Wpf DataGrid：尝试更新数据时出现空引用异常

Question

我有一个DataGrid，它显示来自Access数据库的数据。基本上我要做的是使它在添加或更新数据库时，我想在Datagrid中显示它。

到目前为止我所拥有的计时器将每2秒更新一次：

Public Sub New()
    _UserList = New List(Of Users)
    showUsers()
    Tmr.Interval = 2000
    Tmr.Start()
End Sub

Private Sub TimerEventProcessor(myObject As Object, ByVal myEventArgs As EventArgs) Handles Tmr.Elapsed
    showUsers()
    OnPropertyChanged("SelectedCpuList")
    OnPropertyChanged("UserList")
End Sub

这通常会起作用，但我也在检查那些涉及查看所选行并允许用户删除该记录的数据。这是它中断的地方，基本上每次单击一行时都会设置所选项，但showUsers（）方法会再次将此值设置为空字符串。

Public ReadOnly Property UserList() As List(Of Users)
    Get
        Return _UserList
    End Get
End Property

Public Sub showUsers()

    Dim SelCpuID As String = ""
    If hasRemoved = True Then
        If _SelectedCpuList.Count <= 1 Then
            _SelectedCpuList.Clear()
            OnPropertyChanged("SelectedCpuList")
            OnPropertyChanged("UserList")
            hasRemoved = False
        Else
            OnPropertyChanged("SelectedCpuList")
            OnPropertyChanged("UserList")
            hasRemoved = False
        End If
    End If
    For Each c In _SelectedCpuList
        If String.IsNullOrEmpty(SelCpuID) OrElse SelCpuID.Length = 0 Then
            SelCpuID = c.MachineID
        End If
    Next

    _selUserList = SelCpuID
    _MachineID = SelCpuID

    _UserList = BLL.getUsers()


    OnPropertyChanged("SelectedCpuID")
    OnPropertyChanged("machineID")

End Sub
Private _SelectedCpuList As New List(Of Users)
Public Property SelectedCpuList() As List(Of Users)
    Get
        Return _SelectedCpuList
    End Get
    Set(ByVal value As List(Of Users))

        _SelectedCpuList = value
        OnPropertyChanged("SelectedCpuList")
        showUsers()

    End Set
End Property
Private _SelectedCpuID As Users
Public Property SelectedCpuID() As Users
    Get
        Return _SelectedCpuID
    End Get
    Set(ByVal value As Users)
        _SelectedCpuID = value

        OnPropertyChanged("SelectedCpuID")
        Dim temp As New List(Of Users)
        temp.Add(_SelectedCpuID)
        SelectedCpuList = temp
    End Set
End Property

这将遍历_SelectedCpuList（数据网格中的所选行）中的所有内容，如果所选字符串为空，则它将所选字符串设置为所选行的cpuID。

行SelCpuID = c.MachineID给了我一个Null Reference Exception，我认为可能与更新DataGrid的不断尝试有关。

所以我认为这是一个错误的制作方法，当数据库中的内容被更改或添加时，必须有另一种方法来更新数据？另外，用户必须点击刷新按钮，如果他们忘记这样做可能会很痛苦。