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寬鬆的委派轉換（Visual Basic）

寬鬆的委派轉換可讓您將子和函式指派給委派或處理程式，即使其簽章不相同也一樣。 因此，將委派系結，將與已允許的方法呼叫系結保持一致。

參數和傳回類型

為了取代精確的簽章比對，當Option Strict設為On時，寬鬆的轉換需要滿足以下條件：

  • 從每個委派參數的數據類型到指派函式或 Sub之對應參數的數據類型，都必須有擴大轉換。 在下列範例中，委派Del1有一個參數：。Integer 指派之 Lambda 運算式中的參數 m 必須具有資料類型，而該資料類型有從 Integer的擴展轉換，例如 Long 或 Double。

    ' Definition of delegate Del1.
Delegate Function Del1(ByVal arg As Integer) As Integer

    ' Valid lambda expression assignments with Option Strict on or off:

' Integer matches Integer.
Dim d1 As Del1 = Function(m As Integer) 3

' Integer widens to Long
Dim d2 As Del1 = Function(m As Long) 3

' Integer widens to Double
Dim d3 As Del1 = Function(m As Double) 3

    只有當 Option Strict 設定為 Off 時，才允許縮小轉換。

    ' Valid only when Option Strict is off:

Dim d4 As Del1 = Function(m As String) CInt(m)
Dim d5 As Del1 = Function(m As Short) m

  • 擴大轉換必須與指派函式的傳回型別或 Sub 委派的傳回型別相反方向存在。 在下列範例中，每個指派的 Lambda 運算式主體都必須被評估為能夠拓展成 Integer 的數據類型，因為 del1 的回傳類型是 Integer。

    ' Valid return types with Option Strict on:

' Integer matches Integer.
Dim d6 As Del1 = Function(m As Integer) m

' Short widens to Integer.
Dim d7 As Del1 = Function(m As Long) CShort(m)

' Byte widens to Integer.
Dim d8 As Del1 = Function(m As Double) CByte(m)

如果 Option Strict 設定為 Off，則會雙向移除擴大限制。

' Valid only when Option Strict is set to Off.

' Integer does not widen to Short in the parameter.
Dim d9 As Del1 = Function(n As Short) n

' Long does not widen to Integer in the return type.
Dim d10 As Del1 = Function(n As Integer) CLng(n)

省略參數規格

寬鬆的委派也可讓您在指派的方法中完全省略參數規格：

' Definition of delegate Del2, which has two parameters.
Delegate Function Del2(ByVal arg1 As Integer, ByVal arg2 As String) As Integer

' The assigned lambda expression specifies no parameters, even though
' Del2 has two parameters. Because the assigned function in this 
' example is a lambda expression, Option Strict can be on or off.
' Compare the declaration of d16, where a standard function is assigned.
Dim d11 As Del2 = Function() 3

' The parameters are still there, however, as defined in the delegate.
Console.WriteLine(d11(5, "five"))

' Not valid.
' Console.WriteLine(d11())
' Console.WriteLine(d11(5))

請注意，您無法指定某些參數，並省略其他參數。

' Not valid.
'Dim d12 As Del2 = Function(p As Integer) p

在類似定義事件處理器的情況下，省略參數的特性非常有幫助，尤其是涉及多個複雜的參數時。 某些事件處理程式的參數未被使用。 相反地，處理程式會直接存取註冊事件之控件的狀態，並忽略自變數。 當沒有歧義情況時，寬鬆的代理（delegate）允許您省略這類宣告中的參數。 在下列範例中，完全指定的方法 OnClick 可以重寫為 RelaxedOnClick。

Sub OnClick(ByVal sender As Object, ByVal e As EventArgs) Handles b.Click  
    MessageBox.Show("Hello World from" + b.Text)  
End Sub  
  
Sub RelaxedOnClick() Handles b.Click  
    MessageBox.Show("Hello World from" + b.Text)  
End Sub

AddressOf 範例

Lambda 表達式會用於先前的範例中，讓類型關聯性易於查看。 不過，對於委派任務中使用 AddressOf、Handles 或 AddHandler 的情況，允許相同的放寬。

在下列範例中，函式 f1、 f2、 f3和 f4 全都可以指派給 Del1。

' Definition of delegate Del1.
Delegate Function Del1(ByVal arg As Integer) As Integer

' Definitions of f1, f2, f3, and f4.
Function f1(ByVal m As Integer) As Integer
End Function

Function f2(ByVal m As Long) As Integer
End Function

Function f3(ByVal m As Integer) As Short
End Function

Function f4() As Integer
End Function

' Assignments to function delegate Del1.

' Valid AddressOf assignments with Option Strict on or off:

' Integer parameters of delegate and function match.
Dim d13 As Del1 = AddressOf f1

' Integer delegate parameter widens to Long.
Dim d14 As Del1 = AddressOf f2

' Short return in f3 widens to Integer.
Dim d15 As Del1 = AddressOf f3

下列範例僅在 Option Strict 設定為 Off 時有效。

' If Option Strict is Off, parameter specifications for f4 can be omitted.
Dim d16 As Del1 = AddressOf f4

' Function d16 still requires a single argument, however, as specified
' by Del1.
Console.WriteLine(d16(5))

' Not valid.
'Console.WriteLine(d16())
'Console.WriteLine(d16(5, 3))

丟棄函式返回值

寬鬆的委派轉換可讓您將函式指派給 Sub 委派，有效地忽略函式的傳回值。 不過，您無法將 Sub 指派給函式委派。 在下列範例中，函式的 doubler 位址會指派給 Sub 委派 Del3。

' Definition of Sub delegate Del3.
Delegate Sub Del3(ByVal arg1 As Integer)

' Definition of function doubler, which both displays and returns the
' value of its integer parameter.
Function doubler(ByVal p As Integer) As Integer
    Dim times2 = 2 * p
    Console.WriteLine("Value of p: " & p)
    Console.WriteLine("Double p:   " & times2)
    Return times2
End Function

' You can assign the function to the Sub delegate:
Dim d17 As Del3 = AddressOf doubler

' You can then call d17 like a regular Sub procedure.
d17(5)

' You cannot call d17 as a function. It is a Sub, and has no 
' return value.
' Not valid.
'Console.WriteLine(d17(5))

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