ConditionalWeakTable<TKey,TValue>.GetOrCreateValue(TKey) 方法
定义
重要
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以原子方式在表中搜索指定键，并返回对应的值。 如果表中不存在该键，此方法将调用表示表值的类的无参数构造函数，以创建绑定到指定键的值。
public:
TValue GetOrCreateValue(TKey key);public TValue GetOrCreateValue (TKey key);member this.GetOrCreateValue : 'Key -> 'ValuePublic Function GetOrCreateValue (key As TKey) As TValue参数
- key
- TKey
要搜索的键。
key 表示该属性附加到的对象。
返回
TValue
如果表中已存在 key，则为对应于 key 的值；否则为 TValue 范型类型参数定义的类的无参数构造函数创建的新值。
例外
key 为 null。
表示表中值的类未定义无参数构造函数。
注意：在适用于 Windows 应用商店应用的 .NET 或可移植类库中，改为捕获基类异常 MissingMemberException。
示例
以下示例定义一个 MainClass 类和一个 MainInfo 类，它们提供有关实例 MainClass 的信息。 该示例调用 GetOrCreateValue 方法以将 对象及其附加MainInfo对象添加到MainClassConditionalWeakTable<TKey,TValue>表中。 该示例还演示了调用 Add 和 GetValue 方法以将键/值对添加到表，并调用 TryGetValue 方法以检索现有键的值。
using System;
using System.Reflection;
using System.Runtime.CompilerServices;
public class Example
{
string Name;
public Example(string name)
{
this.Name = name;
}
public override string ToString()
{
return this.Name;
}
}
// Define a class to contain information about each Example instance.
public class ExampleInfo
{
public string Name;
public int Methods;
public int Properties;
public override string ToString()
{
return String.Format("{0}: {1} Methods, {2} Properties",
this.Name, this.Methods, this.Properties);
}
}
public class ExampleTest
{
private static BindingFlags flags = BindingFlags.Public | BindingFlags.Instance;
public static void Main()
{
Example ex1 = new Example("ex1");
Example ex2 = new Example("ex2");
Example ex3 = new Example("ex3");
ExampleInfo exInfo1 = new ExampleInfo();
exInfo1.Name = ex1.ToString();
exInfo1.Methods = ex1.GetType().GetMethods(flags).Length;
exInfo1.Properties = ex1.GetType().GetProperties(flags).Length;
ExampleInfo exInfo3 = new ExampleInfo();
exInfo3.Name = ex3.ToString();
exInfo3.Methods = ex3.GetType().GetMethods(flags).Length;
exInfo3.Properties = ex3.GetType().GetProperties(flags).Length;
var attached = new ConditionalWeakTable<Example, ExampleInfo>();
ExampleInfo value = null;
// Attach a property to ex1 using the Add method, then retrieve it.
attached.Add(ex1, exInfo1);
if (attached.TryGetValue(ex1, out value))
Console.WriteLine("{0}, {1}", ex1, value);
else
Console.WriteLine("{0} does not have an attached property.", ex1);
// Attempt to retrieve the value attached to ex2.
value = attached.GetValue(ex2, ExampleTest.CreateAttachedValue);
if (attached.TryGetValue(ex2, out value))
Console.WriteLine("{0}, {1}", ex2, value);
else
Console.WriteLine("{0} does not have an attached property.", ex2);
// Attempt to retrieve the value attached to ex3.
value = attached.GetOrCreateValue(ex3);
Console.WriteLine("{0}, {1}", ex3, value);
}
public static ExampleInfo CreateAttachedValue(Example ex)
{
ExampleInfo info = new ExampleInfo();
info.Name = ex.ToString();
info.Methods = ex.GetType().GetMethods(flags).Length;
info.Properties = ex.GetType().GetProperties(flags).Length;
return info;
}
}
// The example displays the following output:
// ex1, ex1: 4 Methods, 0 Properties
// ex2, ex2: 4 Methods, 0 Properties
// ex3, : 0 Methods, 0 Properties
Imports System.Reflection
Imports System.Runtime.CompilerServices
Public Class Example
Public Name As String
Public Sub New(name As String)
Me.Name = name
End Sub
Public Overrides Function ToString() As String
Return Me.Name
End Function
End Class
' Define a class to contain information about each Example instance.
Public Class ExampleInfo
Public Name As String
Public Methods As Integer
Public Properties As Integer
Public Overrides Function ToString() As String
Return String.Format("{0}: {1} Methods, {2} Properties", _
Me.Name, Me.Methods, Me.Properties)
End Function
End Class
Module TestExample
Private flags As BindingFlags = BindingFlags.Public Or BindingFlags.Instance
Public Sub Main()
Dim ex1 As New Example("ex1")
Dim ex2 As New Example("ex2")
Dim ex3 As New Example("ex3")
Dim exInfo1 As New ExampleInfo()
exInfo1.Name = ex1.ToString()
exInfo1.Methods = ex1.GetType().GetMethods(flags).Count
exInfo1.Properties = ex1.GetType().GetProperties(flags).Count
Dim exInfo3 As New ExampleInfo()
exInfo3.Name = ex3.ToString()
exInfo3.Methods = ex3.GetType().GetMethods(flags).Count
exInfo3.Properties = ex3.GetType().GetProperties(flags).Count
Dim attached As New ConditionalWeakTable(Of Example, ExampleInfo)
Dim value As ExampleInfo = Nothing
' Attach a property to ex1 using the Add method, then retrieve it.
attached.Add(ex1, exInfo1)
If attached.TryGetValue(ex1, value) Then
Console.WriteLine("{0}, {1}", ex1, value)
Else
Console.WriteLine("{0} does not have an attached property.", ex1)
End If
' Attempt to retrieve the value attached to ex2.
value = attached.GetValue(ex2, AddressOf TestExample.CreateAttachedValue)
If attached.TryGetValue(ex2, value) Then
Console.WriteLine("{0}, {1}", ex2, value)
Else
Console.WriteLine("{0} does not have an attached property.", ex2)
End If
' Atttempt to retrieve the value attached to ex3.
value = attached.GetOrCreateValue(ex3)
Console.WriteLine("{0}, {1}", ex3, value)
End Sub
Public Function CreateAttachedValue(ex As Example) As ExampleInfo
Dim info As New ExampleInfo()
info.Name = ex.ToString()
info.Methods = ex.GetType().GetMethods(flags).Count
info.Properties = ex.GetType().GetProperties(flags).Count
Return info
End Function
End Module
' The example displays the following output:
' ex1, ex1: 4 Methods, 0 Properties
' ex2, ex2: 4 Methods, 0 Properties
' ex3, : 0 Methods, 0 Properties
注解
如果 key 表中不存在，则 方法将添加它，以及通过调用泛型类型参数定义的类的无参数构造函数实例化的对象 TValue 。
TValue如果类没有无参数构造函数，MissingMethodException则会引发 。
如果表值的类定义了无参数构造函数，则这是检索现有值或向 ConditionalWeakTable<TKey,TValue> 表添加新值的建议方法。 如果它未定义无参数构造函数，则可以改为调用 GetValue 方法，该方法依赖于回调提供的方法来实例化表示表值的对象
若要在表中找不到键时检索现有键的值，而不添加键/值对，请调用 TryGetValue 方法。
