VB.NET WinForms中C# Binding.DoNothing等价实现及绑定冲突解决

问题场景 #

在WinForms应用中，有两个单选按钮optVal1（默认选中）和optVal2，通过自定义方法绑定到inputs类的Selection属性。原本功能正常，但添加CheckedChanged事件处理程序后，点击optVal2时optVal1会自动恢复选中状态，调试输出显示inputs.Selection未正确更新，且出现状态来回切换的情况。

绑定代码 #

AddDataBindingForRadioButton(optVal1, "Selection", "Val1")
AddDataBindingForRadioButton(optVal2, "Selection", "Val2")

绑定辅助函数 #

Private Sub AddDataBindingForRadioButton(c As Control, name As String, value As String)
    Dim b As Binding
    b = New Binding("Checked", inputs, name, True, DataSourceUpdateMode.OnPropertyChanged)
    ' Format: model to UI
    AddHandler b.Format, Sub(s, e)
                             e.Value = (CStr(e.Value) = value)
                         End Sub
    ' Parse: UI to model
    AddHandler b.Parse, Sub(s, e)
                            If CBool(e.Value) Then
                                e.Value = value
                            Else
                                e.Value = DBNull.Value
                            End If
                        End Sub
    c.DataBindings.Add(b)
End Sub

CheckedChanged事件代码 #

Private Sub valueChanged(sender As Object, e As EventArgs) Handles optVal1.CheckedChanged, optVal2.CheckedChanged
    If inputs Is Nothing Then
        Exit Sub
    End If

    Debug.WriteLine("optVal1.Checked=" & optVal1.Checked & "  optVal2.Checked=" & optVal2.Checked & "  inputs.Selection=" & If(inputs?.StructureType, "null"))

    Dim opt As RadioButton = DirectCast(sender, RadioButton)
    If Not opt.Checked Then
        Exit Sub
    End If

    If opt Is optVal1 Then
        Debug.Print("Val1")
        grpOtherProp0.Text = "Square"
        inputs.OtherProp1 = 10
        inputs.OtherProp2 = "Square"
    ElseIf opt Is optVal2 Then
        Debug.Print("Val2")
        grpOtherProp0.Text = "Non-Square"
        inputs.OtherProp1 = 18
        inputs.OtherProp2 = "Round"
    End If
End Sub

调试输出 #

optVal1.Checked=False  optVal2.Checked=True  inputs.Selection=Val1
optVal1.Checked=False  optVal2.Checked=True  inputs.Selection=Val1
Val2
optVal1.Checked=True  optVal2.Checked=False  inputs.Selection=Val1
optVal1.Checked=True  optVal2.Checked=False  inputs.Selection=Val1
Val1

问题原因 #

点击optVal2时，其Checked变为True触发CheckedChanged事件，同时绑定的Parse事件触发；而此时optVal1的Checked变为False，它的Parse事件也会执行，将inputs.Selection设为DBNull.Value，导致绑定的Format事件重新执行，把optVal1的Checked又设回True，形成循环。VB.NET WinForms的Binding没有DoNothing选项直接取消Parse时的更新操作，因此需要通过其他方式规避。

解决方案 #

方案1：通过抛出异常终止无效的绑定更新 #

修改AddDataBindingForRadioButton中的Parse事件逻辑，仅在单选按钮被选中时更新数据源，取消选中时抛出OperationCanceledException（WinForms会自动捕获此异常并终止本次绑定更新）：

Private Sub AddDataBindingForRadioButton(c As Control, name As String, value As String)
    Dim b As Binding
    b = New Binding("Checked", inputs, name, True, DataSourceUpdateMode.OnPropertyChanged)
    ' Format: model to UI
    AddHandler b.Format, Sub(s, e)
                             e.Value = (CStr(e.Value) = value)
                         End Sub
    ' Parse: UI to model - 仅选中时更新，取消时终止绑定
    AddHandler b.Parse, Sub(s, e)
                            Dim radioBtn As RadioButton = DirectCast(c, RadioButton)
                            If radioBtn.Checked Then
                                e.Value = value
                            Else
                                Throw New System.ComponentModel.OperationCanceledException()
                            End If
                        End Sub
    c.DataBindings.Add(b)
End Sub

此方案的核心是利用WinForms对OperationCanceledException的特殊处理，取消非选中状态下的数据源更新，避免optVal1的Parse事件覆盖正确的Selection值。

方案2：手动管理绑定更新状态 #

在CheckedChanged事件中，先暂停所有单选按钮的绑定更新，手动设置数据源和UI属性，完成后恢复绑定：

Private Sub valueChanged(sender As Object, e As EventArgs) Handles optVal1.CheckedChanged, optVal2.CheckedChanged
    If inputs Is Nothing Then
        Exit Sub
    End If

    Dim opt As RadioButton = DirectCast(sender, RadioButton)
    If Not opt.Checked Then
        Exit Sub
    End If

    ' 临时禁用绑定的数据源更新
    Dim originalMode = DataSourceUpdateMode.OnPropertyChanged
    For Each binding As Binding In optVal1.DataBindings
        originalMode = binding.DataSourceUpdateMode
        binding.DataSourceUpdateMode = DataSourceUpdateMode.Never
    Next
    For Each binding As Binding In optVal2.DataBindings
        binding.DataSourceUpdateMode = DataSourceUpdateMode.Never
    Next

    Try
        ' 手动设置数据源和UI属性
        If opt Is optVal1 Then
            Debug.Print("Val1")
            grpOtherProp0.Text = "Square"
            inputs.OtherProp1 = 10
            inputs.OtherProp2 = "Square"
            inputs.Selection = "Val1"
        ElseIf opt Is optVal2 Then
            Debug.Print("Val2")
            grpOtherProp0.Text = "Non-Square"
            inputs.OtherProp1 = 18
            inputs.OtherProp2 = "Round"
            inputs.Selection = "Val2"
        End If
    Finally
        ' 恢复绑定的更新模式
        For Each binding As Binding In optVal1.DataBindings
            binding.DataSourceUpdateMode = originalMode
        Next
        For Each binding As Binding In optVal2.DataBindings
            binding.DataSourceUpdateMode = originalMode
        Next
    End Try
End Sub

同时可简化绑定的Parse事件（无需处理，因为手动管理数据源）：

AddHandler b.Parse, Sub(s, e)
                        ' 无需处理，由事件手动更新数据源
                    End Sub

效果验证 #

使用方案1修改后，点击optVal2时，仅它的Parse事件会将inputs.Selection设为Val2，optVal1的Parse事件抛出异常终止更新，绑定的Format事件会正确同步UI状态：optVal1保持False，optVal2保持True，不会出现状态来回切换的问题。

内容的提问来源于stack exchange，提问作者VeryColdAir