CSharp.md

C# 12

Primary constructors

// Old way
public class Person
{
    public Person(string name, int age)
    {
        Name = name;
        Age = age;
    }
    
    public string Name { get; set; }
    public int Age { get; set; }
}

// New way
public class Person(string name, int age)
{
    public string Name { get; set; } = name;
    public int Age { get; set; } = age;
}

Perfect for dependency injection

public class UserController
{
    private readonly UserService _userService;
    private readonly ILogger<UserController> _logger;
    
    public UserController(UserService userService, ILogger<UserController> logger) {
        _userService = userService;
        _logger = logger;
    }
}

public class UserController(UserService userService, ILogger<UserController> logger)
{
}

Collection Expressions

int[] = [1, 2, 3, 4, 5];

// Empty array
public string[] CoolThings { get; set; } = [];

// Works with lists too
public List<string> CoolThings { get; set; } = [];

// Spread operator
int[] numbers = [1, 2, 3, 4, 5];
int[] moreNumbers = [0, ..numbers, 6, 7, 8, 9]; // Note the .. operator

Switch expressions

(Actually C# 11, but still worth mentioning)

public string GetDayOfWeek(int day)
{
    return day switch
    {
        1 or 7 => "Weekend",
        2..6 => "Weekday",
        _ => throw new ArgumentException("Invalid day of week")
    };
}

public string Suffix(int number)
{
    return number switch
    {
        1 => "st",
        2 => "nd",
        3 => "rd",
        >= 4 and <= 20 => "th",
        _ => Suffix(number % 10)
    };
}

public string Family(string[] names) {
    return names switch
    {
        // Array contains exactly one element
        [name] => $"{name} is an only child",
        // Array contains exactly two elements
        [child1, child2] => $"The family has two children: {child1} and {child2}",
        // Array contains more than two elements, but less than four
        [child1, ..others] when others.length < 4 => $"{child1} is the oldest child and has {others.Length} siblings",
        // "else" case
        _ => "The family is too big to keep track of"
    };
}

Null type checking

public class Person
{
    public required string Name { get; set; }
    public string? FavoriteColor { get; set; }
}

?? and ??= operators

string name = null;
string otherName = name ?? "John Doe";

// Same as
string otherName = name != null ? name : "John Doe";

// ??= operator
name ??= "John Doe";

// Same as
if (name == null)
{
    name = "John Doe";
}