.Net/C# 實現真正的唯讀屬性 (ReadOnly Property)

/*

.Net/C# 實現真正的唯讀屬性 (ReadOnly Property)

當類的私人成員是簡單類型時,只需為該成員提供 public { get; } 的訪問器即可實現唯讀屬性。

當類的私人成員不是簡單類型(如: ArrayList、Hashtable 等)時,
如果僅為該成員提供 public { get; } 的訪問器而實現唯讀屬性是遠遠不夠的!
因為該屬性 ArrayList、Hashtable 還可以被執行 Add(..)、Clear()、Remove(...) 等方法!

經 【身披七彩祥雲 腳踏金甲聖衣】的 "思歸 Saucer" 點撥,
參閱 Reflector: ArrayList.ReadOnly(...) static Method
搞定 ReadOnlyHashtable !

*/

//using System;
//using System.Collections;
//using System.Runtime.Serialization;

namespace Microshaoft
{
 public class WithReadOnlyPropertyClass
 {
  public WithReadOnlyPropertyClass()
  {
   this._Hashtable = new System.Collections.Hashtable();
   this._Hashtable.Add("1", "aaa");
   this._Hashtable.Add("2", "bbb");
   this._Hashtable.Add("3", "ccc");

   this._ArrayList = new System.Collections.ArrayList();
   this._ArrayList.Add("1");
   this._ArrayList.Add("2");
   this._ArrayList.Add("3");
  }

  private System.Collections.ArrayList _ArrayList;
  public System.Collections.ArrayList ReadOnlyArrayList
  {
   get
   {
    //.Net Framework 已經實現
    return System.Collections.ArrayList.ReadOnly(this._ArrayList);
   }
  }

  private System.Collections.Hashtable _Hashtable;
  public System.Collections.Hashtable ReadOnlyHashTable
  {
   get
   {
    return ReadOnlyHashtable.ReadOnly(this._Hashtable);
   }
  }

  //.Net Framework 懶得實現 ReadOnlyHashtable ???
  private class ReadOnlyHashtable : System.Collections.Hashtable
  {
   //《Refactoring: Improving the Design of Existing Code》
   // 3.21 Refused Bequest: Replace Inheritance with Delegation
   //如果不想修改superclass,還可以運用 Replace Inheritance with Delegation 來達到目的。
   //也就是以委託取代繼承,在 subclass 中建立一個 Field 來儲存 superclass 對象,
   //去除 subclass 對 superclass 的繼承關係,委託或調用 superclass 的方法來完成目的。
   //這裡的委託不是 .Net 的 Delegation !
   //僅僅就是代理人、替代品、代表的意思!
   private System.Collections.Hashtable _Hashtable;

   private ReadOnlyHashtable(System.Collections.Hashtable Hashtable)
   {
    this._Hashtable = Hashtable;
   }

   public static System.Collections.Hashtable ReadOnly(System.Collections.Hashtable Hashtable)
   {
    if (Hashtable == null)
    {
     throw new System.ArgumentNullException("Hashtable");
    }
    //多態
    return new ReadOnlyHashtable(Hashtable);
   }

   private string _s = "集合是唯讀。";

   //重寫 override 所有 "寫" 操作的方法,執行階段錯誤,如調用該方法則拋出異常
   public override void Add(object key, object value)
   {
    throw new System.NotSupportedException(this._s);
   }

   public override void Clear()
   {
    throw new System.NotSupportedException(this._s);
   }

   public override object Clone()
   {
    ReadOnlyHashtable roht = new ReadOnlyHashtable(this._Hashtable);
    roht._Hashtable = (System.Collections.Hashtable) this._Hashtable.Clone();
    return roht;
   }

   //重寫 override 方法
   public override bool Contains(object key)
   {
    //用代理的 Hashtable Field 對象的執行個體方法重寫
    //return base.Contains(key);
    return this._Hashtable.Contains(key);
   }

   public override bool ContainsKey(object key)
   {
    return this._Hashtable.ContainsKey(key);
   }

   public override bool ContainsValue(object value)
   {
    return this._Hashtable.ContainsValue(value);
   }

   public override void CopyTo(System.Array array, int arrayIndex)
   {
    this._Hashtable.CopyTo(array, arrayIndex);
   }

   public override System.Collections.IDictionaryEnumerator GetEnumerator()
   {
    //經重寫改為調用代理成員對象的同名執行個體方法
    return this._Hashtable.GetEnumerator();
   }

//   protected override int GetHash(object key)
//   {
//    //protected 成員不能通過代理對象的執行個體方法重寫
//    return base.GetHash(key);
//   }
//
//   protected override bool KeyEquals(object item, object key)
//   {
//    return base.KeyEquals(item, key);
//   }

   public override void Remove(object key)
   {
    throw new System.NotSupportedException(this._s);
   }

   public override void GetObjectData(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context)
   {
    this._Hashtable.GetObjectData(info, context);
   }

   public override void OnDeserialization(object sender)
   {
    this._Hashtable.OnDeserialization(sender);
   }

   public override bool IsReadOnly
   {
    get
    {
     return this._Hashtable.IsReadOnly;
    }
   }

   public override bool IsFixedSize
   {
    get
    {
     return this._Hashtable.IsFixedSize;
    }
   }

   public override bool IsSynchronized
   {
    get
    {
     return this._Hashtable.IsSynchronized;
    }
   }

   public override System.Collections.ICollection Keys
   {
    get
    {
     return this._Hashtable.Keys;
    }
   }

   public override System.Collections.ICollection Values
   {
    get
    {
     return this._Hashtable.Values;
    }
   }

   public override object SyncRoot
   {
    get
    {
     return this._Hashtable.SyncRoot;
    }
   }

   public override int Count
   {
    get
    {
     return this._Hashtable.Count;
    }
   }

   //索引器別忘了重寫 override
   public override object this[object key]
   {
    get
    {
     //使用 代理對象
     return this._Hashtable[key];
    }
    set
    {
     throw new System.NotSupportedException(this._s);
    }
   }
  }
 }
}

class AppTest
{
 static void Main(string[] args)
 {
  Microshaoft.WithReadOnlyPropertyClass x = new Microshaoft.WithReadOnlyPropertyClass();

  System.Console.WriteLine("ReadOnlyArrayList Property Test:");
  //多態
  System.Collections.ArrayList al = x.ReadOnlyArrayList;
  foreach (object o in al)
  {
   System.Console.WriteLine("Value: {0}", o);
  }
  System.Console.WriteLine();
  System.Collections.IEnumerator ie = al.GetEnumerator();
  while (ie.MoveNext())
  {
   System.Console.WriteLine("Value: {0}", ie.Current);
  }

  System.Console.WriteLine("按/"y/"健,執行下面寫操作將拋出異常,按其他健跳過寫操作繼續:");
  if (System.Console.ReadLine().ToLower() == "y")
  {
   System.Console.WriteLine("拋出異常...");
   al.Clear();
  }

  System.Console.WriteLine("/nReadOnlyHashTable Property Test:");
  //多態
  System.Collections.Hashtable ht = x.ReadOnlyHashTable;
  foreach (System.Collections.DictionaryEntry e in ht)
  {
   System.Console.WriteLine("Key: {0} , Value: {1}", e.Key, e.Value);
  }
  System.Console.WriteLine();
  System.Collections.IDictionaryEnumerator ide = ht.GetEnumerator();
  while (ide.MoveNext())
  {
   System.Console.WriteLine("Key: {0} , Value: {1}", ide.Key, ide.Value);
  }
  System.Console.WriteLine("按/"y/"健,執行下面寫操作將拋出異常,按其他健跳過寫操作繼續:");
  if (System.Console.ReadLine().ToLower() == "y")
  {
   System.Console.WriteLine("拋出異常...");
   ht.Clear();
  }
  System.Console.ReadLine();
 }
}