在SeclectedItem上绑定组合框无法更改选择

我有一个简单的表单，它使用DataBinding绑定到一组对象。最初我不希望ComboBox显示选择，所以我将其选定索引设置为-1。但是，当组合框变为选中状态时，我无法取消选择它，而无需选取值。在SeclectedItem上绑定组合框无法更改选择

如何在不选取值的情况下取消选择ComboBox（选择其他控件）？

要重新创建，创建一个新的winform，添加一个组合框和文本框，然后使用此代码：当形式开始了ComboBox应选择

Imports System.Collections.Generic 

Public Class Form1 

    Public Property f As Person 

    Public Sub New() 
     ' This call is required by the designer. 
     InitializeComponent() 

     ' Add any initialization after the InitializeComponent() call. 
     Dim db As New People 
     ' ComboBox1.CausesValidation = False 
     ComboBox1.DataSource = db 
     ComboBox1.DisplayMember = "Name" 
     ComboBox1.DataBindings.Add("SelectedItem", Me, "f", False, DataSourceUpdateMode.OnPropertyChanged) 
     ComboBox1.SelectedIndex = -1 
    End Sub 

End Class 

Public Class Person 
    Public Property Name As String 
End Class 

Public Class People 
    Inherits List(Of Person) 

    Public Sub New() 
     Me.Add(New Person With {.Name = "Dave"}) 
     Me.Add(New Person With {.Name = "Bob"}) 
     Me.Add(New Person With {.Name = "Steve"}) 
    End Sub 
End Class

，这是无法选择文本框。

我发现在ComboBox上切换CausesValidation为False可以修复问题，但会中断DataBinding。

来源

2017-07-24 Sam Makin

我认为结合'ComboBox'（和更一般'ListControl'）不能被取消选择列表，因为'CurrencyManager'不允许设置的'Position'为-1时基础列表'Count'是> 0 。 –

解决方法添加无效值的NONE项目。 – Ramankingdom

允许用户选择无效值不是一个好的解决方案 –

找到了解决方案！（谢谢伊凡！）

如果ComboBox作为-1的SelectedIndex，当它是Validating时会暂时禁用绑定。

 AddHandler ComboBox1.Validating, AddressOf DisableBindingWhenNothingSelected 
     AddHandler ComboBox1.Validated, AddressOf DisableBindingWhenNothingSelected 

    ''' <summary> 
    ''' Temporarily disables binding when nothing is selected. 
    ''' </summary> 
    ''' <param name="sender">Sender of the event.</param> 
    ''' <param name="e">Event arguments.</param> 
    ''' <remarks>A list bound ComboBox (and more generically ListControl) cannot be deselected, 
    ''' because CurrencyManager does not allow setting the Position to -1 when the underlying list Count is > 0. 
    ''' This should be bound to the Validating and Validated events of all ComboBoxes that have an initial SelectedIndex of -1.</remarks> 
    Protected Sub DisableBindingWhenNothingSelected(sender As Object, e As EventArgs) 
     Dim cb As ComboBox = DirectCast(sender, ComboBox) 
     If cb.SelectedIndex = -1 Then 
      If (cb.DataBindings.Count = 0) Then Exit Sub 
      If cb.DataBindings(0).DataSourceUpdateMode = DataSourceUpdateMode.Never Then 
       cb.DataBindings(0).DataSourceUpdateMode = DataSourceUpdateMode.OnPropertyChanged 
      Else 
       cb.DataBindings(0).DataSourceUpdateMode = DataSourceUpdateMode.Never 
      End If 
     End If 
    End Sub

来源

2017-07-24 10:49:20

在SeclectedItem上绑定组合框无法更改选择

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