{"id":3835,"date":"2022-08-20T19:18:22","date_gmt":"2022-08-20T13:48:22","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-what-is-a-nullreferenceexception-and-how-do-i-fix-it\/"},"modified":"2022-08-20T19:18:22","modified_gmt":"2022-08-20T13:48:22","slug":"solved-what-is-a-nullreferenceexception-and-how-do-i-fix-it","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-what-is-a-nullreferenceexception-and-how-do-i-fix-it\/","title":{"rendered":"(Solved) What is a NullReferenceException, and how do I fix it?"},"content":{"rendered":"

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What is the cause?<\/h1>\n

Bottom Line<\/h2>\n

You are trying to use something that is null<\/code> (or Nothing<\/code> in VB.NET). This means you either set it to null<\/code>, or you never set it to anything at all.<\/p>\n

Like anything else, null<\/code> gets passed around. If it is null<\/code> in<\/em> method “A”, it could be that method “B” passed a null<\/code> to<\/em> method “A”.<\/p>\n

null<\/code> can have different meanings:<\/p>\n

    \n
  1. Object variables that are uninitialized<\/strong> and hence point to nothing.<\/strong> In this case, if you access members of such objects, it causes a NullReferenceException<\/code>.<\/li>\n
  2. The developer is using null<\/code> intentionally to indicate there is no meaningful value available.<\/strong> Note that C# has the concept of nullable datatypes for variables (like database tables can have nullable fields) – you can assign null<\/code> to them to indicate there is no value stored in it, for example int? a = null;<\/code> (which is a shortcut for Nullable<int> a = null;<\/code>) where the question mark indicates it is allowed to store null<\/code> in variable a<\/code>. You can check that either with if (a.HasValue) {...}<\/code> or with if (a==null) {...}<\/code>. Nullable variables, like a<\/code> this example, allow to access the value via a.Value<\/code> explicitly, or just as normal via a<\/code>.
    Note<\/strong> that accessing it via a.Value<\/code> throws an InvalidOperationException<\/code> instead of a NullReferenceException<\/code> if a<\/code> is null<\/code> – you should do the check beforehand, i.e. if you have another non-nullable variable int b;<\/code> then you should do assignments like if (a.HasValue) { b = a.Value; }<\/code> or shorter if (a != null) { b = a; }<\/code>.<\/li>\n<\/ol>\n

    The rest of this article goes into more detail and shows mistakes that many programmers often make which can lead to a NullReferenceException<\/code>.<\/p>\n

    More Specifically<\/h2>\n

    The runtime<\/code> throwing a NullReferenceException<\/code> always<\/strong> means the same thing: you are trying to use a reference, and the reference is not initialized (or it was once<\/em> initialized, but is no longer<\/em> initialized).<\/p>\n

    This means the reference is null<\/code>, and you cannot access members (such as methods) through a null<\/code> reference. The simplest case:<\/p>\n

    string foo = null;\nfoo.ToUpper();\n<\/code><\/pre>\n
    This will throw a NullReferenceException<\/code> at the second line because you can’t call the instance method ToUpper()<\/code> on a string<\/code> reference pointing to null<\/code>.<\/p>\n
    Debugging<\/h1>\n
    How do you find the source of a NullReferenceException<\/code>? 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<\/a>, either by hovering the mouse over their names, opening a (Quick)Watch window or using the various debugging panels like Locals and Autos.<\/p>\n
    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.<\/p>\n
    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.<\/p>\n
    Examples<\/h1>\n
    Some common scenarios where the exception can be thrown:<\/p>\n
    Generic<\/h3>\n
    ref1.ref2.ref3.member\n<\/code><\/pre>\n
    If ref1 or ref2 or ref3 is null, then you’ll get a NullReferenceException<\/code>. If you want to solve the problem, then find out which one is null by rewriting the expression to its simpler equivalent:<\/p>\n
    var r1 = ref1;\nvar r2 = r1.ref2;\nvar r3 = r2.ref3;\nr3.member\n<\/code><\/pre>\n
    Specifically, in HttpContext.Current.User.Identity.Name<\/code>, the HttpContext.Current<\/code> could be null, or the User<\/code> property could be null, or the Identity<\/code> property could be null.<\/p>\n
    Indirect<\/h3>\n
    public class Person \n{\n    public int Age { get; set; }\n}\npublic class Book \n{\n    public Person Author { get; set; }\n}\npublic class Example \n{\n    public void Foo() \n    {\n        Book b1 = new Book();\n        int authorAge = b1.Author.Age; \/\/ You never initialized the Author property.\n                                       \/\/ there is no Person to get an Age from.\n    }\n}\n<\/code><\/pre>\n
    If you want to avoid the child (Person) null reference, you could initialize it in the parent (Book) object’s constructor.<\/p>\n
    Nested Object Initializers<\/h3>\n
    The same applies to nested object initializers:<\/p>\n
    Book b1 = new Book \n{ \n   Author = { Age = 45 } \n};\n<\/code><\/pre>\n
    This translates to:<\/p>\n
    Book b1 = new Book();\nb1.Author.Age = 45;\n<\/code><\/pre>\n
    While the new<\/code> keyword is used, it only creates a new instance of Book<\/code>, but not a new instance of Person<\/code>, so the Author<\/code> the property is still null<\/code>.<\/p>\n
    Nested Collection Initializers<\/h3>\n
    public class Person \n{\n    public ICollection<Book> Books { get; set; }\n}\npublic class Book \n{\n    public string Title { get; set; }\n}\n<\/code><\/pre>\n
    The nested collection Initializers<\/code> behave the same:<\/p>\n
    Person p1 = new Person \n{\n    Books = {\n         new Book { Title = \"Title1\" },\n         new Book { Title = \"Title2\" },\n    }\n};\n<\/code><\/pre>\n
    This translates to:<\/p>\n
    Person p1 = new Person();\np1.Books.Add(new Book { Title = \"Title1\" });\np1.Books.Add(new Book { Title = \"Title2\" });\n<\/code><\/pre>\n
    The new Person<\/code> only creates an instance of Person<\/code>, but the Books<\/code> collection is still null<\/code>. The collection Initializer<\/code> syntax does not create a collection
    \nfor p1.Books<\/code>, it only translates to the p1.Books.Add(...)<\/code> statements.<\/p>\n
    Array<\/h3>\n
    int[] numbers = null;\nint n = numbers[0]; \/\/ numbers is null. There is no array to index.\n<\/code><\/pre>\n
    Array Elements<\/h3>\n
    Person[] people = new Person[5];\npeople[0].Age = 20 \/\/ people[0] is null. The array was allocated but not\n                   \/\/ initialized. There is no Person to set the Age for.\n<\/code><\/pre>\n
    Jagged Arrays<\/h3>\n
    long[][] array = new long[1][];\narray[0][0] = 3; \/\/ is null because only the first dimension is yet initialized.\n                 \/\/ Use array[0] = new long[2]; first.\n<\/code><\/pre>\n
    Collection\/List\/Dictionary<\/h3>\n
    Dictionary<string, int> agesForNames = null;\nint age = agesForNames[\"Bob\"]; \/\/ agesForNames is null.\n                               \/\/ There is no Dictionary to perform the lookup.\n<\/code><\/pre>\n
    Range Variable (Indirect\/Deferred)<\/h3>\n
    public class Person \n{\n    public string Name { get; set; }\n}\nvar people = new List<Person>();\npeople.Add(null);\nvar names = from p in people select p.Name;\nstring firstName = names.First(); \/\/ Exception is thrown here, but actually occurs\n                                  \/\/ on the line above.  \"p\" is null because the\n                                  \/\/ first element we added to the list is null.\n<\/code><\/pre>\n
    Events (C#)<\/h3>\n
    public class Demo\n{\n    public event EventHandler StateChanged;\n    \n    protected virtual void OnStateChanged(EventArgs e)\n    {        \n        StateChanged(this, e); \/\/ Exception is thrown here \n                               \/\/ if no event handlers have been attached\n                               \/\/ to StateChanged event\n    }\n}\n<\/code><\/pre>\n
    (Note: The VB.NET compiler inserts null checks for event usage, so it’s not necessary to check events for Nothing<\/code> in VB.NET.)<\/p>\n
    Bad Naming Conventions:<\/h3>\n
    If you named fields differently from locals, you might have realized that you never initialized the field.<\/p>\n
    public class Form1\n{\n    private Customer customer;\n    \n    private void Form1_Load(object sender, EventArgs e) \n    {\n        Customer customer = new Customer();\n        customer.Name = \"John\";\n    }\n    \n    private void Button_Click(object sender, EventArgs e)\n    {\n        MessageBox.Show(customer.Name);\n    }\n}\n<\/code><\/pre>\n
    This can be solved by following the convention to prefix fields with an underscore:<\/p>\n
        private Customer _customer;\n<\/code><\/pre>\n
    ASP.NET Page Life cycle:<\/h3>\n
    public partial class Issues_Edit : System.Web.UI.Page\n{\n    protected TestIssue myIssue;\n\n    protected void Page_Load(object sender, EventArgs e)\n    {\n        if (!IsPostBack)\n        {\n             \/\/ Only called on first load, not when button clicked\n             myIssue = new TestIssue(); \n        }\n    }\n        \n    protected void SaveButton_Click(object sender, EventArgs e)\n    {\n        myIssue.Entry = \"NullReferenceException here!\";\n    }\n}\n<\/code><\/pre>\n
    ASP.NET Session Values<\/h3>\n
    \/\/ if the \"FirstName\" session value has not yet been set,\n\/\/ then this line will throw a NullReferenceException\nstring firstName = Session[\"FirstName\"].ToString();\n<\/code><\/pre>\n
    ASP.NET MVC empty view models<\/h3>\n
    If the exception occurs when referencing a property of @Model<\/code> in an ASP.NET MVC View<\/code>, you need to understand that the Model<\/code> gets set in your action method, when you return<\/code> a view. When you return an empty model (or model property) from your controller, the exception occurs when the views access it:<\/p>\n
    \/\/ Controller\npublic class Restaurant:Controller\n{\n    public ActionResult Search()\n    {\n        return View();  \/\/ Forgot the provide a Model here.\n    }\n}\n\n\/\/ Razor view \n@foreach (var restaurantSearch in Model.RestaurantSearch)  \/\/ Throws.\n{\n}\n    \n<p>@Model.somePropertyName<\/p> <!-- Also throws -->\n<\/code><\/pre>\n
    WPF Control Creation Order and Events<\/h3>\n
    WPF<\/code> controls are created during the call to InitializeComponent<\/code> in the order they appear in the visual tree.  A NullReferenceException<\/code> will be raised in the case of early-created controls with event handlers, etc., that fire during InitializeComponent<\/code> which reference late-created controls.<\/p>\n
    For example:<\/p>\n
    <Grid>\n    <!-- Combobox declared first -->\n    <ComboBox Name=\"comboBox1\" \n              Margin=\"10\"\n              SelectedIndex=\"0\" \n              SelectionChanged=\"comboBox1_SelectionChanged\">\n       <ComboBoxItem Content=\"Item 1\" \/>\n       <ComboBoxItem Content=\"Item 2\" \/>\n       <ComboBoxItem Content=\"Item 3\" \/>\n    <\/ComboBox>\n        \n    <!-- Label declared later -->\n    <Label Name=\"label1\" \n           Content=\"Label\"\n           Margin=\"10\" \/>\n<\/Grid>\n<\/code><\/pre>\n
    Here comboBox1<\/code> is created before label1<\/code>. If comboBox1_SelectionChanged<\/code> attempts to reference `label1, it will not yet have been created.<\/p>\n
    private void comboBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)\n{\n    label1.Content = comboBox1.SelectedIndex.ToString(); \/\/ NullReferenceException here!!\n}\n<\/code><\/pre>\n
    Changing the order of the declarations in the XAML<\/code> (i.e., listing label1<\/code> before comboBox1<\/code>, ignoring issues of design philosophy) would at least resolve the NullReferenceException<\/code> here.<\/p>\n
    Cast with as<\/code><\/h3>\n
    var myThing = someObject as Thing;\n<\/code><\/pre>\n
    This doesn’t throw an InvalidCastException<\/code> but returns a null<\/code> when the cast fails (and when someObject<\/code> is itself null). So be aware of that.<\/p>\n
    LINQ FirstOrDefault()<\/code> and SingleOrDefault()<\/code><\/h3>\n
    The plain versions First()<\/code> and Single()<\/code> throw exceptions when there is nothing. The “OrDefault” versions return null<\/code> in that case. So be aware of that.<\/p>\n
    foreach<\/h3>\n
    foreach<\/code> throws when you try to iterate on a null<\/code> collection. Usually caused by unexpected null<\/code> result from methods that return collections.<\/p>\n
    List<int> list = null;    \nforeach(var v in list) { } \/\/ NullReferenceException here\n<\/code><\/pre>\n
    More realistic example – select nodes from XML document. Will throw if nodes are not found but initial debugging shows that all properties valid:<\/p>\n
    foreach (var node in myData.MyXml.DocumentNode.SelectNodes(\"\/\/Data\"))\n<\/code><\/pre>\n
    \n
    Ways to Avoid<\/h1>\n
    Explicitly check for null<\/code> and ignore null<\/code> values.<\/h3>\n
    If you expect the reference sometimes to be null<\/code>, you can check for it being null<\/code> before accessing instance members:<\/p>\n
    void PrintName(Person p)\n{\n    if (p != null) \n    {\n        Console.WriteLine(p.Name);\n    }\n}\n<\/code><\/pre>\n
    Explicitly check for null<\/code> and provide a default value.<\/h3>\n
    Methods you call expecting an instance can return null<\/code>, for example when the object being sought cannot be found. You can choose to return a default value when this is the case:<\/p>\n
    string GetCategory(Book b) \n{\n    if (b == null)\n        return \"Unknown\";\n    return b.Category;\n}\n<\/code><\/pre>\n
    Explicitly check for null<\/code> from method calls and throw a custom exception.<\/h3>\n
    You can also throw a custom exception, only to catch it in the calling code:<\/p>\n
    string GetCategory(string bookTitle) \n{\n    var book = library.FindBook(bookTitle);  \/\/ This may return null\n    if (book == null)\n        throw new BookNotFoundException(bookTitle);  \/\/ Your custom exception\n    return book.Category;\n}\n<\/code><\/pre>\n
    Use Debug.Assert<\/code> if a value should never be null<\/code>, to catch the problem earlier than the exception occurs.<\/h3>\n
    When you know during development that a method could, but never should return null<\/code>, you can use Debug.Assert()<\/code> to break as soon as possible when it does occur:<\/p>\n
    string GetTitle(int knownBookID) \n{\n    \/\/ You know this should never return null.\n    var book = library.GetBook(knownBookID);  \n\n    \/\/ Exception will occur on the next line instead of at the end of this method.\n    Debug.Assert(book != null, \"Library didn't return a book for known book ID.\");\n\n    \/\/ Some other code\n\n    return book.Title; \/\/ Will never throw NullReferenceException in Debug mode.\n}\n<\/code><\/pre>\n
    Though this check will not end up in your release build, causing it to throw the NullReferenceException<\/code> again when book == null<\/code> at runtime in release mode.<\/p>\n
    Use GetValueOrDefault()<\/code> for nullable<\/code> value types to provide a default value when they are null<\/code>.<\/h3>\n
    DateTime? appointment = null;\nConsole.WriteLine(appointment.GetValueOrDefault(DateTime.Now));\n\/\/ Will display the default value provided (DateTime.Now), because appointment is null.\n\nappointment = new DateTime(2022, 10, 20);\nConsole.WriteLine(appointment.GetValueOrDefault(DateTime.Now));\n\/\/ Will display the appointment date, not the default\n<\/code><\/pre>\n
    Use the null coalescing operator: ??<\/code> [C#] or If()<\/code> [VB].<\/h3>\n
    The shorthand to providing a default value when a null<\/code> is encountered:<\/p>\n
    IService CreateService(ILogger log, Int32? frobPowerLevel)\n{\n   var serviceImpl = new MyService(log ?? NullLog.Instance);\n \n   \/\/ Note that the above \"GetValueOrDefault()\" can also be rewritten to use\n   \/\/ the coalesce operator:\n   serviceImpl.FrobPowerLevel = frobPowerLevel ?? 5;\n}\n<\/code><\/pre>\n
    Use the null condition operator: ?.<\/code> or ?[x]<\/code> for arrays (available in C# 6 and VB.NET 14):<\/h3>\n
    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:<\/p>\n
    var title = person.Title.ToUpper();\n<\/code><\/pre>\n
    If the person does not have a title, this will throw an exception because it is trying to call ToUpper<\/code> on a property with a null value.<\/p>\n
    In C# 5<\/code> and below, this can be guarded with:<\/p>\n
    var title = person.Title == null ? null : person.Title.ToUpper();\n<\/code><\/pre>\n
    Now the title variable will be null instead of throwing an exception. C# 6 introduces a shorter syntax for this:<\/p>\n
    var title = person.Title?.ToUpper();\n<\/code><\/pre>\n
    This will result in the title variable being null<\/code>, and the call to ToUpper<\/code> is not made if person.Title<\/code> is null<\/code>.<\/p>\n
    Of course, you still<\/em> have to check title<\/code> for null<\/code> or use the null condition operator together with the null coalescing operator (??<\/code>) to supply a default value:<\/p>\n
    \/\/ regular null check\nint titleLength = 0;\nif (title != null)\n    titleLength = title.Length; \/\/ If title is null, this would throw NullReferenceException\n    \n\/\/ combining the `?` and the `??` operator\nint titleLength = title?.Length ?? 0;\n<\/code><\/pre>\n
    Likewise, for arrays you can use ?[i]<\/code> as follows:<\/p>\n
    int[] myIntArray = null;\nvar i = 5;\nint? elem = myIntArray?[i];\nif (!elem.HasValue) Console.WriteLine(\"No value\");\n<\/code><\/pre>\n
    This will do the following: If myIntArray<\/code> is null<\/code>, the expression returns null<\/code> and you can safely check it. If it contains an array, it will do the same as:
    \nelem = myIntArray[i];<\/code> and returns the ith<\/sup> element.<\/p>\n
    Use null context (available in C# 8):<\/h3>\n
    Introduced in C# 8<\/code>, null contexts and nullable reference types perform static analysis on variables and provide a compiler warning if a value can be potentially null<\/code> or have been set to null<\/code>. The nullable reference types allow types to be explicitly allowed to be null<\/code>.<\/p>\n
    The nullable annotation context and nullable warning context can be set for a project using the Nullable<\/code> element in your csproj<\/code> file. This element configures how the compiler interprets the nullability of types and what warnings are generated. Valid settings are:<\/p>\n