การนำ IEquatable ไปใช้

เหตุใดจึงใช้ IEquatable<T>?

การนำอินเทอร์เฟซไปใช้

using System;

public struct Point : IEquatable<Point>
{
    public int X, Y;
    public Point(int x, int y) { X = x; Y = y; }

    public bool Equals(Point other) => X == other.X && Y == other.Y;
    public override bool Equals(object obj) => obj is Point p && Equals(p);
    public override int GetHashCode() => HashCode.Combine(X, Y);
}

public class Program
{
    public static void Main()
    {
        Console.WriteLine(new Point(1, 2).Equals(new Point(1, 2)));
    }
}

หลีกเลี่ยงการบรรจุค่าสำหรับโครงสร้าง

using System;

public struct Point : IEquatable<Point>
{
    public int X, Y;
    public Point(int x, int y) { X = x; Y = y; }
    public bool Equals(Point other) => X == other.X && Y == other.Y; // no boxing
    public override bool Equals(object obj) => obj is Point p && Equals(p);
    public override int GetHashCode() => HashCode.Combine(X, Y);
}

public class Program
{
    public static void Main()
    {
        Point a = new Point(3, 4);
        Point b = new Point(3, 4);
        Console.WriteLine(a.Equals(b)); // calls the typed overload
    }
}

คอลเลกชันใช้ IEquatable<T>

using System;
using System.Collections.Generic;

public struct Point : IEquatable<Point>
{
    public int X, Y;
    public Point(int x, int y) { X = x; Y = y; }
    public bool Equals(Point other) => X == other.X && Y == other.Y;
    public override bool Equals(object obj) => obj is Point p && Equals(p);
    public override int GetHashCode() => HashCode.Combine(X, Y);
}

public class Program
{
    public static void Main()
    {
        var list = new List<Point> { new Point(0, 0), new Point(1, 1) };
        Console.WriteLine(list.Contains(new Point(1, 1)));
    }
}

ชนิดข้อมูลอ้างอิงกับ IEquatable<T>

using System;

public class Person : IEquatable<Person>
{
    public string Name;
    public Person(string name) { Name = name; }

    public bool Equals(Person other) => other != null && other.Name == Name;
    public override bool Equals(object obj) => Equals(obj as Person);
    public override int GetHashCode() => Name?.GetHashCode() ?? 0;
}

public class Program
{
    public static void Main()
    {
        Console.WriteLine(new Person("Ada").Equals(new Person("Ada")));
        Console.WriteLine(new Person("Ada").Equals((Person)null));
    }
}

ให้ object.Equals เรียกใช้เมธอดที่มีชนิดข้อมูลชัดเจน

using System;

public struct Id : IEquatable<Id>
{
    public int Value;
    public Id(int value) { Value = value; }
    public bool Equals(Id other) => Value == other.Value;
    public override bool Equals(object obj) => obj is Id other && Equals(other);
    public override int GetHashCode() => Value;
}

public class Program
{
    public static void Main()
    {
        object boxed = new Id(5);
        Console.WriteLine(boxed.Equals(new Id(5)));
    }
}

การเปรียบเทียบหลายฟิลด์

using System;

public struct DateRange : IEquatable<DateRange>
{
    public int Start, End;
    public DateRange(int start, int end) { Start = start; End = end; }
    public bool Equals(DateRange other) => Start == other.Start && End == other.End;
    public override bool Equals(object obj) => obj is DateRange d && Equals(d);
    public override int GetHashCode() => HashCode.Combine(Start, End);
}

public class Program
{
    public static void Main()
    {
        Console.WriteLine(new DateRange(1, 5).Equals(new DateRange(1, 5)));
        Console.WriteLine(new DateRange(1, 5).Equals(new DateRange(1, 6)));
    }
}

จับคู่กับตัวดำเนินการ

using System;

public struct Money : IEquatable<Money>
{
    public decimal Amount;
    public Money(decimal a) { Amount = a; }
    public bool Equals(Money other) => Amount == other.Amount;
    public override bool Equals(object obj) => obj is Money m && Equals(m);
    public override int GetHashCode() => Amount.GetHashCode();
    public static bool operator ==(Money a, Money b) => a.Equals(b);
    public static bool operator !=(Money a, Money b) => !a.Equals(b);
}

public class Program
{
    public static void Main()
    {
        Console.WriteLine(new Money(9.99m) == new Money(9.99m));
    }
}

ระเบียนนำสิ่งนี้ไปใช้โดยอัตโนมัติ

using System;

public record Point(int X, int Y);

public class Program
{
    public static void Main()
    {
        var a = new Point(1, 2);
        var b = new Point(1, 2);
        Console.WriteLine(a.Equals(b));
        Console.WriteLine(a == b);
    }
}

เมื่อใดควรนำไปใช้ด้วยตนเอง

using System;

public class CaseInsensitiveTag : IEquatable<CaseInsensitiveTag>
{
    public string Name;
    public CaseInsensitiveTag(string name) { Name = name; }
    public bool Equals(CaseInsensitiveTag other)
        => other != null && string.Equals(Name, other.Name, StringComparison.OrdinalIgnoreCase);
    public override bool Equals(object obj) => Equals(obj as CaseInsensitiveTag);
    public override int GetHashCode() => Name?.ToLowerInvariant().GetHashCode() ?? 0;
}

public class Program
{
    public static void Main()
    {
        Console.WriteLine(new CaseInsensitiveTag("Red").Equals(new CaseInsensitiveTag("RED")));
    }
}

ลองทำด้วยตนเอง

using System;
using System.Collections.Generic;

public class Coordinate : IEquatable<Coordinate>
{
    public int Lat, Lng;
    public Coordinate(int lat, int lng) { Lat = lat; Lng = lng; }
    public bool Equals(Coordinate other) => other != null && other.Lat == Lat && other.Lng == Lng;
    public override bool Equals(object obj) => Equals(obj as Coordinate);
    public override int GetHashCode() => HashCode.Combine(Lat, Lng);
}

public class Program
{
    public static void Main()
    {
        var set = new HashSet<Coordinate> { new Coordinate(10, 20) };
        Console.WriteLine(set.Contains(new Coordinate(10, 20)));
    }
}

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