การห่อหุ้มและเงื่อนไขคงที่

ภาพรวมการห่อหุ้ม

ส่วนที่อ่านได้อย่างเดียว

using System;

// Expose read-only view; mutate via methods only
public class Counter
{
  private int _value;          // hidden state
  public int Value             // read-only to callers
  {
    get { return _value; }
    private set { _value = value; } // keep setter private
  }

  public void Increment()
  {
    Value = Value + 1;
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    Counter c = new Counter();
    c.Increment();
    c.Increment();
    Console.WriteLine("Value = " + c.Value);
  }
}

การป้องกันในตัวสร้าง

using System;

// Enforce rules early (constructor guard)
public class User
{
  public string Name { get; private set; }
  public int Age { get; private set; }

  public User(string name, int age)
  {
    if (string.IsNullOrEmpty(name)) throw new ArgumentException("name");
    if (age < 0) throw new ArgumentOutOfRangeException("age");
    Name = name;
    Age = age;
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    try
    {
      User u = new User("Ada", 28);
      Console.WriteLine(u.Name + " (" + u.Age + ")");
    }
    catch (Exception ex)
    {
      Console.WriteLine("Error: " + ex.Message);
    }
  }
}

อินวาเรียนต์ในการใช้งานจริง

using System;

// Invariant: Balance >= 0 at all times
public class BankAccount
{
  private decimal _balance;

  public decimal Balance
  {
    get { return _balance; }
    private set
    {
      if (value < 0) throw new InvalidOperationException("Balance would be negative");
      _balance = value;
    }
  }

  public BankAccount(decimal opening)
  {
    if (opening < 0) throw new ArgumentOutOfRangeException("opening");
    _balance = opening;
  }

  public void Deposit(decimal amount)
  {
    if (amount <= 0) throw new ArgumentOutOfRangeException("amount");
    Balance = Balance + amount;
  }

  public bool TryWithdraw(decimal amount)
  {
    if (amount <= 0) return false;
    if (Balance - amount < 0) return false;
    Balance = Balance - amount;
    return true;
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    BankAccount a = new BankAccount(50m);
    a.Deposit(20m);
    bool ok = a.TryWithdraw(100m); // fails, invariant protected
    Console.WriteLine("ok=" + ok + " balance=" + a.Balance);
  }
}

ปกป้องคอลเลกชัน

using System;
using System.Collections.Generic;

public class Tags
{
  private readonly List<string> _items = new List<string>();

  public void Add(string tag)
  {
    if (string.IsNullOrWhiteSpace(tag)) return;
    _items.Add(tag.Trim());
  }

  // Expose a copy so callers cannot mutate internal list
  public List<string> GetAllCopy()
  {
    return new List<string>(_items);
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    Tags t = new Tags();
    t.Add(" apple ");
    t.Add("banana");
    List<string> view = t.GetAllCopy();
    view.Add("hacker"); // modifies the copy only
    Console.WriteLine("copy count=" + view.Count);
    Console.WriteLine("original count=" + t.GetAllCopy().Count);
  }
}

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