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寬鬆委派轉換 (Visual Basic)

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寬鬆委派轉換可讓您將 Sub 和函式指派給委派或處理常式，即使其特徵標記不同也一樣。 因此，委派的繫結會變成與已允許用於方法叫用的繫結一致。

參數和傳回型別

為了取代完全相符的特徵標記，寬鬆轉換需要在 Option Strict 設定為 On 時符合下列條件：

  • 必須存在從每個委派參數資料類型到指派函式或 Sub 之對應參數資料類型的放大轉換。 在下列範例中，委派 Del1 具有一個參數 Integer。 指派 Lambda 運算式中的參數 m 必須具有從 Integer (例如 Long 或 Double) 放大轉換而來的資料類型。

    ' 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

    只有在 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

  • 必須存在從指派函式或 Sub 傳回型別到委派傳回型別之相反方向的放大轉換。 在下列範例中，每個指派 Lambda 運算式的主體都必須評估為放大成 Integer 的資料類型，因為 del1 的傳回型別為 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，則兩個方向都會移除放大限制。

' 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

在定義事件處理常式等情況下，由於涉及數個複雜的參數，因此能夠省略參數會很有幫助。 不會使用提供給某些事件處理常式的引數。 相反地，處理常式會直接存取註冊事件所在控制項的狀態，並忽略引數。 寬鬆委派可讓您在沒有模棱兩可的結果時，省略這類宣告中的引數。 在下列範例中，完整指定的方法 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

在先前的範例中，使用 Lambda 運算式以方便查看類型關聯性。 不過，使用 AddressOf、Handles 或 AddHandler 的委派指派允許相同的寬鬆轉換。

在下列範例中，函式 f1、f2、f3 和 f4 全部都可指派給 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

只有在 Option Strict 設定為 Off時，下列範例才有效。

' 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))

卸除函式傳回

寬鬆委派轉換可讓您將函式指派給 Sub 委派，以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，會將函式 doubler 的位址指派給 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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