[C#] the error "Object reference not set to an instance of an object"

會發生這個 error 的原因是: The code is trying to access a member of a reference type variable that is set to null.

當使用automatic properties 的時候，如果沒有初始化 value type 的 property 會是 0, reference type 則會是 null。因此在建構子當中要針對 properties 做初始化的動作 ( "new" )

參考: http://blogs.msdn.com/csharpfaq/archive/2004/05/06/127647.aspx

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[C# 筆記] Collections and Generics 集合和泛型

• Collections 解決 System.Array 只能裝載同一型別之物件和固定長度的容量之限制。Generics 提供更高的 type safety 和 performance。
  
  
• System.Collections 這個 namespace 實現了許多 interfaces 像是: ICollection, IComparer, IDictionary, IDictionaryEnumerator, IEnumerable, IEnumerator, IHashCodeProvider, IList
  
  
• The Role of ICollection
  
  
• ICollection 是最主要的 interface，他允許你 (a) 取得 container 內物件的數量 (b) the thread safety of the container (c) the ability to copy the contents into a System.Array type
  
  
• public interface ICollection : IEnumerable
  {
  int Count { get; }
  bool IsSynchronized { get; }
  object SyncRoot { get; }
  void CopyTo(Array array, int index);
  }
  
  
• The Role of IDictionary
• A dictionary is simply a collection that maintains a set of name/value pairs
  
  
• public interface IDictionary : ICollection, IEnumerable
  {
    bool IsFixedSize { get; }
    bool IsReadOnly { get; }
    // Type indexer; see Chapter 12 for full details.
    object this[object key] { get; set; }
    ICollection Keys { get; }
    ICollection Values { get; }
    void Add(object key, object value);
    void Clear();
    bool Contains(object key);
    IDictionaryEnumerator GetEnumerator();
    void Remove(object key);
  }
  
  
• The Role of IDictionaryEnumerator
  
  
• IDictionaryEnumerator is simply a strongly typed enumerator, given that it extends IEnumerator by adding the following functionality:
  
  public interface IDictionaryEnumerator : IEnumerator
  {
    DictionaryEntry Entry { get; }
    object Key { get; }
    object Value { get; }
  }
  
  
• The Role of IList
  
  
• provides the ability to insert, remove, and index items into (or out of) a container
  
  
• public interface IList : ICollection, IEnumerable
  {
    bool IsFixedSize { get; }
    bool IsReadOnly { get; }
    object this[ int index ] { get; set; }
    int Add(object value);
    void Clear();
    bool Contains(object value);
    int IndexOf(object value);
    void Insert(int index, object value);
    void Remove(object value);
    void RemoveAt(int index);
  }
  
  
• The Class Types of System.Collections
•