当我抛出这行代码时，它引发了错误“对象引用未设置为对象的实例 [英] When I Ran This Lines Of Codes It Was Threw An Error "Object Reference Is Not Set To An Instance Of An Object

问题描述

Imports System.Data.OleDb

Public Class Form1
	'Dim conn As New OleDb.OleDbConnection
	Dim cmd As OleDb.OleDbCommand = Nothing
	
	Dim conString As String = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\OLAKAY\Documents\New folder\DatabaseEneka.accdb"
	


	Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
		Me.Text = "Nominal Role Update Form"

	End Sub

	Private Sub btnSubmit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSubmit.Click
		'Declear the Variables and Data Types 

		Dim Sn, CompNo, SGL As String
		Dim DFA, DOB, DPA, DPS As String 'dateOfPresentAppointment
		Dim StaffName, IDno, HomeTown, StateofOrigin, sex, LGA, Qualification, SubjectS, SubjectT, Rank, LastSchool, Designation, Remark As String


		Sn = txtSerial.Text
		IDno = txtID.Text
		CompNo = txtComputerNumber.Text
		SGL = txtSGL.Text
		DOB = txtDOB.Text
		DFA = txtDfa.Text
		LGA = txtLGA.Text
		DPA = txtDateOfPresent.Text
		DPS = txtDatePostedTo.Text
		StaffName = txtStaffName.Text
		HomeTown = txtHomeTown.Text
		StateofOrigin = txtStateOfOrigin.Text
		sex = txtSex.Text
		Qualification = txtQualification.Text
		SubjectS = txtSubjectS.Text
		SubjectT = txtSubjectT.Text
		Remark = txtRemarks.Text
		Rank = txtRank.Text
		Designation = txtDelegation.Text
		LastSchool = txtLastSchooll.Text
		Try




			Dim conn = New OleDbConnection
			conn.ConnectionString = conString
			cmd.Connection = conn
			conn.Open()

			cmd.CommandText = "INSERT INTO StaffInfo(ID,StaffName,DateofBirth,HomeTown,StateofOrigin,LGA,Sex,DateOfFirstAppointment,ComputerNumber,IDNumber,QualificationWithDates,SubjectSpecialized,SubjectTaught,DateofPresentAppointment,Rank,Designation,DatePostedToSchool,LastSchoolServed,Remarks)VALUE  ('" & IDno & "','" & StaffName & "''" & DOB & "''" & HomeTown & "''" & StateofOrigin & "''" & DOB & "')"
			cmd.ExecuteNonQuery()
			conn.Close()
		Catch ex As Exception
			MsgBox(ex.Message)
			
			'			
			MsgBox("Data Saved")
		End Try
		

	


	End Sub

	Private Function VALUES(ByVal p1 As String, ByVal p2 As Object) As String
		Throw New NotImplementedException
	End Function

	Private Function VALUES(ByVal p1 As String) As String
		Throw New NotImplementedException
	End Function

End Class

推荐答案

PIEBALDconsult在他的作品中给了你几个好主意评论这个问题。



您没有显示带有对象引用未设置为对象实例消息的异常位置。



不用担心。这是检测和修复的最简单的案例之一。它只是意味着某些引用类型的某个成员/变量通过使用和它的实例（非静态）成员解除引用，这要求此成员/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 [ ^ ]。

---

现在，我们来解决您撰写查询的方式问题。不仅重复的字符串连接是低效的（因为字符串是不可变的;我是否必须解释为什么它会使重复连接变坏？），但是有更重要的问题：它打开了通向良好的大门已知的漏洞称为 SQL注入。



这是它的工作原理： http://xkcd.com/327 [ ^ ]。< br $> b $ b

怎么办？只需阅读有关此问题和主要补救措施：参数化语句： http://en.wikipedia.org/ wiki / SQL_injection [ ^ ]。



使用ADO.NET，请使用： http：// msdn.microsoft.com/en-us/library/ff648339.aspx [ ^ ]。



请查看我过去的答案以获取更多详细信息：

EROR IN com.ExecuteNonQuery（）; [ ^ ]，

hi name没有显示在名称中？ [ ^ ]。



祝你好运，

-SA

PIEBALDconsult gave you a couple of good ideas in his comment to the question.

You did not show where the exception with the message "Object reference not set to an instance of an object" is thrown.

Not to worry. This is one of the very easiest cases to detect and fix. 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[^].

---

Now, let's address the problem of the way you compose your query. Not only repeated string concatenation is inefficient (because strings are immutable; do I have to explain why it makes repeated concatenation bad?), but there is way more important issue: it opens the doors to a well-known exploit called SQL injection.

This is how it works: http://xkcd.com/327[^].

What to do? Just read about this problem and the main remedy: parametrized statements: http://en.wikipedia.org/wiki/SQL_injection[^].

With ADO.NET, use this: http://msdn.microsoft.com/en-us/library/ff648339.aspx[^].

Please see my past answers for some more detail:
EROR IN UPATE in com.ExecuteNonQuery();[^],
hi name is not displaying in name?[^].

Good luck,

—SA

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