C Sharp/Encapsulation

封裝(Encapsulation)是把類的方法、屬性、數據成員不讓用戶訪問或者限制其訪問。C#的類的成員默認是private訪問。

public class Frog
{
    private int _height = 0;

    // Methods
    public void JumpLow() { Jump(1); }
    public void JumpHigh() { Jump(10); }

    void Jump(int height) { _height += height; }
}

保護級別

Private

Private成員只有類內的代碼可以訪問。這是缺省的保護級別。

namespace PrivateSample
{
    public class Person
    {
        private string _name;

        // Methods
        public Person(string name)
        {
            // Private members can only be modified by the internal methods or constructors of class
            this._name = name; 
        }
    }

    public class Entry
    {
        static void Main(string[] args)
        {
            Person OnePerson = new Person("Samanta");
            //OnePerson._name = "Sam"; // This causes a error of access level
        }
    }
}

Protected

Protected的成員可以由派生類的代碼訪問。

namespace ProtectedSample
{
    public class Person
    {
        protected string _name;
    }
    /// <summary>
    /// When a class inherits from other class, it can access your protected and public members
    /// above your created members
    /// </summary>
    public class Warrior : Person
    {
        public void SetName(string name)
        {
            // Protected members can be accessed by internal methods or constructors of class
            // so, it can be accessed by inherit class too
            base._name = name;
        }
    }

    public class Entry
    {
        static void Main(string[] args)
        {
            Warrior OnePerson = new Warrior();
            OnePerson.SetName("Glades"); // OK
            // OnePerson._name = "Sam"; // This causes a error of access level too
            // protected members can not be accessed by external scopes
        }
    }
}

Public

Public的成員可以由任何代碼訪問。

namespace PublicSample
{
    public class Person
    {
        public string Name;
    }

    public class Entry
    {
        static void Main(string[] args)
        {
            Person BeautifulPerson = new Person();
            BeautifulPerson.Name = "Debora"; // OK, public member can be accessed by other scopes
        }
    }
}

Internal

Internal保護的成員只能由同一個assembly的代碼看到並訪問。

namespace InternalSample
{
    public class Person
    {
        internal string Name;
    }

    public class Entry
    {
        static void Main(string[] args)
        {
            Person BeautifulPerson = new Person();
            BeautifulPerson.Name = "Debora"; // OK, internal member can be accessed by other 
            // scopes in same assembly supposing that Person is in another assembly, by example a 
            // library, the name cannot be accessed. In another assembly source, this causes an error:
            // BeautifulPerson.Name = "Debora"; // Cannot access internal member
        }
    }
}

Protected Internal

Protected internal的成員可由派生類的代碼訪問，也可由同一個assembly的代碼訪問。即它是protected 或 internal.

namespace InternalSample
{
    public class Person
    {
        protected internal string Name;
    }

    public class Entry
    {
        static void Main(string[] args)
        {
            Person BeautifulPerson = new Person();
            BeautifulPerson.Name = "Debora"; // As above...
        }
    }
}

public class Book : InternalSample.Person
{
    static void Main(string[] args)
    {
        InternalSample.Person BeautifulPerson = new InternalSample.Person();
        string aName = BeautifulPerson.Name; // Can be accessed, as Book is derived from Person
    }
}