System.nullreferenceexception：'对象引用未设置为对象的实例。' [英] System.nullreferenceexception: 'object reference not set to an instance of an object.'

问题描述

错误发生在

error occured on

Dim ext As String = System.IO.Path.GetExtension(imgFileUpload.PostedFile.FileName)

以上。



我尝试过：

above line.

What I have tried:

Protected Sub btnUpload_Click(ByVal sender As Object, ByVal e As System.EventArgs)
      Dim validFileTypes As String() = {"bmp", "gif", "png", "jpg", "jpeg", "doc", "xls"}
      Dim ext As String = System.IO.Path.GetExtension(imgFileUpload.PostedFile.FileName)
      Dim isValidFile As Boolean = False
      For i As Integer = 0 To validFileTypes.Length - 1
          If ext = "." & validFileTypes(i) Then
              isValidFile = True
              Exit For
          End If
      Next

      If Not isValidFile Then
          lblFileUpload.ForeColor = System.Drawing.Color.Red
          lblFileUpload.Text = "Invalid File. Please upload a File with extension " &
          String.Join(",", validFileTypes)
      Else
          lblFileUpload.ForeColor = System.Drawing.Color.Green
          lblFileUpload.Text = "File uploaded successfully."
      End If

  End Sub

推荐答案

这是其中一个我们得到的常见问题，也是我们最不能回答的问题，但你最有能力回答自己。



我来解释一下这个错误是什么意味着：您已尝试使用变量，属性或方法返回值，但它包含null - 这意味着变量中没有类的实例。

它有点像一个口袋：你的衬衫里有一个口袋，用来握笔。如果你进入口袋并发现那里没有笔，你就不能在一张纸上签名 - 如果你尝试的话，你会得到非常有趣的外观！空口袋给你一个空值（这里没有笔！）所以你不能做任何你检索笔通常做的事情。它为什么空？这就是问题 - 可能是你今天早上离开家时忘了拿起你的笔，或者你昨晚把它拿到昨天的衬衫口袋里时可能会把笔留下来。



我们无法分辨，因为我们不在那里，更重要的是，我们甚至看不到你的衬衫，更不用说口袋里的东西了！



回到计算机，你做了同样的事情，不知何故 - 我们看不到你的代码，更不用说运行它了，找不到包含null的东西。

但是你可以 - 而Visual Studio将在这里帮助你。在调试器中运行您的程序，当它失败时，VS将向您显示它发现问题的行。然后，您可以开始查看它的各个部分，以查看哪个值为null，并开始回顾代码以找出原因。因此，在包含错误行的方法的开头放置一个断点，然后从头再次运行程序。这一次，VS会在错误发生前停止，让你通过查看代码来查看你的价值来检查发生了什么。



但我们做不到那 - 我们没有您的代码，如果我们拥有它，我们不知道如何使用它，我们没有您的数据。所以尝试一下 - 看看你能找到多少信息！

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

当你尝试使用NULL对象时，通常会出现NullReferenceExceptions。在这种情况下，您尝试访问未发生的上载的FileName。对此的简单修复是在访问属性之前检查对象（imgFileUpload.PostedFile）是否存在

NullReferenceExceptions generally occur when you are trying to work with an object that is NULL. In this case you are trying to access the FileName of an upload that did not occur. The simple fix for this would be to check if the object (imgFileUpload.PostedFile) exists before you access the property

If Not (imgFileUpload.PostedFile Is Nothing) Then
  Dim ext As String = System.IO.Path.GetExtension(imgFileUpload.PostedFile.FileName)
  ' Continue with processing
End If

imgFileUpload.PostedFile.FileName 链中的项目未正确初始化（ Nothing ，我想）。

理想情况下，您应该确保总是一个有效的链到达您的代码，但是，作为一个简单的解决方法，您可以处理异常（请参阅VB.Net异常处理 [ ^ ]）并可能将其视为'提供的无效文件'。

An item in the imgFileUpload.PostedFile.FileName chain is not properly initialized (Nothing, I suppose).
Ideally you should make sure that always a valid chain reach your code, but, as a simple workaround, you may handle the exception (see VB.Net Exception Handling[^]) and possibly treat it like an 'invalid file provided'.

这篇关于System.nullreferenceexception：'对象引用未设置为对象的实例。'的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！