寬鬆委派轉換

更新：2007 年 11 月

即使在簽章不同的情況下，Visual Basic 2008 中加入的寬鬆委派轉換仍可讓您將子函數和函式指派給委派或處理常式。因此，委派的繫結就會與方法引動中允許的繫結一致。 

參數和傳回型別

寬鬆轉換不需要簽章完全相符，而要求在 Option Strict 設定為 On 時符合下列條件：

• 擴展轉換必須存在，方向是從每個委派參數的資料型別到指派之函式或 Sub 相對應參數的資料型別。在下列範例中，委派 Del1 有一個 Integer 參數。所指派之 Lambda 運算式中的參數 m，其資料型別必須可從 Integer (例如 Long 或 Double) 執行擴展轉換。

  ' 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
  

• 反方向的擴展轉換必須存在，方向是從指派之函式或 Sub 的傳回型別到委派的傳回型別。在下列範例中，因為 del1 的傳回型別為 Integer，每個指派之 Lambda 運算式的主體都必須評估為擴展至 Integer 的資料型別。

  ' 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，則會移除雙向擴展限制。

' 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，您不能指定部分參數而省略其他參數。

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

在定義事件處理常式這類牽涉到多個複雜參數的情況下，省略參數將很有幫助。有時不會用到某些事件處理常式的引數，而是處理常式直接存取註冊事件之控制項的狀態，並忽略引數。寬鬆委派可讓您在不會造成模稜兩可 (Ambiguity) 的情況下，於此類宣告中省略引數。在下列範例中，完整指定的方法 OnClick 可以重新撰寫為 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

透過上述範例中 Lambda 運算式的用法，您可以輕易看出型別關聯性。不過，您也可以針對使用 AddressOf、Handles 或 AddHandler 的委派指派，使用相同的寬鬆原則。

在下列範例中，f1、f2、f3 和 f4 函式都可指派至 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

下列範例只有在 Option Strict 設定為 Off 時才有效。

' 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))

置放函式傳回

寬鬆委派轉換可讓您將函式指派給 Sub 委派，進而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，doubler 函式的位址已指派給 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))

請參閱

工作

HOW TO：在 Visual Basic 中將程序傳遞至其他程序

HOW TO：撰寫事件處理常式

概念

Lambda 運算式

擴展和縮小轉換

委派和 AddressOf 運算子

區域型別推斷

WithEvents 和 Handles 子句

參考

Option Strict 陳述式