c# - What is a NullReferenceException, and how do I fix it? -

i have code , when executes, throws nullreferenceexception, saying:

object reference not set instance of object.

what mean, , can it?

what cause?

bottom line

you trying use null (or nothing in vb.net). means either set null, or never set @ all.

like else, null gets passed around. if null in method "a", method "b" passed null to method "a".

the rest of article goes more detail , shows mistakes many programmers make can lead nullreferenceexception.

more specifically

the runtime throwing nullreferenceexception always means same thing: trying use reference, , reference not initialized (or once initialized, no longer initialized).

this means reference null, , cannot access members (such methods) through null reference. simplest case:

string foo = null; foo.toupper(); 

this throw nullreferenceexception @ second line because can't call instance method toupper() on string reference pointing null.

debugging

how find source of nullreferenceexception? apart looking @ exception itself, thrown @ location occurs, general rules of debugging in visual studio apply: place strategic breakpoints , inspect variables, either hovering mouse on names, opening (quick)watch window or using various debugging panels locals , autos.

if want find out reference or isn't set, right-click name , select "find references". can place breakpoint @ every found location , run program debugger attached. every time debugger breaks on such breakpoint, need determine whether expect reference non-null, inspect variable , and verify points instance when expect to.

by following program flow way, can find location instance should not null, , why isn't set.

examples

some common scenarios exception can thrown:

generic

ref1.ref2.ref3.member 

if ref1 or ref2 or ref3 null, you'll nullreferenceexception. if want solve problem, find out 1 null rewriting expression simpler equivalent:

var r1 = ref1; var r2 = r1.ref2; var r3 = r2.ref3; r3.member 

specifically, in httpcontext.current.user.identity.name, httpcontext.current null, or user property null, or identity property null.

class instances

when creating variable of reference (class) type, default set null.

public class book {     public string title { get; set; } } public class example {     public void foo() {         book b1;         string title = b1.title; // never initialized b1 variable.                                     // there no book title from.     } } 

class type variables must either initialized or set existing class instance. initialization done using new keyword.

book b1 = new book(); 

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; // never initialized author property.                                        // there no person age from.     } } 

if want avoid child (person) null reference, initialize in parent (book) object's constructor.

the same applies nested object initializers:

book b1 = new book { author = { age = 45 } }; 

while new keyword used, creates new instance of book, not new instance of person, author property still null.

array

int[] numbers = null; int n = numbers[0]; // numbers null. there no array index. 

array elements

person[] people = new person[5]; people[0].age = 20 // people[0] null. array allocated not                    // initialized. there no person set age for. 

jagged arrays

long[][] array = new long[1][]; array[0][0] = 3; // null because first dimension yet initialized.                  // use array[0] = new long[2]; first. 

collection/list/dictionary

dictionary<string, int> agesfornames = null; int age = agesfornames["bob"]; // agesfornames null.                                // there no dictionary perform lookup. 

range variable (indirect/deferred)

public class person {     public string name { get; set; } } var people = new list<person>(); people.add(null); var names = p in people select p.name; string firstname = names.first(); // exception thrown here, occurs                                   // on line above.  "p" null because                                   // first element added list null. 

events

public class demo {     public event eventhandler statechanged;      protected virtual void onstatechanged(eventargs e)     {                 statechanged(this, e); // exception thrown here                                 // if no event handlers have been attached                                // statechanged event     } } 

bad naming conventions:

if named fields differently locals, might have realized never initialized 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 solved following convention prefix fields 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)         {             // 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 "firstname" session value has not yet been set, // line throw nullreferenceexception string firstname = session["firstname"].tostring(); 

asp.net mvc empty view models

if exception occurs when referencing property of @model in asp.net mvc view, need understand model gets set in action method, when return view. when return empty model (or model property) controller, exception occurs when views access it:

// controller public class restaurant:controller {     public actionresult search()     {          return view();  // forgot provide model here.     } }  // razor view  @foreach (var restaurantsearch in model.restaurantsearch)  // throws. { }  <p>@model.somepropertyname</p> <!-- throws --> 

wpf control creation order , events

wpf controls created during call initializecomponent in order appear in visual tree. nullreferenceexception raised in case of early-created controls event handlers, etc. , fire during initializecomponent 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 created before label1. if combobox1_selectionchanged attempts reference `label1, not yet have been created.

private void combobox1_selectionchanged(object sender, selectionchangedeventargs e) {     label1.content = combobox1.selectedindex.tostring(); // nullreference here!! } 

changing order of declarations in xaml (i.e., listing label1 before combobox1, ignoring issues of design philosophy, @ least resolve nullreferenceexception here.

cast as

var mything = someobject thing; 

this doesn't throw invalidcastexception returns null when cast fails (and when someobject null). aware of that.

linq firstordefault() , singleordefault()

the plain versions first() , single() throw exceptions when there nothing. "ordefault" versions return null in case. aware of that.

foreach

foreach throws when try iterate null collection. caused unexpected null result methods return collections.

 list<int> list = null;      foreach(var v in list) { } // exception 

more realistic example - select nodes xml document. throw if nodes not found initial debugging shows properties valid:

 foreach (var node in mydata.myxml.documentnode.selectnodes("//data")) 

---

ways avoid

explicitly check null , ignore null values.

if expect reference null, can check being null before accessing instance members:

void printname(person p) {     if (p != null) {         console.writeline(p.name);     } } 

explicitly check null , provide default value.

methods call expect return instance can return null, example when object being sought cannot found. can choose return default value when case:

string getcategory(book b) {     if (b == null)         return "unknown";     return b.category; } 

explicitly check null method calls , throw custom exception.

you can throw custom exception, catch in calling code:

string getcategory(string booktitle) {     var book = library.findbook(booktitle);  // may return null     if (book == null)         throw new booknotfoundexception(booktitle);  // custom exception     return book.category; } 

use debug.assert if value should never null, catch problem earlier exception occurs.

when know during development method maybe can, never should return null, can use debug.assert() break possible when occur:

string gettitle(int knownbookid) {     // know should never return null.     var book = library.getbook(knownbookid);        // exception occur on next line instead of @ end of method.     debug.assert(book != null, "library didn't return book known book id.");      // other code      return book.title; // never throw nullreferenceexception in debug mode. } 

though check will not end in release build, causing throw nullreferenceexception again when book == null @ runtime in release mode.

use getvalueordefault() nullable value types provide default value when null.

datetime? appointment = null; console.writeline(appointment.getvalueordefault(datetime.now)); // display default value provided (datetime.now), because appointment null.  appointment = new datetime(2022, 10, 20); console.writeline(appointment.getvalueordefault(datetime.now)); // display appointment date, not default 

use null coalescing operator: ?? [c#] or if() [vb].

the shorthand providing default value when null encountered:

iservice createservice(ilogger log, int32? frobpowerlevel) {     var serviceimpl = new myservice(log ?? nulllog.instance);      // note above "getvalueordefault()" can rewritten use     // coalesce operator:     serviceimpl.frobpowerlevel = frobpowerlevel ?? 5; } 

use null condition operator: ?. (available in c# 6 , vb.net 14):

this called safe navigation or elvis (after shape) operator. if expression on left side of operator null, right side not evaluated, , null returned instead. means cases this:

var title = person.title.toupper(); 

if person not have title, throw exception because trying call toupper on property null value.

in c# 5 , below, can guarded with:

var title = person.title == null ? null : person.title.toupper(); 

now title variable null instead of throwing exception. c# 6 introduces shorter syntax this:

var title = person.title?.toupper(); 

this result in title variable being null, , call toupper not made if person.title null.

of course, still have check title null or use null condition operator null coalescing operator (??) supply default value:

// regular null check int titlelength = 0; if (title != null)     titlelength = title.length; // if title null, throw nullreferenceexception  // combining `?` , `??` operator int titlelength = title?.length ?? 0;