Dictionary<TKey,TValue>.Item[TKey] 屬性

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

屬性值

與指定鍵相關聯的值。 若找不到指定的鍵，則執行 get 操作拋出 ， KeyNotFoundException集合操作則會產生帶有指定鍵的新元素。

實作

例外狀況

範例

以下程式碼範例使用 Item[] 屬性（C# 中的索引器）來取得值，示範當請求的金鑰不存在時會拋出 a KeyNotFoundException ，並說明與鍵相關的值可以被替換。

範例也展示了當程式經常必須嘗試字典中不存在的鍵值時，如何將此 TryGetValue 方法作為更有效率的取值方法。

此程式碼範例是本類別更大範例 Dictionary<TKey,TValue> 的一部分。 openWith 是本範例中使用的字典名稱。

// Create a new dictionary of strings, with string keys.
//
Dictionary<string, string> openWith =
    new Dictionary<string, string>();

// Add some elements to the dictionary. There are no
// duplicate keys, but some of the values are duplicates.
openWith.Add("txt", "notepad.exe");
openWith.Add("bmp", "paint.exe");
openWith.Add("dib", "paint.exe");
openWith.Add("rtf", "wordpad.exe");

// The Add method throws an exception if the new key is
// already in the dictionary.
try
{
    openWith.Add("txt", "winword.exe");
}
catch (ArgumentException)
{
    Console.WriteLine("An element with Key = \"txt\" already exists.");
}

// Create a new dictionary of strings, with string keys.
let openWith = Dictionary<string, string>()

// Add some elements to the dictionary. There are no
// duplicate keys, but some of the values are duplicates.
openWith.Add("txt", "notepad.exe")
openWith.Add("bmp", "paint.exe")
openWith.Add("dib", "paint.exe")
openWith.Add("rtf", "wordpad.exe")

// The Add method throws an exception if the new key is
// already in the dictionary.
try
    openWith.Add("txt", "winword.exe")
with :? ArgumentException ->
    printfn "An element with Key = \"txt\" already exists."

' Create a new dictionary of strings, with string keys.
'
Dim openWith As New Dictionary(Of String, String)

' Add some elements to the dictionary. There are no 
' duplicate keys, but some of the values are duplicates.
openWith.Add("txt", "notepad.exe")
openWith.Add("bmp", "paint.exe")
openWith.Add("dib", "paint.exe")
openWith.Add("rtf", "wordpad.exe")

' The Add method throws an exception if the new key is 
' already in the dictionary.
Try
    openWith.Add("txt", "winword.exe")
Catch 
    Console.WriteLine("An element with Key = ""txt"" already exists.")
End Try

// 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.
printfn $"""For key = "rtf", value = {openWith["rtf"]}"""

// The indexer can be used to change the value associated
// with a key.
openWith["rtf"] <- "winword.exe"
printfn $"""For key = "rtf", value = {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
    printfn $"""For key = "tif", value = {openWith["tif"]}"""
with :? KeyNotFoundException ->
    printfn "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", 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.
match openWith.TryGetValue "tif" with
| true, value -> printfn $"For key = \"tif\", value = {value}."
| _ -> printfn "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

此特性允許使用以下 C# 語法存取集合中的特定元素：myCollection[key]（在 Visual Basic 中為 myCollection(key)）。

你也可以利用這個 Item[] 屬性來新增元素，方法是設定一個不存在於 Dictionary<TKey,TValue>的鍵值。 當你設定屬性值時，如果鍵值在 Dictionary<TKey,TValue>，該鍵的值會被分配值取代。 若鍵不在 中 Dictionary<TKey,TValue>，則將鍵與值加入字典。 相較之下，該 Add 方法不會修改現有元素。

若值null類型是參考型態，鍵值不可能是 TValue，但值可以是。

C# 語言使用 this 關鍵字來定義索引器，而非實作該 Item[] 屬性。 Visual Basic 作為 Item[] 預設屬性實作，提供相同的索引功能。

取得或設定此性質的值接近 O（1） 運算。

另請參閱

• Add(TKey, TValue)