View and Download Psion Teklogix 9160 G2 user manual online. Wireless Gateway. 9160 G2 Gateway pdf manual download. Convert MySQL to JSON using PHP. Learn how to convert the mysql query result set to json format or file with PHP Programming Language. Easily export mysql to json. A website with various source code and resources for Visual Basic and Visual Basic. Net. What is a Null. Reference. Exception, and how do I fix it Null. Reference Exception Visual Basic. The Null. Reference Exception for Visual Basic is no different from the one in C. After all, they are both reporting the same exception defined in the. NET Framework which they both use. Causes unique to Visual Basic are rare perhaps only one. This answer will use Visual Basic terms, syntax, and context. The examples used come from a large number of past Stack Overflow questions. This is to maximize relevance by using the kinds of situations often seen in posts. A bit more explanation is also provided for those who might need it. An example similar to yours is very likely listed here. Note This is concept based there is no code for you to paste into your project. It is intended to help you understand what causes a Null. Reference. Exception NRE, how to find it, how to fix it, and how to avoid it. An NRE can be caused many ways so this is unlikely to be your sole encounter. The examples from Stack Overflow posts do not always show the best way to do something in the first place. Typically, the simplest remedy is used. Basic Meaning. The message Object not set to an instance of Object means you are trying to use an object which has not been initialized. This boils down to one of these Your code declared an object variable, but it did not initialize it create an instance or instantiate itSomething which your code assumed would initialize an object, did not. Possibly, other code prematurely invalidated an object still in use. Finding The Cause. Since the problem is an object reference which is Nothing, the answer is to examine them to find out which one. Then determine why it is not initialized. Hold the mouse over the various variables and Visual Studio VS will show their values the culprit will be Nothing. You should also remove any TryCatch blocks from the relevant code, especially ones where there is nothing in the Catch block. This will cause your code to crash when it tries to use an object which is Nothing. This is what you want because it will identify the exact location of the problem, and allow you to identify the object causing it. A Msg. Box in the Catch which displays Error while. This method also leads to very bad Stack Overflow questions, because you cant describe the actual exception, the object involved or even the line of code where it happens. You can also use the Locals Window Debug Windows Locals to examine your objects. Once you know what and where the problem is, it is usually fairly easy to fix and faster than posting a new question. See also Examples and Remedies. Class Objects Creating an Instance. Dim reg As Cash. Register. Text. Box. 1. Text reg. Amount NRE. The problem is that Dim does not create a Cash. Register object it only declares a variable named reg of that Type. Declaring an object variable and creating an instance are two different things. Remedy. The New operator can often be used to create the instance when you declare it Dim reg As New Cash. Register New creates instance, invokes the constructor. Longer, more explicit form. Dim reg As Cash. Register New Cash. Register. When it is only appropriate to create the instance later Private reg As Cash. Register Declare. New Cash. Register Create instance. Note Do not use Dim again in a procedure, including the constructor Sub New Private reg As Cash. Register. Public Sub New. Dim reg As New Cash. Register. This will create a local variable, reg, which exists only in that context sub. The reg variable with module level Scope which you will use everywhere else remains Nothing. Missing the New operator is the 1 cause of Null. Reference Exceptions seen in the Stack Overflow questions reviewed. Visual Basic tries to make the process clear repeatedly using New Using the New Operator creates a new object and calls Sub New the constructor where your object can perform any other initialization. To be clear, Dim or Private only declares a variable and its Type. The Scope of the variable whether it exists for the entire moduleclass or is local to a procedure is determined by where it is declared. Private Friend Public defines the access level, not Scope. For more information, see Arrays. Arrays must also be instantiated Private arr as String. This array has only been declared, not created. There are several ways to initialize an array Private arr as String New String1. Private arr As String New String1. For a local array in a procedure and using Option Infer. Dim arr New String1. Note Beginning with VS 2. Option Infer, the As lt Type and New elements are optional Dim my. Dbl As Double 1. Dim my. Dbl New Double 1. Dim my. Dbl 1. The data Type and array size are inferred from the data being assigned. ClassModule level declarations still require As lt Type with Option Strict Private my. Doubles As Double 1. Example Array of class objects. Dim arr. Foo5 As Foo. For i As Integer 0 To arr. Foo. Count 1. arr. Fooi. Bar i 1. Exception. The array has been created, but the Foo objects in it have not. Remedy. For i As Integer 0 To arr. Foo. Count 1. arr. Fooi New Foo Create Foo instance. Fooi. Bar i 1. Using a ListOf T will make it quite difficult to have an element without a valid object Dim Foo. List As New ListOf Foo List created, but it is empty. Dim f As Foo Temporary variable for the loop. For i As Integer 0 To 5. New Foo Foo instance created. Bar i 1. 0. Foo. List. Addf Foo object added to list. For more information, see Lists and Collections. NET collections of which there are many varieties Lists, Dictionary, etc. must also be instantiated or created. Private my. List As ListOf String. List. Addziggy Null. Reference. You get the same exception for the same reason my. List was only declared, but no instance created. The remedy is the same my. List New ListOf String. Or create an instance when declared. Private my. List As New ListOf String. A common oversight is a class which uses a collection Type Public Class Foo. Private bar. List As ListOf Bar. Friend Function Bar. Count As Integer. Return bar. List. Count. End Function. Friend Sub Add. Itemnew. Bar As Bar. If bar. List. Containsnew. Bar False Then. List. Addnew. Bar. End Function. Either procedure will result in an NRE, because bar. List is only declared, not instantiated. Creating an instance of Foo will not also create an instance of the internal bar. List. It may have been the intent to do this in the constructor Public Sub New Constructor. Stuff to do when a new Foo is created. List New ListOf Bar. As before, this is incorrect Public Sub New. Creates another bar. List local to this procedure. Dim bar. List As New ListOf Bar. For more information, see ListOf T Class. Data Provider Objects. Working with databases presents many opportunities for a Null. Reference because there can be many objects Command, Connection, Transaction, Dataset, Data. Table, Data. Rows. in use at once. Note It does not matter which data provider you are using My. SQL, SQL Server, Ole. DB, etc. the concepts are the same. Example 1. Dim da As Ole. Db. Data. Adapter. Dim ds As Data. Set. Dim Max. Rows As Integer. Dim sql SELECT FROM tblfoobarList. New Ole. Db. Data. Adaptersql, con. Fillds, foobar. Max. Rows ds. Tablesfoobar. Rows. Count Error. As before, the ds Dataset object was declared, but an instance was never created. The Data. Adapter will fill an existing Data. Set, not create one. In this case, since ds is a local variable, the IDE warns you that this might happen When declared as a moduleclass level variable, as appears to be the case with con, the compiler cant know if the object was created by an upstream procedure. Do not ignore warnings. Remedy. Dim ds As New Data. Set. Example 2ds New Data. Set. da New Ole. DBData. Adaptersql, con. Fillds, Employees. ID. Text ds. TablesEmployee. Rows0. Item1. ID. Name ds. TablesEmployee. Rows0. Item2. A typo is a problem here Employees vs Employee. There was no Data. Table named Employee created, so a Null. Reference. Exception results trying to access it. Another potential problem is assuming there will be Items which may not be so when the SQL includes a WHERE clause. Remedy. Since this uses one table, using Tables0 will avoid spelling errors.
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