IDictionary<TKey,TValue>.TryGetValue(TKey, TValue) 方法

定義

命名空間:

System.Collections.Generic

組件:

System.Runtime.dll

取得與指定索引鍵關聯的值。

public:
 bool TryGetValue(TKey key, [Runtime::InteropServices::Out] TValue % value);

public bool TryGetValue (TKey key, out TValue value);

abstract member TryGetValue : 'Key * 'Value -> bool

Public Function TryGetValue (key As TKey, ByRef value As TValue) As Boolean

參數

key

TKey

要取得其值的索引鍵。

value

TValue

這個方法傳回時，如果找到索引鍵，則為與指定索引鍵關聯的值，否則為 value 參數的型別預設值。 這個參數會以未初始化的狀態傳遞。

傳回

Boolean

如果實作 IDictionary<TKey,TValue> 之物件包含具有指定索引鍵的元素，則為 true，否則為 false。

例外狀況

ArgumentNullException

key 為 null。

範例

此範例示範如何使用 TryGetValue 方法來擷取值。 如果程式經常嘗試不在字典中的索引鍵值， TryGetValue 方法比在 Item[] C#) 中使用屬性 (索引器更有效率，這會在嘗試擷取不存在的索引鍵時擲回例外狀況。

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>。

// 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

// 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

備註

這個方法結合了 方法和 Item[] 屬性的功能ContainsKey。

如果找不到索引鍵，則 value 參數會取得型 TValue別的適當預設值;例如，整數類型的零 (0) 、 false 布爾值型別，以及 null 參考型別。

另請參閱

• ContainsKey(TKey)
• Item[TKey]