C# 9 - What's new?

0- C# 9 - What's new?.md

Examples of the majority of the new features in C# 9

1. Top level statements
2. Init only setters
3. Record types
4. Pattern matching enhancements
5. Target-typed new expressions
6. Lambda discard parameters
7. Static anonymous functions
8. Attributes on local functions

Reference https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-9

1-top-level-statements.cs

// C# 8
namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            System.Console.WriteLine("Hello World!");
        }
    }
}

// C# 9
System.Console.WriteLine("Hello World!");

2-init-only-setters.cs

var person1 = new Person("Danny", "Skoog"); // OK
var person2 = new Person("Danny", "Skoog") { FirstName = "Mike" }; // OK
person1.FirstName = "Mike"; // NOT OK

public class Person
{
    public string FirstName { get; init; }
    public string LastName { get; init; }

    public Person(string firstName, string lastName) => (FirstName, LastName) = (firstName, lastName);
}

3-record-types.cs

// Declaration
public record Person(string FirstName, string LastName);

// Equality
var person1 = new Person("Danny", "Skoog");
var person2 = new Person("Danny", "Skoog");
Console.WriteLine(person1 == person2); // Outputs "True"

// Immutability
var person3 = new Person("Danny", "Skoog"); // OK
var person4 = new Person("Danny", "Skoog") { FirstName = "Mike", LastName = "Johnson" }; // OK
var person5 = person1 with { FirstName = "Mike" }; // OK
person3.FirstName = "Mike"; // NOT OK

// ToString
Console.WriteLine(person1.ToString()); // Outputs "Person { FirstName = Danny, LastName = Skoog }"

// Deconstruction
var (firstName, lastName) = person1;

4-pattern-matching-enhancements.cs

// Type patterns - Example 1
// C# 8
static int GetSpeed(Vehicle vehicle) => vehicle switch
{
    Bus _ => 75,
    Car _ => 100,
    _ => 50
};
// C# 9
static int GetSpeed2(Vehicle vehicle) => vehicle switch
{
    Bus => 75,
    Car => 100,
    _ => 50
};

// Type patterns - Example 2
// C# 8
static bool IsDannyTuple((object, object) tuple)
{
    return tuple is (string _, int _);
}
// C# 9
static bool IsDannyTuple2((object, object) tuple)
{
    return tuple is (string, int);
}

// Relational patterns - Example 1
// C# 8
static int GetTax(int muncipalityId) => muncipalityId switch
{
    0 => 20,
    int n when n < 5 => 21,
    int n when n <= 10 => 22,
    int n when n > 21 => 23,
    _ => 20
};
// C# 9
static int GetTax2(int muncipalityId) => muncipalityId switch
{
    0 => 20,
    < 5 => 21,
    <= 10 => 22,
    > 21 => 23,
    _ => 20
};

// Relational patterns - Example 2
// C# 8
bool IsLetter(char c) => c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z';
// C# 9
bool IsLetter2(char c) => c is >= 'a' and <= 'z' or >= 'A' and <= 'Z';

// Logical patterns - Example 1
// C# 8
static int GetTax3(int muncipalityId) => muncipalityId switch
{
    int n when n == 0 || n == 1 => 20,
    int n when n > 1 && n < 5 => 21,
    int n when n > 5 && n != 7 => 22,
    7 => 23,
    _ => 20
};
// C# 9
static int GetTax4(int muncipalityId) => muncipalityId switch
{
    0 or 1 => 20,
    > 1 and < 5 => 21,
    > 5 and not 7 => 22,
    7 => 23,
    _ => 20
};

// Logical patterns - Example 2
// C# 8
static int GetSpeed3(Vehicle vehicle)
{
    if (!(vehicle is null))
    {
        if (!(vehicle is Bus))
            return 100;

        return 70;
    }

    return 0;
}
// C# 9
static int GetSpeed4(Vehicle vehicle)
{
    if (vehicle is not null)
    {
        if (vehicle is not Bus)
            return 100;

        return 70;
    }

    return 0;
}

// Logical patterns - Example 3
// C# 8
static int GetSpeed5(Vehicle vehicle) => !(vehicle is Bus) ? 100 : 70;
// C# 9
static int GetSpeed6(Vehicle vehicle) => vehicle is not Bus ? 100 : 70;

// Parenthesized patterns
// C# 9
bool IsLetter3(char c) => c is (>= 'a' and <= 'z') or (>= 'A' and <= 'Z');

5-target-typed-new-expressions.cs

// Example 1
// C# 8
Person person = new Person();
// C# 9
Person person2 = new();

// Example 2
// C# 8
DoIt(new Person());
// C# 9
DoIt(new());

// Example 3
// C# 8
Person GetPerson()
{
    return new Person();
}
// C# 9
Person GetPerson2()
{
    return new();
}

6-lambda-discard-parameters.cs

// C# 8
Console.CancelKeyPress += (_, b) => Console.WriteLine("Cancel key was pressed");
// C# 9
Console.CancelKeyPress += (_, _) => Console.WriteLine("Cancel key was pressed");

7-static-anonymous-functions.cs

var x = 5;

// C# 8
Func<int, int> func = (int num) => x + 5; // OK
// C# 9
Func<int, int> func2 = static (int num) => x + 5; // NOT OK

8-attributes-on-local-functions.cs

int Calculate(int? num)
{
    int CalculateLocal([DisallowNull] int? num)
    {
        return num.GetValueOrDefault() + 10;
    }

    return CalculateLocal(num);
}

Calculate(100);