C#: الأنواع القابلة للإلغاء ومطابقة الأنماط

int? a = 42;
int? b = null;
bool? flag = null;
double? price = 9.99;

Console.WriteLine(a.HasValue);
Console.WriteLine(b.HasValue);
Console.WriteLine(flag.GetValueOrDefault());
Console.WriteLine(price.GetValueOrDefault(0.0));

True
False
False
9.99

int? score = null;

Console.WriteLine(score.HasValue);
Console.WriteLine(score.GetValueOrDefault());
Console.WriteLine(score.GetValueOrDefault(60));

score = 88;
Console.WriteLine(score.Value);

False
0
60
88

int? x = null;
int y = x ?? 0;

string? name = null;
int? len = name?.Length;

string s = null!;
Console.WriteLine(s is null);

True

string? input = null;

string display = input ?? "(empty)";
Console.WriteLine(display);

int? length = input?.Length;
Console.WriteLine(length ?? -1);

string? city = "Beijing";
Console.WriteLine(city?.Length ?? 0);

(empty)
-1
7

#nullable enable
string? maybeNull = null;
string notNull = "hello";

maybeNull = null;
notNull = null!;

#nullable enable
string Greet(string? name)
{
    return name is null ? "Hello, stranger" : $"Hello, {name}";
}

Console.WriteLine(Greet(null));
Console.WriteLine(Greet("Alice"));

Hello, stranger
Hello, Alice

string? GetName() => null;

string? name = GetName();

if (name is null)
{
    Console.WriteLine("Name is null");
}

string upper = name?.ToUpper() ?? "DEFAULT";
Console.WriteLine(upper);

Name is null
DEFAULT

object obj = 42;

if (obj is int i)
{
    Console.WriteLine($"Is integer: {i}");
}

Is integer: 42

object value = "hello";

if (value is int n)
{
    Console.WriteLine(n);
}
else if (value is string s)
{
    Console.WriteLine($"String length: {s.Length}");
}

String length: 5

string Describe(object obj)
{
    return obj switch
    {
        int i => $"Integer {i}",
        double d => $"Double {d}",
        string s => $"String \"{s}\"",
        bool b => $"Boolean {b}",
        null => "null",
        _ => "Unknown type"
    };
}

Console.WriteLine(Describe(10));
Console.WriteLine(Describe(3.14));
Console.WriteLine(Describe("hi"));
Console.WriteLine(Describe(true));

Integer 10
Double 3.14
String "hi"
Boolean True

var person = new { Name = "Alice", Age = 20 };

if (person is { Age: > 18 })
{
    Console.WriteLine("Adult");
}

string Category(object p) => p switch
{
    { Age: < 12 } => "Child",
    { Age: >= 12 and < 18 } => "Teenager",
    { Age: >= 18 } => "Adult",
    _ => "Unknown"
};

Console.WriteLine(Category(person));

Adult
Adult

record Person(string Name, int Age);

record Person(string Name, int Age);

var p1 = new Person("Alice", 18);
var p2 = new Person("Alice", 18);

Console.WriteLine(p1 == p2);
Console.WriteLine(ReferenceEquals(p1, p2));
Console.WriteLine(p1);

True
False
Person { Name = Alice, Age = 18 }

public class Student : IEquatable<Student>
{
    public string Name { get; }
    public int Age { get; }
    public string Grade { get; }
    public Student(string name, int age, string grade)
    {
        Name = name;
        Age = age;
        Grade = grade;
    }
    public Student With(int? age = null, string grade = null)
    {
        return new Student(Name, age ?? this.Age, grade ?? this.Grade);
    }
    public bool Equals(Student other)
    {
        return other != null && Name == other.Name && Age == other.Age && Grade == other.Grade;
    }
    public override bool Equals(object obj) => Equals(obj as Student);
    public override int GetHashCode() => System.HashCode.Combine(Name, Age, Grade);
    public override string ToString() => $"Student {{ Name = {Name}, Age = {Age}, Grade = {Grade} }}";
}

class Program
{
    static void Main()
    {
        var s1 = new Student("Bob", 20, "A");
        var s2 = s1.With(age: 21, grade: "A+");
        System.Console.WriteLine(s1);
        System.Console.WriteLine(s2);
    }
}

Student { Name = Bob, Age = 20, Grade = A }
Student { Name = Bob, Age = 21, Grade = A+ }

(int Id, string Name) person = (1, "Charlie");
Console.WriteLine(person.Id);
Console.WriteLine(person.Name);

var coords = (X: 3.0, Y: 4.0);
double distance = Math.Sqrt(coords.X * coords.X + coords.Y * coords.Y);
Console.WriteLine(distance);

(int x, int y) = (10, 20);
Console.WriteLine($"x={x}, y={y}");

1
Charlie
5
x=10, y=20

string Classify(int score, bool hasBonus) => (score, hasBonus) switch
{
    (>= 90, true) => "Excellent+Bonus",
    (>= 90, false) => "Excellent",
    (>= 60, true) => "Pass+Bonus",
    (>= 60, false) => "Pass",
    _ => "Fail"
};

Console.WriteLine(Classify(95, true));
Console.WriteLine(Classify(75, false));
Console.WriteLine(Classify(50, true));

Excellent+Bonus
Pass
Fail