What is a NullReferenceException, and how do I fix it?
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
What is the cause?
Bottom Line
You are trying to use something that is null (or Nothing in VB.NET). This means you either set it to null, or you never set it to anything at all.
Like anything else, null gets passed around. If it is null in method "A", it could be that method "B" passed a null to method "A".
null can have different meanings:
- Object variables that are uninitialized and hence point to nothing. In this case, if you access members of such objects, it causes a
NullReferenceException. - The developer is using
nullintentionally to indicate there is no meaningful value available. Note that C# has the concept of nullable datatypes for variables (like database tables can have nullable fields) - you can assignnullto them to indicate there is no value stored in it, for exampleint? a = null;(which is a shortcut forNullable<int> a = null;) where the question mark indicates it is allowed to storenullin variablea. You can check that either withif (a.HasValue) {...}or withif (a==null) {...}. Nullable variables, likeathis example, allow to access the value viaa.Valueexplicitly, or just as normal viaa.
Note that accessing it viaa.Valuethrows anInvalidOperationExceptioninstead of aNullReferenceExceptionifaisnull- you should do the check beforehand, i.e. if you have another non-nullable variableint b;then you should do assignments likeif (a.HasValue) { b = a.Value; }or shorterif (a != null) { b = a; }.
The rest of this article goes into more detail and shows mistakes that many programmers often make which can lead to a NullReferenceException.
More Specifically
The runtime throwing a NullReferenceException always means the same thing: you are trying to use a reference, and the reference is not initialized (or it was once initialized, but is no longer initialized).
This means the reference is null, and you cannot access members (such as methods) through a null reference. The simplest case:
string foo = null;
foo.ToUpper();
This will throw a NullReferenceException at the second line because you can't call the instance method ToUpper() on a string reference pointing to null.
Debugging
How do you find the source of a NullReferenceException? Apart from looking at the exception itself, which will be thrown exactly at the location where it occurs, the general rules of debugging in Visual Studio apply: place strategic breakpoints and inspect your variables, either by hovering the mouse over their names, opening a (Quick)Watch window or using the various debugging panels like Locals and Autos.
If you want to find out where the reference is or isn't set, right-click its name and select "Find All References". You can then place a breakpoint at every found location and run your program with the debugger attached. Every time the debugger breaks on such a breakpoint, you need to determine whether you expect the reference to be non-null, inspect the variable, and verify that it points to an instance when you expect it to.
By following the program flow this way, you can find the location where the instance should not be null, and why it isn't properly set.
Examples
Some common scenarios where the exception can be thrown:
Generic
ref1.ref2.ref3.member
If ref1 or ref2 or ref3 is null, then you'll get a NullReferenceException. If you want to solve the problem, then find out which one is null by rewriting the expression to its simpler equivalent:
var r1 = ref1;
var r2 = r1.ref2;
var r3 = r2.ref3;
r3.member
Specifically, in HttpContext.Current.User.Identity.Name, the HttpContext.Current could be null, or the User property could be null, or the Identity property could be null.
Indirect
public class Person
{
public int Age { get; set; }
}
public class Book
{
public Person Author { get; set; }
}
public class Example
{
public void Foo()
{
Book b1 = new Book();
int authorAge = b1.Author.Age; // You never initialized the Author property.
// there is no Person to get an Age from.
}
}
If you want to avoid the child (Person) null reference, you could initialize it in the parent (Book) object's constructor.
Nested Object Initializers
The same applies to nested object initializers:
Book b1 = new Book
{
Author = { Age = 45 }
};
This translates to:
Book b1 = new Book();
b1.Author.Age = 45;
While the new keyword is used, it only creates a new instance of Book, but not a new instance of Person, so the Author the property is still null.
Nested Collection Initializers
public class Person
{
public ICollection<Book> Books { get; set; }
}
public class Book
{
public string Title { get; set; }
}
The nested collection Initializers behave the same:
Person p1 = new Person
{
Books = {
new Book { Title = "Title1" },
new Book { Title = "Title2" },
}
};
This translates to:
Person p1 = new Person();
p1.Books.Add(new Book { Title = "Title1" });
p1.Books.Add(new Book { Title = "Title2" });
The new Person only creates an instance of Person, but the Books collection is still null. The collection Initializer syntax does not create a collection
for p1.Books, it only translates to the p1.Books.Add(...) statements.
Array
int[] numbers = null;
int n = numbers[0]; // numbers is null. There is no array to index.
Array Elements
Person[] people = new Person[5];
people[0].Age = 20 // people[0] is null. The array was allocated but not
// initialized. There is no Person to set the Age for.
Jagged Arrays
long[][] array = new long[1][];
array[0][0] = 3; // is null because only the first dimension is yet initialized.
// Use array[0] = new long[2]; first.
Collection/List/Dictionary
Dictionary<string, int> agesForNames = null;
int age = agesForNames["Bob"]; // agesForNames is null.
// There is no Dictionary to perform the lookup.
Range Variable (Indirect/Deferred)
public class Person
{
public string Name { get; set; }
}
var people = new List<Person>();
people.Add(null);
var names = from p in people select p.Name;
string firstName = names.First(); // Exception is thrown here, but actually occurs
// on the line above. "p" is null because the
// first element we added to the list is null.
Events (C#)
public class Demo
{
public event EventHandler StateChanged;
protected virtual void OnStateChanged(EventArgs e)
{
StateChanged(this, e); // Exception is thrown here
// if no event handlers have been attached
// to StateChanged event
}
}
(Note: The VB.NET compiler inserts null checks for event usage, so it's not necessary to check events for Nothing in VB.NET.)
Bad Naming Conventions:
If you named fields differently from locals, you might have realized that you never initialized the field.
public class Form1
{
private Customer customer;
private void Form1_Load(object sender, EventArgs e)
{
Customer customer = new Customer();
customer.Name = "John";
}
private void Button_Click(object sender, EventArgs e)
{
MessageBox.Show(customer.Name);
}
}
This can be solved by following the convention to prefix fields with an underscore:
private Customer _customer;
ASP.NET Page Life cycle:
public partial class Issues_Edit : System.Web.UI.Page
{
protected TestIssue myIssue;
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
// Only called on first load, not when button clicked
myIssue = new TestIssue();
}
}
protected void SaveButton_Click(object sender, EventArgs e)
{
myIssue.Entry = "NullReferenceException here!";
}
}
ASP.NET Session Values
// if the "FirstName" session value has not yet been set,
// then this line will throw a NullReferenceException
string firstName = Session["FirstName"].ToString();
ASP.NET MVC empty view models
If the exception occurs when referencing a property of @Model in an ASP.NET MVC View, you need to understand that the Model gets set in your action method, when you return a view. When you return an empty model (or model property) from your controller, the exception occurs when the views access it:
// Controller
public class Restaurant:Controller
{
public ActionResult Search()
{
return View(); // Forgot the provide a Model here.
}
}
// Razor view
@foreach (var restaurantSearch in Model.RestaurantSearch) // Throws.
{
}
<p>@Model.somePropertyName</p> <!-- Also throws -->
WPF Control Creation Order and Events
WPF controls are created during the call to InitializeComponent in the order they appear in the visual tree. A NullReferenceException will be raised in the case of early-created controls with event handlers, etc., that fire during InitializeComponent which reference late-created controls.
For example:
<Grid>
<!-- Combobox declared first -->
<ComboBox Name="comboBox1"
Margin="10"
SelectedIndex="0"
SelectionChanged="comboBox1_SelectionChanged">
<ComboBoxItem Content="Item 1" />
<ComboBoxItem Content="Item 2" />
<ComboBoxItem Content="Item 3" />
</ComboBox>
<!-- Label declared later -->
<Label Name="label1"
Content="Label"
Margin="10" />
</Grid>
Here comboBox1 is created before label1. If comboBox1_SelectionChanged attempts to reference `label1, it will not yet have been created.
private void comboBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
label1.Content = comboBox1.SelectedIndex.ToString(); // NullReferenceException here!!
}
Changing the order of the declarations in the XAML (i.e., listing label1 before comboBox1, ignoring issues of design philosophy) would at least resolve the NullReferenceException here.
Cast with as
var myThing = someObject as Thing;
This doesn't throw an InvalidCastException but returns a null when the cast fails (and when someObject is itself null). So be aware of that.
LINQ FirstOrDefault() and SingleOrDefault()
The plain versions First() and Single() throw exceptions when there is nothing. The "OrDefault" versions return null in that case. So be aware of that.
foreach
foreach throws when you try to iterate on a null collection. Usually caused by unexpected null result from methods that return collections.
List<int> list = null;
foreach(var v in list) { } // NullReferenceException here
More realistic example - select nodes from XML document. Will throw if nodes are not found but initial debugging shows that all properties valid:
foreach (var node in myData.MyXml.DocumentNode.SelectNodes("//Data"))
Ways to Avoid
Explicitly check for null and ignore null values.
If you expect the reference sometimes to be null, you can check for it being null before accessing instance members:
void PrintName(Person p)
{
if (p != null)
{
Console.WriteLine(p.Name);
}
}
Explicitly check for null and provide a default value.
Methods you call expecting an instance can return null, for example when the object being sought cannot be found. You can choose to return a default value when this is the case:
string GetCategory(Book b)
{
if (b == null)
return "Unknown";
return b.Category;
}
Explicitly check for null from method calls and throw a custom exception.
You can also throw a custom exception, only to catch it in the calling code:
string GetCategory(string bookTitle)
{
var book = library.FindBook(bookTitle); // This may return null
if (book == null)
throw new BookNotFoundException(bookTitle); // Your custom exception
return book.Category;
}
Use Debug.Assert if a value should never be null, to catch the problem earlier than the exception occurs.
When you know during development that a method could, but never should return null, you can use Debug.Assert() to break as soon as possible when it does occur:
string GetTitle(int knownBookID)
{
// You know this should never return null.
var book = library.GetBook(knownBookID);
// Exception will occur on the next line instead of at the end of this method.
Debug.Assert(book != null, "Library didn't return a book for known book ID.");
// Some other code
return book.Title; // Will never throw NullReferenceException in Debug mode.
}
Though this check will not end up in your release build, causing it to throw the NullReferenceException again when book == null at runtime in release mode.
Use GetValueOrDefault() for nullable value types to provide a default value when they are null.
DateTime? appointment = null;
Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now));
// Will display the default value provided (DateTime.Now), because appointment is null.
appointment = new DateTime(2022, 10, 20);
Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now));
// Will display the appointment date, not the default
Use the null coalescing operator: ?? [C#] or If() [VB].
The shorthand to providing a default value when a null is encountered:
IService CreateService(ILogger log, Int32? frobPowerLevel)
{
var serviceImpl = new MyService(log ?? NullLog.Instance);
// Note that the above "GetValueOrDefault()" can also be rewritten to use
// the coalesce operator:
serviceImpl.FrobPowerLevel = frobPowerLevel ?? 5;
}
Use the null condition operator: ?. or ?[x] for arrays (available in C# 6 and VB.NET 14):
This is also sometimes called the safe navigation or Elvis (after its shape) operator. If the expression on the left side of the operator is null, then the right side will not be evaluated, and null is returned instead. That means cases like this:
var title = person.Title.ToUpper();
If the person does not have a title, this will throw an exception because it is trying to call ToUpper on a property with a null value.
In C# 5 and below, this can be guarded with:
var title = person.Title == null ? null : person.Title.ToUpper();
Now the title variable will be null instead of throwing an exception. C# 6 introduces a shorter syntax for this:
var title = person.Title?.ToUpper();
This will result in the title variable being null, and the call to ToUpper is not made if person.Title is null.
Of course, you still have to check title for null or use the null condition operator together with the null coalescing operator (??) to supply a default value:
// regular null check
int titleLength = 0;
if (title != null)
titleLength = title.Length; // If title is null, this would throw NullReferenceException
// combining the `?` and the `??` operator
int titleLength = title?.Length ?? 0;
Likewise, for arrays you can use ?[i] as follows:
int[] myIntArray = null;
var i = 5;
int? elem = myIntArray?[i];
if (!elem.HasValue) Console.WriteLine("No value");
This will do the following: If myIntArray is null, the expression returns null and you can safely check it. If it contains an array, it will do the same as:
elem = myIntArray[i]; and returns the ith element.
Use null context (available in C# 8):
Introduced in C# 8, null contexts and nullable reference types perform static analysis on variables and provide a compiler warning if a value can be potentially null or have been set to null. The nullable reference types allow types to be explicitly allowed to be null.
The nullable annotation context and nullable warning context can be set for a project using the Nullable element in your csproj file. This element configures how the compiler interprets the nullability of types and what warnings are generated. Valid settings are:
enable: The nullable annotation context is enabled. The nullable warning context is enabled. Variables of a reference type, string, for example, are non-nullable. All nullability warnings are enabled.disable: The nullable annotation context is disabled. The nullable warning context is disabled. Variables of a reference type are oblivious, just like earlier versions of C#. All nullability warnings are disabled.safeonly: The nullable annotation context is enabled. The nullable warning context is safeonly. Variables of a reference type are non-nullable. All safety nullability warnings are enabled.warnings: The nullable annotation context is disabled. The nullable warning context is enabled. Variables of a reference type are oblivious. All nullability warnings are enabled.safeonlywarnings: The nullable annotation context is disabled. The nullable warning context is safeonly. Variables of a reference type are oblivious. All safety nullability warnings are enabled.
A nullable reference type is noted using the same syntax as nullable value types: a ? is appended to the type of the variable.
Special techniques for debugging and fixing null derefs in iterators
C# supports "iterator blocks" (called "generators" in some other popular languages). NullReferenceException can be particularly tricky to debug in iterator blocks because of deferred execution:
public IEnumerable<Frob> GetFrobs(FrobFactory f, int count)
{
for (int i = 0; i < count; ++i)
yield return f.MakeFrob();
}
...
FrobFactory factory = whatever;
IEnumerable<Frobs> frobs = GetFrobs();
...
foreach(Frob frob in frobs) { ... }
If whatever results in null then MakeFrob will throw. Now, you might think that the right thing to do is this:
// DON'T DO THIS
public IEnumerable<Frob> GetFrobs(FrobFactory f, int count)
{
if (f == null)
throw new ArgumentNullException("f", "factory must not be null");
for (int i = 0; i < count; ++i)
yield return f.MakeFrob();
}
Why is this wrong? Because the iterator block does not actually run until the foreach! The call to GetFrobs simply returns an object which when iterated will run the iterator block.
By writing a null check like this you prevent the NullReferenceException, but you move the NullArgumentException to the point of the iteration, not to the point of the call, and that is very confusing to debug.
The correct fix is:
// DO THIS
public IEnumerable<Frob> GetFrobs(FrobFactory f, int count)
{
// No yields in a public method that throws!
if (f == null)
throw new ArgumentNullException("f", "factory must not be null");
return GetFrobsForReal(f, count);
}
private IEnumerable<Frob> GetFrobsForReal(FrobFactory f, int count)
{
// Yields in a private method
Debug.Assert(f != null);
for (int i = 0; i < count; ++i)
yield return f.MakeFrob();
}
That is, make a private helper method that has the iterator block logic and a public surface method that does the null check and returns the iterator. Now when GetFrobs is called, the null check happens immediately, and then GetFrobsForReal executes when the sequence is iterated.
If you examine the reference source for LINQ to Objects you will see that this technique is used throughout. It is slightly more clunky to write, but it makes debugging nullity errors much easier. Optimize your code for the convenience of the caller, not the convenience of the author.
A note on null dereferences in unsafe code
C# has an "unsafe" mode which is, as the name implies, extremely dangerous because the normal safety mechanisms which provide memory safety and type safety are not enforced. You should not be writing unsafe code unless you have a thorough and deep understanding of how memory works.
In unsafe mode, you should be aware of two important facts:
- dereferencing a null pointer produces the same exception as dereferencing a null reference
- dereferencing an invalid non-null pointer can produce that exception in some circumstances
To understand why that is, it helps to understand how .NET produces NullReferenceException in the first place. (These details apply to .NET running on Windows; other operating systems use similar mechanisms.)
Memory is virtualized in Windows; each process gets a virtual memory space of many "pages" of memory that are tracked by the operating system. Each page of memory has flags set on it that determine how it may be used: read from, written to, executed, and so on. The lowest page is marked as "produce an error if ever used in any way".
Both a null pointer and a null reference in C# are internally represented as the number zero, and so any attempt to dereference it into its corresponding memory storage causes the operating system to produce an error. The .NET runtime then detects this error and turns it into the NullReferenceException.
That's why dereferencing both a null pointer and a null reference produces the same exception.
What about the second point? Dereferencing any invalid pointer that falls in the lowest page of virtual memory causes the same operating system error, and thereby the same exception.
Why does this make sense? Well, suppose we have a struct containing two ints, and an unmanaged pointer equal to null. If we attempt to dereference the second int in the struct, the CLR will not attempt to access the storage at location zero; it will access the storage at location four. But logically this is a null dereference because we are getting to that address via the null.
If you are working with unsafe code and you get a NullReferenceException, just be aware that the offending pointer need not be null. It can be any location in the lowest page, and this exception will be produced.
Another scenario is when you cast a null object into a value type. For example, the code below:
object o = null;
DateTime d = (DateTime)o;
It will throw a NullReferenceException on the cast. It seems quite obvious in the above sample, but this can happen in more "late-binding" intricate scenarios where the null object has been returned from some code you don't own, and the cast is for example generated by some automatic system.
One example of this is this simple ASP.NET binding fragment with the Calendar control:
<asp:Calendar runat="server" SelectedDate="<%#Bind("Something")%>" />
Here, SelectedDate is in fact a property - of DateTime type - of the Calendar Web Control type, and the binding could perfectly return something null. The implicit ASP.NET Generator will create a piece of code that will be equivalent to the cast code above. And this will raise a NullReferenceException that is quite difficult to spot, because it lies in ASP.NET generated code which compiles fine...