如何在DataGridView绑定时在BindingList中编辑空值？

Question

对于使用Visual Basic .NET的Windows Forms中的数据绑定，我仍然很陌生，但试图熟悉它。我试图寻找这方面的信息，但无济于事。

我想建立双向一DataGridView控制和对象的列表之间的绑定（假设他们被称为MyListElementClass一个虚构的类型），以类似于我在this answer看到了另一个问题的方式。下面是我的实施MyListElementClass，在一个名为MyListElementClass.vb文件：

Imports System.ComponentModel 
Imports System.Runtime.CompilerServices 

<Serializable> 
Public NotInheritable Class MyListElementClass 
    Implements INotifyPropertyChanged 
    Implements IMyListElementClass 

#Region "Fields" 
    Private _a As UShort 
    Private _b As Double 
    Private _c, _d, _e As Boolean 

    ' End fields region. 
#End Region 

#Region "INotifyPropertyChanged implementation" 
    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged 

    Private Sub NotifyPropertyChanged(<CallerMemberName()> Optional ByVal propertyName As String = Nothing) 
     RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propertyName)) 
    End Sub 

    ' End INotifyPropertyChanged implementation region. 
#End Region 

#Region "IMyListElementClass implementation" 
    Public Property PropertyA As UShort Implements IMyListElementClass.PropertyA 
     Get 
      Return _a 
     End Get 
     Set(value As UShort) 
      If _a <> value Then 
       _a = value 
       NotifyPropertyChanged() 
      End If 
     End Set 
    End Property 

    Public Property PropertyB As Double Implements IMyListElementClass.PropertyB 
     Get 
      Return _b 
     End Get 
     Set(value As Double) 
      If _b <> value Then 
       _b = value 
       NotifyPropertyChanged() 
      End If 
     End Set 
    End Property 

    Public Property PropertyC As Boolean Implements IMyListElementClass.PropertyC 
     Get 
      Return _c 
     End Get 
     Set(value As Boolean) 
      If _c <> value Then 
       _c = value 
       NotifyPropertyChanged() 
      End If 
     End Set 
    End Property 

    Public Property PropertyD As Boolean Implements IMyListElementClass.PropertyD 
     Get 
      Return _d 
     End Get 
     Set(value As Boolean) 
      If _d <> value Then 
       _d = value 
       NotifyPropertyChanged() 
      End If 
     End Set 
    End Property 

    Public Property PropertyE As Boolean Implements IMyListElementClass.PropertyE 
     Get 
      Return _e 
     End Get 
     Set(value As Boolean) 
      If _e <> value Then 
       _e = value 
       NotifyPropertyChanged() 
      End If 
     End Set 
    End Property 

    ' End IMyListElementClass implementation region. 
#End Region 

#Region "Constructors" 
    Public Sub New() 
     PropertyA = 0 
     PropertyB = 0 
     PropertyC = False 
     PropertyD = False 
     PropertyE = False 
    End Sub 

    Public Sub New(a As UShort, b As Double, c As Boolean, d As Boolean, e As Boolean) 
     Me.PropertyA = a 
     Me.PropertyB = b 
     Me.PropertyC = c 
     Me.PropertyD = d 
     Me.PropertyE = e 
    End Sub 

    Public Sub New(other As IMyListElementClass) 
     If other Is Nothing Then Throw New ArgumentNullException(NameOf(other)) 
     CopyFrom(other) 
    End Sub 

    ' End constructors region. 
#End Region 

#Region "Methods" 
    Public Sub CopyFrom(other As IMyListElementClass) 
     If other Is Nothing Then Throw New ArgumentNullException(NameOf(other)) 
     With other 
      PropertyA = .PropertyA 
      PropertyB = .PropertyB 
      PropertyC = .PropertyC 
      PropertyD = .PropertyD 
      PropertyE = .PropertyE 
     End With 
    End Sub 

    ' End methods region. 
#End Region 
End Class 

这里的想法是，DataGridView控制将显示的可用列表“槽”（行）认为MyListElementClass实例可以进入到。 但是，其中一些插槽可能为空，可能需要稍后填写或清除。表中的行数由其他地方输入的数字指定，因此用户无法随时添加或删除行;他们必须与所给的空间一起工作。

我在这个当前的尝试是有结合到BindingList(Of MyListElementClass)，其中其大小总是等于可用时隙和空的时隙的数目是由空元素表示的DataGridView控制。但是，我发现如果我在BindingList(Of MyListElementClass)中存在这些空值，那么这些行不能由编辑的DataGridView控件绑定到它，我不确定如何处理这些。

的我想要在其中包含我的用户控制做一个例子，DataGridView（这里命名为dgvDataGridView，并已通过设计师开设专栏）：

Public Class MyUserControl 

    Private _myBindingList As BindingList(Of MyListElementClass) 

    Public Sub New() 
     ' This call is required by the designer. 
     InitializeComponent() 
     ' Add any initialization after the InitializeComponent() call. 
     dgvDataGridView.AutoGenerateColumns = False ' Columns already created through the Visual Studio designer with the ordering and header text I want. 
     SetUpTableDataBinding() 
    End Sub 

    Private Sub SetUpTableDataBinding() 
     colA.DataPropertyName = NameOf(MyListElementClass.PropertyA) 
     colB.DataPropertyName = NameOf(MyListElementClass.PropertyB) 
     colC.DataPropertyName = NameOf(MyListElementClass.PropertyC) 
     colD.DataPropertyName = NameOf(MyListElementClass.PropertyD) 
     colE.DataPropertyName = NameOf(MyListElementClass.PropertyE) 

     Dim initialList As New List(Of MyListElementClass)(Enumerable.Repeat(Of MyListElementClass)(Nothing, 1)) ' First row will contain a null value, and hence be "empty". 
     _myBindingList = New BindingList(Of MyListElementClass)(initialList) 
     Dim source = New BindingSource(_myBindingList, Nothing) 
     dgvDataGridView.DataSource = source 

     ' Some test data for data binding. 
     _myBindingList.AddNew() ' Adds a new MyListElementClass instance with default property values. 
     _myBindingList.Add(New MyListElementClass(2345, 7.4, False, True, False)) ' Just some sample values. 
    End Sub 

End Class 

这个用户控件加载后，我可以看到一个空行，一行为默认值为MyListElementClass，并且有一行包含一些示例值，总共为三行。我可以编辑第二行和第三行，但不是第一行（我输入的任何值立即消失）。

再一次，在这个完全陌生的领域里，请耐心等待。如果我无法得到这个工作，那么我会放弃这个想法，然后回到手动设置和检索数据，就像我以前一直做的那样。

Answer 1

无法编辑空值，将其替换为空字符串，而我认为您会发现它将按预期工作。