vb代码背后的问题 - ASP.NET 4.0 [英] Problem in vb code behind - ASP.NET 4.0

问题描述

当我尝试在asp.net我们的Web应用程序服务器中加载页面时，我在确定导致对象引用未设置为对象实例错误的原因时遇到问题，但是当我运行时，相同的代码运行正常它在IDE（Microsoft Visual Web Developer 2010 Express）中。

这是aspx的摘录

I have a problem in determining what causes an 'object reference not set to an instance of an object' error when I try to load a page in asp.net our web application server, but the same code runs fine when I run it in the IDE (Microsoft Visual Web Developer 2010 Express).
Here is an extract from the aspx

   <div id="div1"
   style="border: thin none #000000; font-family: Arial, Helvetica, sans-serif; font-weight: bold; font-size: inherit; font-style: normal; color: #000000;">

       <asp:Label ID="lblSN" runat="server" Text="Serial Number:" Height="22px" ></asp:Label> <br />
       <asp:Label ID="lblFamDesc" runat="server" Text="Family Description:" Height="22px" ></asp:Label> <br />
       <asp:Label ID="lblCusNam" runat="server"  Text="Select Customer Name:" Height="22px"></asp:Label> <br />
       <asp:Label ID="lblWONumber" runat="server" Text="Works Order Number:" Height="22px"></asp:Label> <br />
       <asp:Label ID="lblSONumber" runat="server" Text="Sales Order Number:" Height="22px"></asp:Label> <br />
       <asp:Label ID="lblCusCod" runat="server" Text="Customer Code:" Height="22px"></asp:Label> <br />
       <asp:Label ID="lblModelNo" runat="server" Text="Model Number:" Height="22px"></asp:Label> <br />
       <asp:Label ID="lblCaldBy" runat="server" Text="Calibrator's Name:" Height="22px"></asp:Label> <br />
       <asp:Label ID="lblStartDatePrompt" runat="server" Text="From: "></asp:Label>
       <asp:Label ID="lblStartDateValue" runat="server" Height="22px"></asp:Label> <br />
       <asp:Label ID="lblEndDatePrompt" runat="server" Text="To: "></asp:Label>
       <asp:Label ID="Label8" runat="server" Height="22px" Width="78px"></asp:Label>

       <asp:Button ID="btnRefresh" runat="server" Text="Refresh" />
    </div>

<div id="divStatus" style="background-color: #FFFBF0;">

    <asp:Label ID="lblStatus" runat="server" Visible="False" Height="54px"
         Font-Bold="True" ForeColor="Red" Width="380px"></asp:Label>
 </div>

Private Sub Page_PreRender(sender As Object, e As System.EventArgs) Handles Me.PreRender



       ' to prevent the list being populated multiple times on page refreshes.
       If Not Page.IsPostBack Then


           Dim yr As Integer
           Dim strYear As String
           Dim TodaysDate As Date

           TodaysDate = New Date
           TodaysDate = Today


           'lblCurrentDate.Text = DateTime.Now.ToString("dd-MM-yyyy")


           ' Assign a default Start Date
           If lblStartDateValue.Text = "" Then
               DateStart.SelectedDate = "2009-11-30"
               lblStartDateValue.Text = "30/11/2009"
           End If

           Try
               ' Assign a default End Date
               If Me.Label8.Text = "" Then
                   Me.Label8.Text = DateTime.Now.ToString("dd/MM/yyyy")
                   DateEnd.SelectedDate = Today
               End If
           Catch Excptn As Exception
               Me.lblStatus.Visible = True
               Me.lblStatus.Text = "Stack Trace: " & vbCrLf & Excptn.StackTrace
           End Try
           DropDownListFamilyDesc.Items.Clear()
           DropDownListFamilyDesc.Items.Insert(0, "ANY")

           ' Populate the Family Description Drop Down List manually (so that the ANY item is followed by the list of valid Families).
           Dim con As New SqlConnection(WebConfigurationManager.ConnectionStrings("Norbar_ProductionConnectionString").ConnectionString)
           Dim cmd As New SqlCommand("SELECT [Family_Description], [FamilyID] FROM [vwFamily] ORDER BY [Family_Description], [FamilyID]", con)

           Using con
               con.Open()
               Dim reader As SqlDataReader = cmd.ExecuteReader()
               While reader.Read()
                   DropDownListFamilyDesc.Items.Add(CType(reader("Family_Description"), String))
               End While
           End Using

           If DropDownListStartYear.Items.Count = 0 Then
               strYear = Date.Parse(Today).ToString("yyyy")
               yr = strYear
               For theYear As Integer = yr To 2009 Step -1
                   DropDownListStartYear.Items.Add(theYear.ToString)
               Next
           End If

           If DropDownListEndYear.Items.Count = 0 Then
               strYear = Date.Parse(Today).ToString("yyyy")
               yr = strYear
               For theYear As Integer = yr To 2009 Step -1
                   DropDownListEndYear.Items.Add(theYear.ToString)
               Next
           End If

       End If
   End Sub

该页面抛出错误行

Me.lblStatus.Visible = True。



注意lblStartDateValue.Text设置确定错误之前的行，并且据我所知，它的设置方式大致相同。此外，下拉列表在IDE中工作正常（我不知道在应用程序服务器上，因为页面永远不会被渲染。）



此代码在运行时工作正常在我的IDE中，但只在Web应用程序服务器上运行时抛出错误。有没有人有什么建议？ （另外。一旦被捕获，我该如何输出错误？）

The page throws the error on the line
"Me.lblStatus.Visible = True".

Note that lblStartDateValue.Text is set ok in the lines before the one that errors, and as far as I can see it is set up in much the same way. Also the dropdown list works fine in the IDE (I don't know about on the application server because the page never gets rendered.)

This code works fine when ran in my IDE but only throws the error when run on a web application server. Does anyone have any suggestions? (Also. How should I output the error once it has been trapped?)

推荐答案

今天或 lblCurrentDate 为空，检查一下。你可以在调试器下完成它。



这是最容易检测和修复的案例之一，因此，你不是一遍又一遍地提出类似的问题，而是需要培养处理这种情况的基本技能。



它只是意味着某些引用类型的某个成员/变量通过使用和实例来解除引用（非-static）成员，要求此成员/变量为非null，但实际上它似乎为null。只需在调试器下执行它，它将停止抛出异常的执行。在该行上设置一个断点，重新启动应用程序并再次到达这一点。评估下一行中涉及的所有引用，并查看哪一个为null，而不需要为null。解决这个问题之后，修复代码：要么确保将成员/变量正确初始化为非空引用，要么将其检查为null，如果为null，则执行其他操作。



另请参阅：想要在按钮点击时显示下一条记录。但是如果下一条记录功能的条件对象引用没有设置为对象的实例 [ ^ ]。



有时候，你不能在调试器下，由于一个或另一个原因做到这一点。一个非常讨厌的情况是，只有在调试信息不​​可用时构建软件时才会出现问题。在这种情况下，你必须使用更难的方式。首先，你需要确保你永远不会通过静默处理异常来阻止异常的传播（这是开发人员对自己的犯罪，但很常见）。您需要在每个线程的最顶层堆栈帧上捕获绝对所有异常。如果处理类型 System.Exception 的异常，则可以执行此操作。在处理程序中，您需要记录所有异常信息，尤其是 System.Exception.StackTrace ：

http://msdn.microsoft.com/en-us/library/system.exception.aspx [ ^ ]，

http://msdn.microsoft.com/en-us/library/system.exception。 stacktrace.aspx [ ^ ]。



堆栈跟踪只是一个字符串，显示从throw语句到处理程序的异常传播的完整路径。通过阅读，您总能找到目的。对于日志记录，使用类 System.Diagnostics.EventLog ：

http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx [ ^ ]。

-SA

Either Today or lblCurrentDate is null, check it up. You can do it under the debugger.

This is one of the very easiest cases to detect and fix, so, instead of asking similar questions over and over, you rather need to develop an elementary skills of dealing with such situations.

It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object"[^].

Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type System.Exception. In the handler, you need to log all the exception information, especially the System.Exception.StackTrace:
http://msdn.microsoft.com/en-us/library/system.exception.aspx[^],
http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx[^].

The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it's the best (in most cases) to use the class System.Diagnostics.EventLog:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx[^].

Good luck,

—SA

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