
{
// static field
static Dictionary<Person, Professor> enrollees = new Dictionary<Person, Professor>();
// instance method
public void Enroll(Professor supervisor) { enrollees[this] = supervisor; }
// instance property
public Professor? Supervisor => enrollees.TryGetValue(this, out var supervisor) ? supervisor : null;
// static property
public static ICollection<Person> Students => enrollees.Keys;
// instance constructor
public Person(string name, Professor supervisor) : this(name) { this.Enroll(supervisor); }
}
class Professor { … }
interface IStudent
{
string Name { get; }
Professor? Supervisor { get; }
void Enroll(Professor supervisor);
}
24 .extension Student extends Person : IStudent
{
// static field
static Dictionary<Person, Professor> enrollees = new Dictionary<Person, Professor>();
// instance method
public void Enroll(Professor supervisor) { enrollees[this] = supervisor; }
// instance property
public Professor? Supervisor => enrollees.TryGetValue(this, out var supervisor) ? supervisor : null;
// static property
public static ICollection<Person> Students => enrollees.Keys;
// instance constructor
public Person(string name, Professor supervisor) : this(name) { this.Enroll(supervisor); }
}
class Professor { … }
interface IStudent
{
string Name { get; }
Professor? Supervisor { get; }
void Enroll(Professor supervisor);
}
25 .interface IGroup<T>
{
static T Zero { get; }
static T operator +(T t1, T t2);
}
extension IntGroup extends int : IGroup<int>
{
public static int T Zero => 0;
}
public static T AddAll<T>(T[] ts) where T : IGroup<T>
{
T result = T.Zero;
foreach (T t in ts) { result += t; }
return result;
}
int sixtyThree = AddAll(new [] { 1, 2, 4, 8, 16, 32 });
26 .docs.microsoft.com/dotnet/csharp
blogs.msdn.microsoft.com/dotnet
github.com/dotnet/csharplang
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