ค่าคงที่และฟิลด์ readonly

สองวิธีในการทำให้ค่าคงที่

using System;

class Config
{
    public const double Pi = 3.14159;
    public static readonly DateTime StartedAt = DateTime.Now;
}

class Program
{
    static void Main()
    {
        Console.WriteLine("Pi = " + Config.Pi);
    }
}

const: ค่าคงที่ขณะคอมไพล์

using System;

class Circle
{
    public const double Pi = 3.14159;

    public static double Area(double r) => Pi * r * r;
}

class Program
{
    static void Main()
    {
        Console.WriteLine(Circle.Area(2).ToString("0.00"));
    }
}

const ต้องเป็นค่าตามตัวอักษร

using System;

class Limits
{
    public const int MaxRetries = 3;
    public const int Timeout = MaxRetries * 1000;   // const expression OK
}

class Program
{
    static void Main()
    {
        Console.WriteLine(Limits.MaxRetries + ", " + Limits.Timeout);
    }
}

readonly: ค่าคงที่ขณะทำงาน

using System;

class Session
{
    public readonly string Id;

    public Session(string id)
    {
        Id = id;   // allowed: inside constructor
    }
}

class Program
{
    static void Main()
    {
        var s = new Session("abc-123");
        Console.WriteLine("Session: " + s.Id);
    }
}

readonly ใช้ค่าขณะทำงานได้

using System;

class Order
{
    public readonly Guid OrderId;

    public Order()
    {
        OrderId = Guid.NewGuid();
    }
}

class Program
{
    static void Main()
    {
        var o = new Order();
        Console.WriteLine("Has id: " + (o.OrderId != Guid.Empty));
    }
}

static readonly

using System;

class AppInfo
{
    public static readonly string[] Roles = { "admin", "user", "guest" };
}

class Program
{
    static void Main()
    {
        Console.WriteLine("Roles: " + Roles());
    }

    static int Roles() => AppInfo.Roles.Length;
}

เหตุใดค่าคงที่จึงอาจมีความเสี่ยงเมื่อข้ามชุดแอสเซมบลี

using System;

class Lib
{
    public const int Version = 2;              // inlined into callers
    public static readonly int Build = 100;     // read at runtime
}

class Program
{
    static void Main()
    {
        Console.WriteLine(Lib.Version + "." + Lib.Build);
    }
}

ไม่สามารถกำหนดค่าใหม่ให้ทั้งสองแบบได้

using System;

class Demo
{
    public const int A = 10;
    public readonly int B;

    public Demo() { B = 20; }
    // A = 11;  // would not compile
    // B = 21;  // would not compile outside constructor
}

class Program
{
    static void Main()
    {
        var d = new Demo();
        Console.WriteLine(Demo.A + ", " + d.B);
    }
}

การเลือกใช้งานระหว่างสองแบบ

using System;

class Physics
{
    public const double Gravity = 9.81;          // universal literal
    public readonly double Mass;                  // per object

    public Physics(double mass) { Mass = mass; }
    public double Weight() => Mass * Gravity;
}

class Program
{
    static void Main()
    {
        var p = new Physics(10);
        Console.WriteLine("Weight: " + p.Weight());
    }
}

ข้อควรระวังเกี่ยวกับการอ้างอิงแบบอ่านได้อย่างเดียว

using System;
using System.Collections.Generic;

class Cart
{
    public readonly List<string> Items = new List<string>();
}

class Program
{
    static void Main()
    {
        var c = new Cart();
        c.Items.Add("Pen");   // allowed: mutating the object
        c.Items.Add("Book");
        Console.WriteLine("Items: " + c.Items.Count);
    }
}

สรุปการใช้งานร่วมกัน

using System;

class Account
{
    public const decimal MinBalance = 0m;
    public readonly string Owner;

    public Account(string owner)
    {
        Owner = owner;
    }
}

class Program
{
    static void Main()
    {
        var a = new Account("Rin");
        Console.WriteLine(a.Owner + " min=" + Account.MinBalance);
    }
}

ตรวจสอบความเข้าใจอย่างรวดเร็ว

ทบทวน

using System;

class Demo
{
    public const int Max = 100;
    public readonly int Created;
    public Demo(int c) { Created = c; }
}

class Program
{
    static void Main()
    {
        var d = new Demo(7);
        Console.WriteLine(Demo.Max + ", " + d.Created);
    }
}