ConditionalWeakTable<TKey,TValue>.GetOrCreateValue(TKey) 方法
定義
重要
部分資訊涉及發行前產品，在發行之前可能會有大幅修改。 Microsoft 對此處提供的資訊，不做任何明確或隱含的瑕疵擔保。
以不可部分完成的方式搜尋數據表中的指定索引鍵，並傳回對應的值。 如果索引鍵不存在於數據表中，方法會叫用 類別的無參數建構函式，此建構函式代表數據表的值，以建立系結至指定索引鍵的值。
public:
TValue GetOrCreateValue(TKey key);public TValue GetOrCreateValue(TKey key);member this.GetOrCreateValue : 'Key -> 'ValuePublic Function GetOrCreateValue (key As TKey) As TValue參數
- key
- TKey
要搜尋的鍵。
key 代表該屬性所附著的物件。
傳回
TValue
對應於的 key值（若 key 表格中已存在）;否則，則由類別的無參數建構器創造一個由泛 TValue 型參數參數所定義的新值。
例外狀況
key 為 null。
代表資料表值的類別並不定義無參數建構子。
範例
以下範例定義了一個 MainClass 類別和一個 MainInfo 類別，後者提供了關於該實例的 MainClass 資訊。 範例中呼叫GetOrCreateValue將物件及其附加MainInfo物件加入MainClassConditionalWeakTable<TKey,TValue>資料表的方法。 範例同時說明 Add 呼叫 和 GetValue 方法以將鍵值對加入資料表，以及呼叫 TryGetValue 方法以取得現有鍵值。
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
備註
如果 key 表格中不存在，方法會將它加入，並連同由泛型參數定義 TValue 的類別中無參數建構子實例化的物件一起。 若類別 TValue 沒有無參數建構子，則拋出 a MissingMethodException 。
這是擷取現有值或在表中類別定義無參數建構子時新增值 ConditionalWeakTable<TKey,TValue> 的推薦方法。 如果它沒有定義無參數建構子，你可以呼叫該 GetValue 方法，該方法依賴回調提供的方法來實例化代表該資料表值的物件
若在表格中找不到該鍵，若要取得現有鍵值的值而不加入鍵值對，請呼叫該 TryGetValue 方法。
