VB.NET - NullReferenceException 未处理

Question

通常我自己处理错误，但这次我需要专家的帮助！ 这从来没有发生在我身上，而且（通常）拥有的数据越少，你就越难说出发生了什么。

我正在尝试编写一个简单的查询分析器。我随机收到这些类型的崩溃：

1）我从以下函数开始：

Dim thd As New Thread(AddressOf StartSub)
thd.Start()

然后 Startsub 如下：

 Public Sub StartSub()
    CheckForIllegalCrossThreadCalls = False
    txtExecution.Text = "Executing query..."
    Dim query As String = QueryBuilder()
    UpdateView(query)
End Sub

然后 updateview 更新我拥有的数据网格：

    Dim da As New SqlCeDataAdapter(query, connStr)
    Dim dt As New DataTable()
    Try
        da.Fill(dt)
        txtExecution.Text = "Query executed successfully."
        dgTickets.DataSource = dt
    Catch ex As Exception
        txtExecution.Text = "Query failed."
        tbGrid.BeginInvoke(Sub() tbGrid.SelectedTab = tbGrid.TabPages(1))
    End Try

2）代码在下一行的 UpdateQuery 中崩溃（调试器没有说）它在这里崩溃了，我通过选择所有行并逐一浏览它来猜测它：

dgTickets.DataSource = dt

3）调试器说的是： NullReferenceException 未处理（...） 使用 new 关键字创建对象实例

堆栈跟踪：

at System.Windows.Forms.DataGridViewCell.GetEditedFormattedValue(Object value, Int32 rowIndex, DataGridViewCellStyle& dataGridViewCellStyle, DataGridViewDataErrorContexts context)
   at System.Windows.Forms.DataGridViewCell.PaintWork(Graphics graphics, Rectangle clipBounds, Rectangle cellBounds, Int32 rowIndex, DataGridViewElementStates cellState, DataGridViewCellStyle cellStyle, DataGridViewAdvancedBorderStyle advancedBorderStyle, DataGridViewPaintParts paintParts)
   at System.Windows.Forms.DataGridViewRow.PaintCells(Graphics graphics, Rectangle clipBounds, Rectangle rowBounds, Int32 rowIndex, DataGridViewElementStates rowState, Boolean isFirstDisplayedRow, Boolean isLastVisibleRow, DataGridViewPaintParts paintParts)
   at System.Windows.Forms.DataGridViewRow.Paint(Graphics graphics, Rectangle clipBounds, Rectangle rowBounds, Int32 rowIndex, DataGridViewElementStates rowState, Boolean isFirstDisplayedRow, Boolean isLastVisibleRow)
   at System.Windows.Forms.DataGridView.PaintRows(Graphics g, Rectangle boundingRect, Rectangle clipRect, Boolean singleHorizontalBorderAdded)
   at System.Windows.Forms.DataGridView.PaintGrid(Graphics g, Rectangle gridBounds, Rectangle clipRect, Boolean singleVerticalBorderAdded, Boolean singleHorizontalBorderAdded)
   at System.Windows.Forms.DataGridView.OnPaint(PaintEventArgs e)
   at System.Windows.Forms.Control.PaintWithErrorHandling(PaintEventArgs e, Int16 layer)
   at System.Windows.Forms.Control.WmPaint(Message& m)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.DataGridView.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.Run(ApplicationContext context)
   at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.OnRun()
   at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.DoApplicationModel()
   at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.Run(String[] commandLine)
   at SQLquery.My.MyApplication.Main(String[] Args) in 17d14f5c-a337-4978-8281-53493378c1071.vb:line 81 

这实际上非常模糊。上面指定的文件不存在。崩溃的地方用Try-End Try包裹起来。此外，是的，我设置了绘画事件，但它不应该涉及它（或者也许它涉及？）。

就这一点而言，我真的非常非常感谢任何提示。我必须补充一点，我使用的是 Visual Basic Express 版本。该错误偶尔会发生 - 有时当我很幸运时什么也没有发生，而当我不幸运时，我就会遇到此崩溃。

皮特.

Answer 1

您永远不应该触摸/更新后台线程内的任何 GUI 控件。因此，像：

txtExecution.Text = "Executing query..."

和

dgTickets.DataSource = dt

在后台线程中这样的行注定会失败。这应该始终使用 Control.BeginInvoke 在主 GUI 线程上完成。

您似乎正确执行的唯一 GUI 更新是 catch 中的 tbGrid：

tbGrid.BeginInvoke(Sub() tbGrid.SelectedTab = tbGrid.TabPages(1))

您应该阅读有关 UI WinForms 线程调用。