宽松委托转换

更新：2007 年 11 月

通过进行 Visual Basic 2008 中引入的宽松委托转换，可以将 Sub 和函数分配给委托或处理程序，即使在签名不同时仍可如此。因此，委托绑定将与方法调用中已允许的绑定保持一致。

参数和返回类型

为替代签名完全匹配，宽松转换要求在将 Option Strict 设置为 On 的时满足以下条件：

• 必须存在从每个委托参数的数据类型到所分配函数或 Sub 的相应参数数据类型的扩大转换。在下面的代码中，委托 Del1 具一个参数 Integer。所分配的 lambda 表达式中的参数 m 必须具有一个数据类型（对于该数据类型，存在一个以 Integer 为转换来源的扩大转换），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
  

  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
  

• 反方向中必须存在从所分配函数或 Sub 的返回类型到委托的返回类型的扩大转换。在下面的示例中，所分配的每个 lambda 表达式体的计算结果都必须是扩大到 Integer 的数据类型，因为 del1 的返回类型为 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)
  

如果将 Option Strict 设置为 Off，则会在两个方向上移除扩大限制。

' 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)

忽略参数规范

宽松委托还允许完全忽略分配的方法中的参数规范：

' 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，不能指定一些参数而忽略其他参数。

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

忽略参数功能在涉及一些复杂参数的情况下（如定义事件处理程序）非常有用。未使用某些事件处理程序的参数。但处理程序可以直接访问注册事件的控件的状态，并忽略这些参数。在不产生多义性时，宽松委托允许忽略此类声明中的参数。在下面的示例中，可以将完全指定的方法 OnClick 重新编写为 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

在先前的示例中使用了 lambda 表达式，以便轻松地查看类型关系。但是，对于使用 AddressOf、Handles 或 AddHandler 的委托分配，允许相同的宽松转换。

在下面的示例中，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

仅当 Option Strict 设置为 Off 时，下面的示例才是有效的。

' 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

通过进行宽松委托转换，可以为 Sub 委托分配函数，并有效地忽略该函数的返回值。但是，不能将 Sub 分配给函数委托。在下面的示例中，将函数 doubler 的地址分配给 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))

请参见

任务

如何：在 Visual Basic 中将过程传递给另一过程

如何：编写事件处理程序

概念

lambda 表达式

扩大转换和收缩转换

委托和 AddressOf 运算符

局部类型推理

WithEvents 语句和 Handles 子句

参考

Option Strict 语句