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

定义

命名空间:

System.Collections.Generic

程序集:

System.Runtime.dll

获取与指定键关联的值。

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

参数

key

TKey

要获取其值的键。

value

TValue

当此方法返回时，如果找到指定键，则返回与该键相关联的值；否则，将返回 value 参数的类型的默认值。 此参数未经初始化即被传递。

返回

Boolean

如果实现 IDictionary<TKey,TValue> 的对象包含具有指定键的元素，则为 true；否则，为 false。

例外

ArgumentNullException

key 为 null。

示例

该示例演示如何使用 TryGetValue 方法检索值。 如果程序经常尝试不在字典中的键值，则 TryGetValue 方法可能比在 Item[] C#) 中使用属性 (索引器更有效，这在尝试检索不存在的键时会引发异常。

此代码是可以编译和执行的较大示例的一部分。 请参阅 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

注解

此方法结合了 方法和 Item[] 属性的功能ContainsKey。

如果未找到键，则 value 参数将为类型 TValue 获取适当的默认值；例如，为整数类型获取零 (0)，为布尔值类型获取 false，为引用类型获取 null。

另请参阅

• ContainsKey(TKey)
• Item[TKey]