Hashtable.Item[Object] 屬性

定義

取得或設定與指定之索引鍵相關聯的值。Gets or sets the value associated with the specified key.

public:
 virtual property System::Object ^ default[System::Object ^] { System::Object ^ get(System::Object ^ key); void set(System::Object ^ key, System::Object ^ value); };
public virtual object this[object key] { get; set; }
public virtual object? this[object key] { get; set; }
member this.Item(obj) : obj with get, set
Default Public Overridable Property Item(key As Object) As Object

參數

key
Object

索引鍵,要讀取或設定其值。The key whose value to get or set.

屬性值

Object

與指定之索引鍵關聯的值。The value associated with the specified key. 如果找不到指定的索引鍵,嘗試取得將傳回 null,並且嘗試設定會使用指定的索引鍵建立新的元素。If the specified key is not found, attempting to get it returns null, and attempting to set it creates a new element using the specified key.

實作

例外狀況

keynullkey is null.

屬性已設定,而且 Hashtable 是唯讀的。The property is set and the Hashtable is read-only.

-或--or- 屬性已設定、key 不在於集合中,而且 Hashtable 的大小固定。The property is set, key does not exist in the collection, and the Hashtable has a fixed size.

備註

這個屬性可透過下列語法存取集合中的特定元素:myCollection[key]This property provides the ability to access a specific element in the collection by using the following syntax: myCollection[key].

您也可以使用 Item[] 屬性來加入新的專案,方法是設定不存在於中的索引鍵值 Hashtable ; 例如 myCollection["myNonexistentKey"] = myValueYou can also use the Item[] property to add new elements by setting the value of a key that does not exist in the Hashtable; for example, myCollection["myNonexistentKey"] = myValue. 但是,如果指定的索引鍵已經存在於中 Hashtable ,則設定 Item[] 屬性會覆寫舊的值。However, if the specified key already exists in the Hashtable, setting the Item[] property overwrites the old value. 相反地,此 Add 方法不會修改現有的元素。In contrast, the Add method does not modify existing elements.

索引鍵不能是 null ,但值可以是。A key cannot be null, but a value can be. 為了區分所 null 傳回的,因為找不到指定的索引鍵,而且 null 因為指定的索引鍵值是而傳回,所以 null 請使用 Contains 方法或 ContainsKey 方法來判斷索引鍵是否存在於清單中。To distinguish between null that is returned because the specified key is not found and null that is returned because the value of the specified key is null, use the Contains method or the ContainsKey method to determine if the key exists in the list.

這個屬性的值是一項作業,而 O(1) 設定屬性也是一項作業 O(1)Retrieving the value of this property is an O(1) operation; setting the property is also an O(1) operation.

C # 語言使用 this 關鍵字來定義索引子,而不是執行 Item[] 屬性。The C# language uses the this keyword to define the indexers instead of implementing the Item[] property. Visual Basic 會將 Item[] 實作為預設屬性,這樣會提供相同的索引功能。Visual Basic implements Item[] as a default property, which provides the same indexing functionality.

適用於

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