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宽松委托转换 (Visual Basic)

  • 项目

使用宽松委托转换可将子元素和函数分配给委托或处理程序，即使这些委托或处理程序的签名不同。 因此，与委托的绑定将与已允许用于方法调用的绑定相一致。

参数和返回类型

作为取代完全匹配签名的一种方式，宽松转换要求当 Option Strict 设置为 On 时满足以下条件：

  • 必须存在从每个委托参数的数据类型，到已分配函数或 Sub 的相应参数的数据类型的扩大转换。 在以下示例中，委托 Del1 有一个参数，即 Integer。 分配的 lambda 表达式中的参数 m 必须具有一个数据类型（例如 Long 或 Double），该类型存在从 Integer 进行的扩大转换。

    ' 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

    仅当 Option Strict 设置为 Off 时，才允许进行收缩转换。

    ' 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

  • 在相反的方向（从已分配函数或 Sub 的返回类型到委托的返回类型）必须存在一个扩大转换。 在以下示例中，每个已分配的 lambda 表达式的主体必须计算为扩大为 Integer 的数据类型，因为 del1 的返回类型是 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 设置为 Off，则会去除两个方向的扩大限制。

' 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)

省略参数规范

宽松委托还允许在分配的方法中完全省略参数规范：

' 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))

请注意，不能指定一些参数，同时省略其他一些参数。

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

在定义事件处理程序等场合下（涉及到多个复杂参数），省略参数的功能很有帮助。 不会使用某些事件处理程序的自变量。 处理程序将直接访问事件所注册到的控件的状态，并忽略自变量。 宽松委托允许在不产生歧义结果的情况下省略此类声明中的自变量。 在以下示例中，完全指定的方法 OnClick 可重写为 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 示例

为了轻松发现类型关系，以上示例中使用了 Lambda 表达式。 但是，对于使用 AddressOf、Handles 或 AddHandler 的委托分配，允许相同的宽松转换。

在以下示例中，函数 f1、f2、f3 和 f4 都可以分配给 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

以下示例仅在 Option Strict 设置为 Off 时才有效。

' 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))

删除函数返回值

使用宽松委托转换可将函数分配给 Sub 委托，从而有效地忽略函数的返回值。 但是，不能将 Sub 分配给函数委托。 在以下示例中，函数 doubler 的地址将分配给 Sub 委托 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))

请参阅