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

定义

获取与指定键关联的值。

public:
 virtual bool TryGetValue(TKey key, [Runtime::InteropServices::Out] TValue % value);
public bool TryGetValue (TKey key, out TValue value);
abstract member TryGetValue : 'Key * 'Value -> bool
override this.TryGetValue : 'Key * 'Value -> bool
Public Function TryGetValue (key As TKey, ByRef value As TValue) As Boolean

参数

key
TKey

要获取的值的键。

value
TValue

当此方法返回值时,如果找到该键,便会返回与指定的键相关联的值;否则,则会返回 value 参数的类型默认值。 此参数未经初始化即被传递。

返回

如果 true 包含具有指定键的元素,则为 Dictionary<TKey,TValue>;否则为 false

实现

例外

keynull

示例

该示例演示如何使用 TryGetValue 方法作为一种更有效的方法来检索经常尝试不在字典中的键的程序中的值。 相比之下,该示例还演示了属性 (C#) 在尝试检索不存在的键时如何 Item[] 引发异常。

此代码示例是为 Dictionary<TKey,TValue> 类提供的更大示例的一部分, (openWith 是此示例中使用的字典的名称) 。

// 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 0(零),为布尔值类型获取 false,为引用类型获取 null

TryGetValue如果代码经常尝试访问不在字典中的键,请使用 方法。 使用此方法比捕获 KeyNotFoundException 属性引发的 Item[] 更有效。

此方法接近 O (1) 操作。

适用于

另请参阅