IDictionary<TKey,TValue>.Item[TKey] 屬性

定義

命名空間:

System.Collections.Generic

組件:

System.Runtime.dll

取得或設定具有指定索引鍵的項目。

public:
 property TValue default[TKey] { TValue get(TKey key); void set(TKey key, TValue value); };

public TValue this[TKey key] { get; set; }

member this.Item('Key) : 'Value with get, set

Default Public Property Item(key As TKey) As TValue

參數

key

TKey

要取得或設定之項目的索引鍵。

屬性值

TValue

具有指定索引鍵的項目。

例外狀況

ArgumentNullException

key 為 null。

KeyNotFoundException

屬性已擷取，並且找不到 key。

NotSupportedException

屬性已設定，而且 IDictionary<TKey,TValue> 是唯讀的。

範例

下列程式代碼範例會 Item[] 使用屬性 (C# ) 中的索引器來擷取值，示範 KeyNotFoundException 當要求的索引鍵不存在時擲回 ，並顯示可以取代與索引鍵相關聯的值。

如果程式通常必須嘗試不在字典中的索引鍵值，此範例也會示範如何使用 TryGetValue 方法作為更有效率的方式來擷取值。

此程式代碼是可編譯和執行之較大範例的一部分。 請參閱 System.Collections.Generic.IDictionary<TKey,TValue>。

// The Item property is another name for the indexer, so you
// can omit its name when accessing elements.
Console::WriteLine("For key = \"rtf\", value = {0}.",
    openWith["rtf"]);

// The indexer can be used to change the value associated
// with a key.
openWith["rtf"] = "winword.exe";
Console::WriteLine("For key = \"rtf\", value = {0}.",
    openWith["rtf"]);

// If a key does not exist, setting the indexer for that key
// adds a new key/value pair.
openWith["doc"] = "winword.exe";

// The Item property is another name for the indexer, so you
// can omit its name when accessing elements.
Console.WriteLine("For key = \"rtf\", value = {0}.",
    openWith["rtf"]);

// The indexer can be used to change the value associated
// with a key.
openWith["rtf"] = "winword.exe";
Console.WriteLine("For key = \"rtf\", value = {0}.",
    openWith["rtf"]);

// If a key does not exist, setting the indexer for that key
// adds a new key/value pair.
openWith["doc"] = "winword.exe";

' The Item property is the default property, so you 
' can omit its name when accessing elements. 
Console.WriteLine("For key = ""rtf"", value = {0}.", _
    openWith("rtf"))

' The default Item property can be used to change the value
' associated with a key.
openWith("rtf") = "winword.exe"
Console.WriteLine("For key = ""rtf"", value = {0}.", _
    openWith("rtf"))

' If a key does not exist, setting the default item property
' for that key adds a new key/value pair.
openWith("doc") = "winword.exe"

// The indexer throws an exception if the requested key is
// not in the dictionary.
try
{
    Console::WriteLine("For key = \"tif\", value = {0}.",
        openWith["tif"]);
}
catch (KeyNotFoundException^)
{
    Console::WriteLine("Key = \"tif\" is not found.");
}

// The indexer throws an exception if the requested key is
// not in the dictionary.
try
{
    Console.WriteLine("For key = \"tif\", value = {0}.",
        openWith["tif"]);
}
catch (KeyNotFoundException)
{
    Console.WriteLine("Key = \"tif\" is not found.");
}

' The default Item property throws an exception if the requested
' key is not in the dictionary.
Try
    Console.WriteLine("For key = ""tif"", value = {0}.", _
        openWith("tif"))
Catch 
    Console.WriteLine("Key = ""tif"" is not found.")
End Try

// When a program often has to try keys that turn out not to
// be in the dictionary, TryGetValue can be a more efficient
// way to retrieve values.
String^ value = "";
if (openWith->TryGetValue("tif", value))
{
    Console::WriteLine("For key = \"tif\", value = {0}.", value);
}
else
{
    Console::WriteLine("Key = \"tif\" is not found.");
}

// When a program often has to try keys that turn out not to
// be in the dictionary, TryGetValue can be a more efficient
// way to retrieve values.
string value = "";
if (openWith.TryGetValue("tif", out value))
{
    Console.WriteLine("For key = \"tif\", value = {0}.", value);
}
else
{
    Console.WriteLine("Key = \"tif\" is not found.");
}

' When a program often has to try keys that turn out not to
' be in the dictionary, TryGetValue can be a more efficient 
' way to retrieve values.
Dim value As String = ""
If openWith.TryGetValue("tif", value) Then
    Console.WriteLine("For key = ""tif"", value = {0}.", value)
Else
    Console.WriteLine("Key = ""tif"" is not found.")
End If

備註

此屬性提供使用下列語法存取集合中特定元素的能力： myCollection[key] 在 Visual Basic 中 (myCollection(key)) 。

您也可以使用 Item[] 屬性來新增元素，方法是設定字典中不存在的索引鍵值;例如， myCollection["myNonexistentKey"] = myValue 在 Visual Basic) 的 C# (myCollection("myNonexistentKey") = myValue 中。 不過，如果指定的索引鍵已存在於字典中，則設定 Item[] 屬性會覆寫舊的值。 相反地， Add 方法不會修改現有的專案。

實作可能會因判斷物件的相等方式而有所不同;例如， List<T> 類別會使用 Comparer<T>.Default，而 Dictionary<TKey,TValue> 類別可讓使用者指定要 IComparer<T> 用於比較索引鍵的實作。

C# 語言會使用此 關鍵詞來定義索引器，而不是實作 Item[] 屬性。 Visual Basic 會將 Item[] 實作為預設屬性，這樣會提供相同的索引功能。

實作可能會因是否允許 key 為 null而有所不同。

另請參閱

• Add(TKey, TValue)
• ContainsKey(TKey)
• IsReadOnly