The one NullReferenceExceptionin Visual Basic is no different than the one in C# . After all, they both report the same exception defined in the .Net Framework that both languages ​​are based on. The causes that may be unique to Visual Basic are few (perhaps only one).

This answer will use Visual Basic terms, syntax, and context. The examples used are taken from a large number of old questions on Stack Overflow. In this way, its relevance is maximized, using the kind of situations that often arise in posts. Some more explanations are also attached for those who may need them. An example similar to yours is very likely to be found here.

Grades:

1. This is a conceptual answer: there is no code that you can copy and paste into your project. It's all about helping you understand what causes an NullReferenceExceptionNRE, how to find it, how to fix it, and how to avoid it. An NRE can have many causes so don't expect to find all of them here.
2. The examples (from the Stack Overflow posts) don't always show the best way to do something to start with.
3. Normally we will use the simplest solution.

Basic Meaning

The message "Object reference not set to an instance of an object" means that you are trying to use an object that has not been initialized. This can be narrowed down to one of these causes:

• In your code you have declared an object variable, but you have not initialized it (you have not instantiated or ' instantiated ' it)
• Something in your code that was assumed to initialize an object did not.
• Perhaps another piece of code invalidated an object that was still in use prematurely

Finding the cause

Assuming that the problem is an object reference that is Nothing, the answer is to examine the objects to find which one is the problem, and then determine why it is not initialized. Hold your mouse cursor over the variables and Visual Studio (VS) will show you their values ​​- the culprit will be Nothing.

You should also remove any Try/Catch blocks from the relevant parts of your code, especially those that have an empty Catch block. This will cause your code to fail when it tries to use an object that is Nothing. This is what you want , as this will pinpoint the exact location of the problem, allowing you to identify which object is causing it.

An MsgBoxin the Catch block that shows Error en...is very unhelpful. This method of catching exceptions also causes very bad questions on Stack Overflow, since you can't describe the actual exception, what object is causing it, and not even the line of code where it happened.

You can also use the window Variables Locales( Debug -> Windows -> Locals ) to examine your objects.

Once you know what the problem is and where it is, it's usually very easy to fix and faster than adding a new question.

See also:

• breakpoints
• MSDN: How To: Use the Try/Catch Block to Catch Exceptions
• MSDN: Best Practices for Exceptions

Examples and solutions

Class Objects / Creating an Instance

Dim reg As Registradora
...
TextBox1.Text = reg.Cantidad         ' NRE

The problem is that Dimit doesn't create an object of type Registradora; it just declares a named variable regof that Type. Declaring an object variable and creating an instance of it are two different things.

Solution

The operator Newis the one normally used to create the instance when you declare the object:

Dim reg As New Registradora        ' [New] crea la instancia, invoca el constructor

' Otra forma mas larga y explícita:
Dim reg As Registradora = New Registradora        

When the instance should be created later:

Private reg As Registradora         ' Declaración
  ...
reg = New Registradora()            ' Creamos la instancia

Note: Do not use Dimagain in a procedure, including the constructor ( Sub New):

Private reg As Registradora
'...

Public Sub New()
   '...
   Dim reg As New Registradora
End Sub

This will create a local variable , reg, which exists only in this (sub) context. The variable regthat you have as ámbitothe module and that you will use everywhere else remains as Nothing.

Forgetting the operator Newis the #1 cause of questions onNullReferenceExceptions the Stack Overflow questions reviewed.

Visual Basic tries to make the process clear by repeatedly using New: Using the operator Newcreates a new object and calls Sub New-- the constructor -- where your object can perform any other initialization.

To make this clear, Dim(o Private) just declares a variable and its Type( type ). The scope of the variable - whether it exists for the entire module/class or whether it is local to a procedure - is determined by where it is declared. Private | Friend | Publicdefines the level of access, not its Scope .

For more information, see:

• new operator
• Scope in Visual Basic
• Access Levels in Visual Basic
• Value Types and Reference Types

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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){}

' Para un array local (en un procedimiento) y usando 'Option Infer':
Dim arr = New String(10) {}

Note: As of VS 2010, when initializing a local array using a literal and Option Infer, the elements As <Type>and Neware 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 type and size of the array size are inferred from the data that is allocated. Class/Module level declarations still require As <Type>with Option Strict:

Private misDoubles 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       ' Excepción
Next

The array has been created, but the objects of type Fooin it have not.

Solution

For i As Integer = 0 To arrFoo.Count - 1
    arrFoo(i) = New Foo()         ' Crear instancia de Foo
    arrFoo(i).Bar = i * 10
Next

Using one List(Of T)will make it quite difficult to have an element without a valid object:

Dim FooList As New List(Of Foo)     ' Lista creadad, pero está vacia
Dim f As Foo                        ' Variable temporal para el bucle
For i As Integer = 0 To 5
    f = New Foo()                    ' Instancia de Foo creada
    f.Bar =  i * 10
    FooList.Add(f)                   ' objeto de tipo Foo añadido a la lista
Next

For more information, see:

• Option Infer Statement
• Scope in Visual Basic
• Arrays in Visual Basic

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Lists and Collections

Collections in .NET (of which there are many varieties - Lists, Dictionaries, etc.) must also be instantiated or created.

Private miLista As List(Of String)
..
miLista.Add("prueba")           ' NullReference

You get the same exception for the same reason - it miListawas just declared, but no instance was created. The solution is the same:

miLista = New List(Of String)

' O crear la instancia al declararla:
Private miLista As New List(Of String)

A common mistake is a class that uses a Typecollection 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

Both procedures will cause an NRE, because barListit was only declared, not instantiated. Creating an instance of Foowill not also create an instance of the barListinner one. Maybe the intention was to do it in the constructor:

Public Sub New         ' Constructor
    ' Cosas que hacer cuando se crea un nuevo Foo...
    barList = New List(Of Bar)
End Sub

As before, this is not correct:

Public Sub New()
    ' Crea otra barList local a este procedimiento
     Dim barList As New List(Of Bar)
End Sub

For more information, see ClassList<T> .

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Data Provider Objects

Working with databases means there are many opportunities for a NullReferenceException since there can be many objects ( Command, Connection, Transaction, Dataset, DataTable, DataRows....) in use at the same time. Note: It doesn't matter what data provider you use -- 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 Dataset dshas been declared, but an instance of it has never been created. He DataAdapterfills an DataSetexisting one, he does not create one. In this case, since it dsis a local variable, the IDE gives you a warning that this could happen:

When declared as a variable at the module/class level, as seems to be the case with con, the compiler cannot tell if the object was created by a bottom-up procedure. Don't ignore the warnings.

Solution

Dim ds As New DataSet

Example 2

ds = New DataSet
da = New OleDBDataAdapter(sql, con)
da.Fill(ds, "Empleados")

txtID.Text = ds.Tables("Empleado").Rows(0).Item(1)
txtID.Name = ds.Tables("Empleado").Rows(0).Item(2)

A typo is the problem here: Empleados-> Empleado. There is no DataTableone called "Employee", so one NullReferenceExceptionresults from trying to access it. Another potential problem is assuming that they will exist Items, which may not be true when the SQL query contains a WHERE clause.

Solution

Since only one table is being used in the example, using Tables(0)will avoid misspellings. Browse Rows.Countcan 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

Fillis a function that returns the number of Rowsaffected rows( ) that can also be checked:

If da.Fill(ds, "Empleados") > 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 will DataAdapterprovide TableNamesas seen in the example above, but does not parse the database table names. As a result, it ds.Tables("TICKET_RESERVATION")references a table that does not exist.

The solution is the same, reference the table by its index:

If ds.Tables(0).Rows.Count > 0 Then

See also DataTable class .

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Object Paths ( Object Paths ) / Nested ( Nested )

If myFoo.Bar.Items IsNot Nothing Then
   ...

The code is just testing Itemswhile myFooand Barcould also be Nothing. The solution is to try the entire chain or the ture of the objects one at a time:

If (myFoo IsNot Nothing) AndAlso
    (myFoo.Bar IsNot Nothing) AndAlso
    (myFoo.Bar.Items IsNot Nothing) Then
    ....

AndAlsoIt is important. Subsequent tests will not run once the first condition is met False. This allows code to safely check objects from level to level, evaluating myFoo.Baronly after (and if) it is determined to myFoobe valid. Object chains or paths can get quite long when dealing with complex objects:

myBase.myNodes(3).Layer.SubLayer.Foo.Files.Add("somefilename")

It is also not possible to reference anything below a null object. This also applies to controls:

myWebBrowser.Document.GetElementById("formfld1").InnerText = "un valor"

Here, they could myWebBrowsereither Documentbe Nothing, or the element formfld1could 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 might not have selected anything in one or more controls. ListBox1.SelectedItemcould very well be Nothing, so it will ListBox1.SelectedItem.ToStringcause an NRE.

Solution

Validate the data before using it (it is also recommended to use Option Strictand SQL parameters):

Dim expiry As DateTime         ' para validar un texto de una fecha    If (ComboBox5.SelectedItems.Count > 0) AndAlso
    (ListBox1.SelectedItems.Count > 0) AndAlso
    (ComboBox2.SelectedItems.Count > 0) AndAlso
    (DateTime.TryParse(expiry.Text, expiry) Then

    '... hacer cosas
Else
    MessageBox.Show(...mensaje de error...)
End If

Alternatively, you could also use(ComboBox5.SelectedItem IsNot Nothing) AndAlso...

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Visual Basic Forms

Public Class Form1

    Private NameBoxes = New TextBox(5) {Controls("TextBox1"), _
                   Controls("TextBox2"), Controls("TextBox3"), _
                   Controls("TextBox4"), Controls("TextBox5"), _
                   Controls("TextBox6")}

    ' lo mismo en un formato diferente:
    Private boxList As New List(Of TextBox) From {TextBox1, TextBox2, TextBox3 ...}

    ' NRE Inmediato:
    Private unaVariable As String = Me.Controls("TextBox1").Text

This is a very common way to obtain an NRE. In C#, depending on how it's programmed, the IDE would report that Controlsit doesn't exist in the current context, or "Cannot access a non-static member from a static scope." So in a way this is a situation that can only happen in VB. It is also a complex situation, because it can cause a cascading failure.

Arrays and collections cannot be initialized this way. This initialization code will be executed before the constructor creates the Form or controls( Controls). As a result:

• Las Listas y Colecciones estarán simplemente vacías
• El Array contendrá cinco elementos de Nothing
• La asignación a unaVariable resultará en una NRE inmediata, porque Nothing no tiene una propiedad .Text

Referenciar los elementos del array mas tarde resultará en una NRE. Si lo haces en el Form_Load, debido a un bug extraño, el IDE podría no reportar la excepción cuando ocurra. La excepción aparecerá mas tarde cuando tu código intent usar el array. Esta "excepción silenciosa" es detallada en este post (en inglés). Para nuestro propósito, la clave es que cuando algo catastrófico ocurre al crear un formulario (Sub New o evento Form Load), las excepciones podrían no reportarse. El flujo sale del procedimiento y simplemente muestra el formulario.

Dado que ningún código mas en tu Sub New o en tu evento Form Load se ejecutará después de la NRE, un montón de otras cosas podrían quedarse sin inicializar.

Sub Form_Load(..._
   '...
   Dim name As String = NameBoxes(2).Text        ' NRE
   ' ...
   ' Mas código (que probablemente no se ejecutará)
   ' ...
End Sub

Nota esto aplica para cualquiera y todas las referencias a controles y componentes haciendo esto ilegal donde se encuentre:

Public Class Form1

    Private myFiles() As String = Me.OpenFileDialog1.FileName & ...
    Private dbcon As String = OpenFileDialog1.FileName & ";Jet Oledb..."
    Private studentName As String = TextBox13.Text

Solución parcial

Es curioso que VB no de al menos un aviso, pero la solución es declarar los contenedores a nivel de formulario, pero inicializarles en el evento form load cuando los controles realmente existen. Esto se puede hacer también en Sub New siempre y cuando tu código esté después de la llamada a InitializeComponent:

' Declaración a nivel de módulo
Private NameBoxes as TextBox()
Private studentName As String

' Form Load, Form Shown or Sub New:
'
' (ilegal usando OPTION STRICT)
NameBoxes = New TextBox() {Me.Controls("TextBox1"), Me.Controls("TestBox2"), ...)
studentName = TextBox32.Text           

El código del array podría no estar a salvo todavía. Cualquier control que esté en un control contenedor (como un GroupBox o Panel) no se encontrará en Me.Controls; estarán en la colección Controls de ese Panel o GroupBox. Tampoco se devolverá ningun control si su nombre está mal escrito ("TeStBox2"). En esos casos, en esos elementos del array se almacenará Nothing y resultará en una NRE cuando intentes referenciarlos.

Estos deberían ser fáciles de encontrar ahora que ya sabes lo que estás buscando:

"Button2" está contenido en un Panel

Solución

En lugar de usar referencias indirectas por nombre usando la colección Controls del formulario, usa la referencia del control:

' Declaración
Private NameBoxes As TextBox()

' Inicializacion -  simple y fácil de leer, difícil de "chapucear":
NameBoxes = New TextBox() {TextBox1, TextBox2, ...)

' Inicializar una lista
NamesList = New List(Of TextBox)({TextBox1, TextBox2, TextBox3...})
' o
NamesList = New List(Of TextBox)
NamesList.AddRange({TextBox1, TextBox2, TextBox3...})

---

Function que devuelve Nothing

Private bars As New List(Of Bars)        ' Declarado y creado

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

Este es un caso donde el IDE mostrará un aviso de que 'no todas las rutas devuelve un valor lo que puede resultar en una NullReferenceException'. Puedes suprimir el aviso sustituyendo Exit Function con Return Nothing, pero eso no resuelve el problema. Cualquier cosa que intente usar el valor devuelto cuando someCondition = False resultará en una NRE:

bList = myFoo.BarList()
For Each b As Bar in bList      ' EXCEPCION
      ...

Solución

Cambia Exit Function en la función por Return bList. Devolver una lista(List) vacía no es lo mismo que devolver Nothing. Si existe la posibilidad de que un objeto devuelto pueda ser Nothing, compruébalo antes de usarlo:

 bList = myFoo.BarList()
 If bList IsNot Nothing Then...

---

Try/Catch mal implementado

Un Try/Catch mal implementado puede ocultar donde está el problema y provocar como consecuencia otros nuevos:

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)

    'Mas código

    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

Este caso es el de un objeto que no ha sido creado como se esperaba, pero también demuestra la "extrema" utilidad de un Catch vacío.

Hay una coma de más en el SQL (después de 'mailaddress') que causa una excepción en .ExecuteReader. Después de que Catch no haga nada, Finally intenta hacer limpieza, pero como no puedes cerrar(Close) un DataReader nulo, una nueva NullReferenceException es lanzada.

Un bloque Catch vacío es muy peligroso y nada recomendable. Este OP estaba sorprendido intentando entender porqué estaba obteniendo una NRE en el bloque Finally. En otras situaciones, un Catch vacío podría resultar en algo mucho mas grave y hacerte perder mucho tiempo probando cosas equivocadas en el lugar equivocado para encontrar el problema. (La "excepción silenciosa" descrita anteriormente proporciona el mismo entretenimiento añadido.)

Solución

No uses bloques Try/Catch vacios - deja que el código falle para que puedas a) identificar la causa b) indetificar la localización y c) aplicar la solución correcta. Los bloques Try/Catch no se hicieron para ocultar las excepciones de la persona cualificada para solucionarlas - el desarrollador.

---

DBNull no es lo mismo que Nothing

For Each row As DataGridViewRow In dgvPlanning.Rows
    If Not IsDBNull(row.Cells(0).Value) Then
        ...

La funciónIsDBNull se usa para comprobar si un valor es igual que System.DBNull: De MSDN:

El valor System.DBNull indica que el objeto representa datos que faltan o no existen. DBNull no es lo mismo que Nothing, que indica que una variable aun no ha sido inicializada.

Solución

If row.Cells(0) IsNot Nothing Then ...

Como antes, puedes comprobar primero Nothing, y después por un valor específico:

If (row.Cells(0) IsNot Nothing) AndAlso (IsDBNull(row.Cells(0).Value) = False) Then

Ejemplo 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 devuelve el primer elemento o el valor por defecto, que en el caso de tipos de referencia es Nothing y nunca DBNull:

If getFoo IsNot Nothing Then...

---

Controles

Dim chk As CheckBox

chk = CType(Me.Controls(chkName), CheckBox)
If chk.Checked Then
    Return chk
End If

Si un CheckBox con chkName no se encuentra (o existe en un GroupBox), entonces chk será Nothing y el intento de referenciar cualquier propiedad resultará en una excepción.

Solución

If (chk IsNot Nothing) AndAlso (chk.Checked) Then ...

El DataGridView

El DGV tiene algunos "caprichos" que se ven periódicamente:

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"

Si dgvBooks tiene AutoGenerateColumns = True, creará las columnas pero no las pondrá nombre, así que el código de arriba falla cuando trata de referenciarlas por nombre.

Solución

Pon nombre a las columnas manualmente, o referéncialas por índice:

dgvBooks.Columns(0).Visible = True

Ejemplo 2 — Cuidado con 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

Cuando tu DataGridView tiene AllowUserToAddRows a True (pro defecto), las Cells en la fila vacía/nueva al final contendrán todas Nothing. Los intentos de usar los contenidos de ésta (por ejemplo, ToString) resultarán en una NRE.

Solución

Usa un bucle For/Each y pregunta por la propiedad IsNewRow para determinar si es la última fila. Esto funciona aunque AllowUserToAddRows sea true o false:

For Each r As DataGridViewRow in myDGV.Rows
    If r.IsNewRow = False Then
         ' puedes usar esta fila

Si usas un bucle For n, modifica el contador de filas o usa Exit For cuando IsNewRow es true.

---

My.Settings (StringCollection)

Bajo ciertas circunstancias, intentado usar un elemento de My.Settings que es un StringCollection puede resultar en una NullReference la primera vez que lo usas. La solución es la misma, aunque no tan obvia. Considera:

My.Settings.FooBars.Add("ziggy")         ' foobars es una string collection

Ya que VB está gestionando Settings por ti, es razonable esperar que inicialice la colección. Lo hará, pero solo si previamente has añadido un elemento inicial a la colección (en el editor de Settings). Ya que la colección esta siendo (aparentemente) inicializada cuando un elemento es añadido, permanece a Nothing cuando no hay elementos en el editor de Settings para añadir.

Solución

Inicializa la colección de settings en el manejador del evento Load del formulario, si es necesario:

If My.Settings.FooBars Is Nothing Then
    My.Settings.FooBars = New System.Collections.Specialized.StringCollection
End If

Typically, the collection Settingswould only need to be initialized the first time the application is run. An alternative solution is to add an initial value to the collection in Project -> Settings | FooBars , save the project, and then remove the false value.

---

Key points

You probably forgot the operator New.

either

Something you assumed would work perfectly to return an initialized object hasn't.

Don't ignore compiler warnings (never) and use Option Strict On(always).

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MSDN Class NullReferenceException

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This answer is a translation of this Stack Overflow answer