Conversão de delegado reduzida (Visual Basic)

Relaxada a conversão de delegado, introduzido no Visual Basic 2008, permite que você atribua sub-rotinas e funções a delegados ou manipuladores, mesmo quando suas assinaturas não são idênticas. Portanto, a ligação a delegados fica consistente com a ligação já é permitida para chamadas de método . 

Parameters and Return Type

No lugar da assinatura de correspondência exata, a conversão reduzida requer que as seguintes condições sejam atendidos quando Option Strict for definido como On:

• Uma conversão de expansão deve existir do tipo de dados de cada parâmetro do delegado ao tipo de dados do parâmetro correspondente da função atribuída ou Sub. No exemplo a seguir, o delegado Del1 tem um parâmetro, um Integer. Parameter m in the assigned lambda expressions must have a data type for which there is a widening conversion from Integer, such as Long or 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
  

  Narrowing conversions are permitted only when Option Strict is set to Off.

  ' 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
  

• Uma conversão de expansão deve existir na direção oposta do tipo de retorno da função atribuída ou Sub o tipo de retorno do delegado. Nos exemplos a seguir, o corpo de cada expressão lambda atribuído deve ser avaliada como um tipo de dados que amplia a Integer porque o tipo de retorno de del1 é 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)
  

If Option Strict is set to Off, the widening restriction is removed in both directions.

' 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)

Omitting Parameter Specifications

Relaxed delegates also allow you to completely omit parameter specifications in the assigned method:

' 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))

Note that you cannot specify some parameters and omit others.

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

The ability to omit parameters is helpful in a situation such as defining an event handler, where several complex parameters are involved. The arguments to some event handlers are not used. Instead, the handler directly accesses the state of the control on which the event is registered, and ignores the arguments. Relaxed delegates allow you to omit the arguments in such declarations when no ambiguities result. In the following example, the fully specified method OnClick can be rewritten as 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 Examples

Lambda expressions are used in the previous examples to make the type relationships easy to see. However, the same relaxations are permitted for delegate assignments that use AddressOf, Handles, or AddHandler.

In the following example, functions f1, f2, f3, and f4 can all be assigned to 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

O exemplo a seguir é válido somente quando Option Strict for definido como Off.

' 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))

Dropping Function Returns

Relaxed delegate conversion enables you to assign a function to a Sub delegate, effectively ignoring the return value of the function. However, you cannot assign a Sub to a function delegate. In the following example, the address of function doubler is assigned to Sub delegate 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))

Consulte também

Tarefas

Como: Passar Procedimentos para outro Procedimento em Visual Basic.

Referência

Opção declaração estrito

Conceitos

Expressões Lambda (Visual Basic)

Conversões de expansão e restrição (Visual Basic)

Inferência de tipo de variável local (Visual Basic)

Outros recursos

Delegados (Visual Basic)