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

定义

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

keynull

示例

此示例演示如何使用 TryGetValue 该方法检索值。 如果程序经常尝试不在字典中的键值,则 TryGetValue 该方法比使用 Item[] C#) 中的索引器 (属性更高效,这会在尝试检索不存在的键时引发异常。

此代码是可以编译和执行的更大示例的一部分。 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

此方法合并了该方法和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

适用于

另请参阅