continue With Throw Null Exception C
NullReference Exception — Visual Basic
The NullReference 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
NullReferenceException(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' it)
- Something 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 Try/Catch 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 MsgBox in the Catch which displays Error while... will be of little help. This method also leads to very bad Stack Overflow questions, because you can't 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:
- Breakpoints
- MSDN: How to: Use the Try/Catch Block to Catch Exceptions
- MSDN: Best Practices for Exceptions
Examples and Remedies
Class Objects / Creating an Instance
Dim reg As CashRegister ... TextBox1.Text = reg.Amount ' NRE The problem is that Dim does not create a CashRegister 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 CashRegister ' [New] creates instance, invokes the constructor ' Longer, more explicit form: Dim reg As CashRegister = New CashRegister When it is only appropriate to create the instance later:
Private reg As CashRegister ' Declare ... reg = New CashRegister() ' Create instance Note: Do not use Dim again in a procedure, including the constructor (Sub New):
Private reg As CashRegister '... Public Sub New() '... Dim reg As New CashRegister End Sub 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
Newoperator is the #1 cause ofNullReference Exceptionsseen in the Stack Overflow questions reviewed.Visual Basic tries to make the process clear repeatedly using
New: Using theNewOperator creates a new object and callsSub 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 module/class 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:
- New Operator
- Scope in Visual Basic
- Access Levels in Visual Basic
- Value Types and Reference Types
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 String(10){} ' or Private arr() As String = New String(10){} ' For a local array (in a procedure) and using 'Option Infer': Dim arr = New String(10) {} Note: Beginning with VS 2010, when initializing a local array using a literal and Option Infer, the As <Type> and New elements are optional:
Dim myDbl As Double() = {1.5, 2, 9.9, 18, 3.14} Dim myDbl = New Double() {1.5, 2, 9.9, 18, 3.14} Dim myDbl() = {1.5, 2, 9.9, 18, 3.14} The data Type and array size are inferred from the data being assigned. Class/Module level declarations still require As <Type> with Option Strict:
Private myDoubles As Double() = {1.5, 2, 9.9, 18, 3.14} Example: Array of class objects
Dim arrFoo(5) As Foo For i As Integer = 0 To arrFoo.Count - 1 arrFoo(i).Bar = i * 10 ' Exception Next The array has been created, but the Foo objects in it have not.
Remedy
For i As Integer = 0 To arrFoo.Count - 1 arrFoo(i) = New Foo() ' Create Foo instance arrFoo(i).Bar = i * 10 Next Using a List(Of T) will make it quite difficult to have an element without a valid object:
Dim FooList As New List(Of Foo) ' List created, but it is empty Dim f As Foo ' Temporary variable for the loop For i As Integer = 0 To 5 f = New Foo() ' Foo instance created f.Bar = i * 10 FooList.Add(f) ' Foo object added to list Next For more information, see:
- Option Infer Statement
- Scope in Visual Basic
- Arrays in Visual Basic
Lists and Collections
.NET collections (of which there are many varieties - Lists, Dictionary, etc.) must also be instantiated or created.
Private myList As List(Of String) .. myList.Add("ziggy") ' NullReference You get the same exception for the same reason - myList was only declared, but no instance created. The remedy is the same:
myList = New List(Of String) ' Or create an instance when declared: Private myList As New List(Of String) A common oversight is a class which uses a collection Type:
Public Class Foo Private barList As List(Of Bar) Friend Function BarCount As Integer Return barList.Count End Function Friend Sub AddItem(newBar As Bar) If barList.Contains(newBar) = False Then barList.Add(newBar) End If End Function Either procedure will result in an NRE, because barList is only declared, not instantiated. Creating an instance of Foo will not also create an instance of the internal barList. 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... barList = New List(Of Bar) End Sub As before, this is incorrect:
Public Sub New() ' Creates another barList local to this procedure Dim barList As New List(Of Bar) End Sub For more information, see List(Of T) Class.
Data Provider Objects
Working with databases presents many opportunities for a NullReference because there can be many objects (Command, Connection, Transaction, Dataset, DataTable, DataRows....) in use at once. Note: It does not matter which data provider you are using -- MySQL, SQL Server, OleDB, etc. -- the concepts are the same.
Example 1
Dim da As OleDbDataAdapter Dim ds As DataSet Dim MaxRows As Integer con.Open() Dim sql = "SELECT * FROM tblfoobar_List" da = New OleDbDataAdapter(sql, con) da.Fill(ds, "foobar") con.Close() MaxRows = ds.Tables("foobar").Rows.Count ' Error As before, the ds Dataset object was declared, but an instance was never created. The DataAdapter will fill an existing DataSet, not create one. In this case, since ds is a local variable, the IDE warns you that this might happen:
When declared as a module/class level variable, as appears to be the case with con, the compiler can't know if the object was created by an upstream procedure. Do not ignore warnings.
Remedy
Dim ds As New DataSet Example 2
ds = New DataSet da = New OleDBDataAdapter(sql, con) da.Fill(ds, "Employees") txtID.Text = ds.Tables("Employee").Rows(0).Item(1) txtID.Name = ds.Tables("Employee").Rows(0).Item(2) A typo is a problem here: Employees vs Employee. There was no DataTable named "Employee" created, so a NullReferenceException 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 Tables(0) will avoid spelling errors. Examining Rows.Count can also help:
If ds.Tables(0).Rows.Count > 0 Then txtID.Text = ds.Tables(0).Rows(0).Item(1) txtID.Name = ds.Tables(0).Rows(0).Item(2) End If Fill is a function returning the number of Rows affected which can also be tested:
If da.Fill(ds, "Employees") > 0 Then... Example 3
Dim da As New OleDb.OleDbDataAdapter("SELECT TICKET.TICKET_NO, TICKET.CUSTOMER_ID, ... FROM TICKET_RESERVATION AS TICKET INNER JOIN FLIGHT_DETAILS AS FLIGHT ... WHERE [TICKET.TICKET_NO]= ...", con) Dim ds As New DataSet da.Fill(ds) If ds.Tables("TICKET_RESERVATION").Rows.Count > 0 Then The DataAdapter will provide TableNames as shown in the previous example, but it does not parse names from the SQL or database table. As a result, ds.Tables("TICKET_RESERVATION") references a non-existent table.
The Remedy is the same, reference the table by index:
If ds.Tables(0).Rows.Count > 0 Then See also DataTable Class.
Object Paths / Nested
If myFoo.Bar.Items IsNot Nothing Then ... The code is only testing Items while both myFoo and Bar may also be Nothing. The remedy is to test the entire chain or path of objects one at a time:
If (myFoo IsNot Nothing) AndAlso (myFoo.Bar IsNot Nothing) AndAlso (myFoo.Bar.Items IsNot Nothing) Then .... AndAlso is important. Subsequent tests will not be performed once the first False condition is encountered. This allows the code to safely 'drill' into the object(s) one 'level' at a time, evaluating myFoo.Bar only after (and if) myFoo is determined to be valid. Object chains or paths can get quite long when coding complex objects:
myBase.myNodes(3).Layer.SubLayer.Foo.Files.Add("somefilename") It is not possible to reference anything 'downstream' of a null object. This also applies to controls:
myWebBrowser.Document.GetElementById("formfld1").InnerText = "some value" Here, myWebBrowser or Document could be Nothing or the formfld1 element may not exist.
UI Controls
Dim cmd5 As New SqlCommand("select Cartons, Pieces, Foobar " _ & "FROM Invoice where invoice_no = '" & _ Me.ComboBox5.SelectedItem.ToString.Trim & "' And category = '" & _ Me.ListBox1.SelectedItem.ToString.Trim & "' And item_name = '" & _ Me.ComboBox2.SelectedValue.ToString.Trim & "' And expiry_date = '" & _ Me.expiry.Text & "'", con) Among other things, this code does not anticipate that the user may not have selected something in one or more UI controls. ListBox1.SelectedItem may well be Nothing, so ListBox1.SelectedItem.ToString will result in an NRE.
Remedy
Validate data before using it (also use Option Strict and SQL parameters):
Dim expiry As DateTime ' for text date validation If (ComboBox5.SelectedItems.Count > 0) AndAlso (ListBox1.SelectedItems.Count > 0) AndAlso (ComboBox2.SelectedItems.Count > 0) AndAlso (DateTime.TryParse(expiry.Text, expiry) Then '... do stuff Else MessageBox.Show(...error message...) End If Alternatively, you can use (ComboBox5.SelectedItem IsNot Nothing) AndAlso...
Visual Basic Forms
Public Class Form1 Private NameBoxes = New TextBox(5) {Controls("TextBox1"), _ Controls("TextBox2"), Controls("TextBox3"), _ Controls("TextBox4"), Controls("TextBox5"), _ Controls("TextBox6")} ' same thing in a different format: Private boxList As New List(Of TextBox) From {TextBox1, TextBox2, TextBox3 ...} ' Immediate NRE: Private somevar As String = Me.Controls("TextBox1").Text This is a fairly common way to get an NRE. In C#, depending on how it is coded, the IDE will report that Controls does not exist in the current context, or "cannot reference non-static member". So, to some extent, this is a VB-only situation. It is also complex because it can result in a failure cascade.
The arrays and collections cannot be initialized this way. This initialization code will run before the constructor creates the Form or the Controls. As a result:
- Lists and Collection will simply be empty
- The Array will contain five elements of Nothing
- The
somevarassignment will result in an immediate NRE because Nothing doesn't have a.Textproperty
Referencing array elements later will result in an NRE. If you do this in Form_Load, due to an odd bug, the IDE may not report the exception when it happens. The exception will pop up later when your code tries to use the array. This "silent exception" is detailed in this post. For our purposes, the key is that when something catastrophic happens while creating a form (Sub New or Form Load event), exceptions may go unreported, the code exits the procedure and just displays the form.
Since no other code in your Sub New or Form Load event will run after the NRE, a great many other things can be left uninitialized.
Sub Form_Load(..._ '... Dim name As String = NameBoxes(2).Text ' NRE ' ... ' More code (which will likely not be executed) ' ... End Sub Note this applies to any and all control and component references making these illegal where they are:
Public Class Form1 Private myFiles() As String = Me.OpenFileDialog1.FileName & ... Private dbcon As String = OpenFileDialog1.FileName & ";Jet Oledb..." Private studentName As String = TextBox13.Text Partial Remedy
It is curious that VB does not provide a warning, but the remedy is to declare the containers at the form level, but initialize them in form load event handler when the controls do exist. This can be done in Sub New as long as your code is after the InitializeComponent call:
' Module level declaration Private NameBoxes as TextBox() Private studentName As String ' Form Load, Form Shown or Sub New: ' ' Using the OP's approach (illegal using OPTION STRICT) NameBoxes = New TextBox() {Me.Controls("TextBox1"), Me.Controls("TestBox2"), ...) studentName = TextBox32.Text ' For simple control references The array code may not be out of the woods yet. Any controls which are in a container control (like a GroupBox or Panel) will not be found in Me.Controls; they will be in the Controls collection of that Panel or GroupBox. Nor will a control be returned when the control name is misspelled ("TeStBox2"). In such cases, Nothing will again be stored in those array elements and an NRE will result when you attempt to reference it.
These should be easy to find now that you know what you are looking for:
"Button2" resides on a Panel
Remedy
Rather than indirect references by name using the form's Controls collection, use the control reference:
' Declaration Private NameBoxes As TextBox() ' Initialization - simple and easy to read, hard to botch: NameBoxes = New TextBox() {TextBox1, TextBox2, ...) ' Initialize a List NamesList = New List(Of TextBox)({TextBox1, TextBox2, TextBox3...}) ' or NamesList = New List(Of TextBox) NamesList.AddRange({TextBox1, TextBox2, TextBox3...}) Function Returning Nothing
Private bars As New List(Of Bars) ' Declared and created Public Function BarList() As List(Of Bars) bars.Clear If someCondition Then For n As Integer = 0 to someValue bars.Add(GetBar(n)) Next n Else Exit Function End If Return bars End Function This is a case where the IDE will warn you that 'not all paths return a value and a NullReferenceException may result'. You can suppress the warning, by replacing Exit Function with Return Nothing, but that does not solve the problem. Anything which tries to use the return when someCondition = False will result in an NRE:
bList = myFoo.BarList() For Each b As Bar in bList ' EXCEPTION ... Remedy
Replace Exit Function in the function with Return bList. Returning an empty List is not the same as returning Nothing. If there is a chance that a returned object can be Nothing, test before using it:
bList = myFoo.BarList() If bList IsNot Nothing Then... Poorly Implemented Try/Catch
A badly implemented Try/Catch can hide where the problem is and result in new ones:
Dim dr As SqlDataReader Try Dim lnk As LinkButton = TryCast(sender, LinkButton) Dim gr As GridViewRow = DirectCast(lnk.NamingContainer, GridViewRow) Dim eid As String = GridView1.DataKeys(gr.RowIndex).Value.ToString() ViewState("username") = eid sqlQry = "select FirstName, Surname, DepartmentName, ExtensionName, jobTitle, Pager, mailaddress, from employees1 where username='" & eid & "'" If connection.State <> ConnectionState.Open Then connection.Open() End If command = New SqlCommand(sqlQry, connection) 'More code fooing and barring dr = command.ExecuteReader() If dr.Read() Then lblFirstName.Text = Convert.ToString(dr("FirstName")) ... End If mpe.Show() Catch Finally command.Dispose() dr.Close() ' <-- NRE connection.Close() End Try This is a case of an object not being created as expected, but also demonstrates the counter usefulness of an empty Catch.
There is an extra comma in the SQL (after 'mailaddress') which results in an exception at .ExecuteReader. After the Catch does nothing, Finally tries to perform clean up, but since you cannot Close a null DataReader object, a brand new NullReferenceException results.
An empty Catch block is the devil's playground. This OP was baffled why he was getting an NRE in the Finally block. In other situations, an empty Catch may result in something else much further downstream going haywire and cause you to spend time looking at the wrong things in the wrong place for the problem. (The "silent exception" described above provides the same entertainment value.)
Remedy
Don't use empty Try/Catch blocks - let the code crash so you can a) identify the cause b) identify the location and c) apply a proper remedy. Try/Catch blocks are not intended to hide exceptions from the person uniquely qualified to fix them - the developer.
DBNull is not the same as Nothing
For Each row As DataGridViewRow In dgvPlanning.Rows If Not IsDBNull(row.Cells(0).Value) Then ... The IsDBNull function is used to test if a value equals System.DBNull: From MSDN:
The System.DBNull value indicates that the Object represents missing or non-existent data. DBNull is not the same as Nothing, which indicates that a variable has not yet been initialized.
Remedy
If row.Cells(0) IsNot Nothing Then ... As before, you can test for Nothing, then for a specific value:
If (row.Cells(0) IsNot Nothing) AndAlso (IsDBNull(row.Cells(0).Value) = False) Then Example 2
Dim getFoo = (From f In dbContext.FooBars Where f.something = something Select f).FirstOrDefault If Not IsDBNull(getFoo) Then If IsDBNull(getFoo.user_id) Then txtFirst.Text = getFoo.first_name Else ... FirstOrDefault returns the first item or the default value, which is Nothing for reference types and never DBNull:
If getFoo IsNot Nothing Then... Controls
Dim chk As CheckBox chk = CType(Me.Controls(chkName), CheckBox) If chk.Checked Then Return chk End If If a CheckBox with chkName can't be found (or exists in a GroupBox), then chk will be Nothing and be attempting to reference any property will result in an exception.
Remedy
If (chk IsNot Nothing) AndAlso (chk.Checked) Then ... The DataGridView
The DGV has a few quirks seen periodically:
dgvBooks.DataSource = loan.Books dgvBooks.Columns("ISBN").Visible = True ' NullReferenceException dgvBooks.Columns("Title").DefaultCellStyle.Format = "C" dgvBooks.Columns("Author").DefaultCellStyle.Format = "C" dgvBooks.Columns("Price").DefaultCellStyle.Format = "C" If dgvBooks has AutoGenerateColumns = True, it will create the columns, but it does not name them, so the above code fails when it references them by name.
Remedy
Name the columns manually, or reference by index:
dgvBooks.Columns(0).Visible = True Example 2 — Beware of the NewRow
xlWorkSheet = xlWorkBook.Sheets("sheet1") For i = 0 To myDGV.RowCount - 1 For j = 0 To myDGV.ColumnCount - 1 For k As Integer = 1 To myDGV.Columns.Count xlWorkSheet.Cells(1, k) = myDGV.Columns(k - 1).HeaderText xlWorkSheet.Cells(i + 2, j + 1) = myDGV(j, i).Value.ToString() Next Next Next When your DataGridView has AllowUserToAddRows as True (the default), the Cells in the blank/new row at the bottom will all contain Nothing. Most attempts to use the contents (for example, ToString) will result in an NRE.
Remedy
Use a For/Each loop and test the IsNewRow property to determine if it is that last row. This works whether AllowUserToAddRows is true or not:
For Each r As DataGridViewRow in myDGV.Rows If r.IsNewRow = False Then ' ok to use this row If you do use a For n loop, modify the row count or use Exit For when IsNewRow is true.
My.Settings (StringCollection)
Under certain circumstances, trying to use an item from My.Settings which is a StringCollection can result in a NullReference the first time you use it. The solution is the same, but not as obvious. Consider:
My.Settings.FooBars.Add("ziggy") ' foobars is a string collection Since VB is managing Settings for you, it is reasonable to expect it to initialize the collection. It will, but only if you have previously added an initial entry to the collection (in the Settings editor). Since the collection is (apparently) initialized when an item is added, it remains Nothing when there are no items in the Settings editor to add.
Remedy
Initialize the settings collection in the form's Load event handler, if/when needed:
If My.Settings.FooBars Is Nothing Then My.Settings.FooBars = New System.Collections.Specialized.StringCollection End If Typically, the Settings collection will only need to be initialized the first time the application runs. An alternate remedy is to add an initial value to your collection in Project -> Settings | FooBars, save the project, then remove the fake value.
Key Points
You probably forgot the New operator.
or
Something you assumed would perform flawlessly to return an initialized object to your code, did not.
Don't ignore compiler warnings (ever) and use Option Strict On (always).
MSDN NullReference Exception
Source: https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it
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