string foo = null;
foo.ToUpper();

ref1.ref2.ref3.member

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

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.
    }
}

Book b1 = new Book { Author = { Age = 45 } };

int[] numbers = null;
int n = numbers[0]; // numbers is null. There is no array to index.

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.

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.

Dictionary<string, int> agesForNames = null;
int age = agesForNames["Bob"]; // agesForNames is null.
                               // There is no Dictionary to perform the lookup.

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.

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
    }
}

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);
    }
}

private Customer _customer;

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!";
    }
}

// if the "FirstName" session value has not yet been set,
// then this line will throw a NullReferenceException
string firstName = Session["FirstName"].ToString();

// 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 -->

<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>

private void comboBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    label1.Content = comboBox1.SelectedIndex.ToString(); // NullReference here!!
}

var myThing = someObject as Thing;

void PrintName(Person p) {
    if (p != null) {
        Console.WriteLine(p.Name);
    }
}

string GetCategory(Book b) {
    if (b == null)
        return "Unknown";
    return b.Category;
}

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;
}

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.
}

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

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;
}

var title = person.Title.ToUpper();

var title = person.Title == null ? null : person.Title.ToUpper();

var title = person.Title?.ToUpper();

// 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;

object o = null;
    DateTime d = (DateTime)o;

<asp:Calendar runat="server" SelectedDate="<%#Bind("Something")%>" />

SqlConnection connection = null;
connection.Open();

if (myvar != null)
{
    // Go ahead and use myvar
    myvar.property = ...
}
else
{
    // Whoops! myvar is null and cannot be used without first
    // assigning it to an instance reference
    // Attempting to use myvar here will result in NullReferenceException
}

string testString = null; //Because it doesn't have a value (i.e. it's null; "Length" cannot do what it needs to do)

if (testString.Length == 0) // Throws a nullreferenceexception
{
    //Do something
}

Person p = null;
p.Name = "Harry"; // NullReferenceException occurs here.

Person p = null;
if (p!=null)
{
    p.Name = "Harry"; // Not going to run to this point
}

class Book {
    public string Name { get; set; }
}
class Car { }

Car mycar = new Car();
Book mybook = mycar as Book;   // Incompatible conversion --> mybook = null

Console.WriteLine(mybook.Name);   // NullReferenceException

ComicBook cb = (ComicBook)specificBook;

ComicBook cb = specificBook as ComicBook;
if (cb != null) {
   // ...
}

string value = null;
if (value.Length == 0) // <-- Causes exception
{
    Console.WriteLine(value); // <-- Never reached
}

object o = null;
DateTime d = (DateTime)o;  // NullReferenceException

DateTime? d = null;
var s = d.ToString();  // OK, no exception (no boxing), returns ""
var t = d.GetType();   // Bang! d is boxed, NullReferenceException

public static void MyExtension(this object x)
{
  x.ToString();
}

DateTime? d = null;
d.MyExtension();  // Leads to boxing, NullReferenceException occurs inside the body of the called method, not here.

Contact contact = new Contact { Name = "Abhinav"};
var context = new DataContext();
context.Contacts.Add(contact);
context.SaveChanges(); // NullReferenceException at this line

public class DataContext : DbContext
{
    public DbSet<Contact> Contacts {get; set;}
}

public partial class Contact
{
    public string Name {get; set;}
}

var x = myString.Trim();

var x = str1.Trim() + str2.Trim();

Public Class MyController
{
    private ServiceA serviceA;
    private ServiceB serviceB;

    public MyController(ServiceA serviceA, ServiceB serviceB)
    {
          this.serviceA = serviceA;
          this.serviceB = serviceB;

    }

    public void MyMethod()
    {
          //We don't need to check null because the dependency injection container injects it, provided you took care of bootstrapping it.
          var someObject = serviceA.DoThis();
    }
}

string str = string.Empty;
str.ToLower(); // throw null reference exception

Public Class Person {
    public string Name { get; set; }
}
Person objPerson;
objPerson.Name  /// throw Null refernce Exception

@{
    MyEntity M = new MyEntity();
}
@RenderPage("_MyOtherView.cshtml", M); // error in _MyOtherView, the Model was Null

@{
    MyEntity M = new MyEntity();
}
@Html.Partial("_MyOtherView.cshtml", M);

@inherits System.Web.Mvc.WebViewPage
@{
    ViewBag.Title = "Entity Index";
    List<MyEntity> MyEntities = new List<MyEntity>();
    MyEntities.Add(new MyEntity());
    MyEntities.Add(new MyEntity());
    MyEntities.Add(new MyEntity());
}
<div>
    @{
        foreach(var M in MyEntities)
        {
            // Squiggly lines below. Hovering says: cannot convert method group 'partial' to non-delegate type Object, did you intend to envoke the Method?
            @Html.Partial("MyOtherView.cshtml");
        }
    }
</div>

@foreach(var M in MyEntities){
    ...
}

public X { get; set; }

public void InvokeX()
{
  X.DoSomething(); // if X value is null, you will get a NullReferenceException
}

//Using code contracts:
[ContractInvariantMethod]
protected void ObjectInvariant ()
{
  Contract.Invariant ( X != null );
  //...
}

public void DoSomething(MyObject obj) {
    if(obj == null)
    {
        throw new ArgumentNullException("obj", "Need a reference to obj.");
    }
}

public void DoSometing([NotNull] obj)

[System.Diagnostics.DebuggerNonUserCode]
public struct NotNull<T> where T : class
{
    private T _value;

    public T Value
    {
        get
        {
            if (_value == null)
            {
                throw new Exception("null value not allowed");
            }

            return _value;
        }
        set
        {
            if (value == null)
            {
                throw new Exception("null value not allowed.");
            }

            _value = value;
        }
    }

    public static implicit operator T(NotNull<T> notNullValue)
    {
        return notNullValue.Value;
    }

    public static implicit operator NotNull<T>(T value)
    {
        return new NotNull<T> { Value = value };
    }
}

NotNull<Person> person = null; // throws exception
NotNull<Person> person = new Person(); // OK
NotNull<Person> person = GetPerson(); // throws exception if GetPerson() returns null

Person person = new Person { Name = "John" };
WriteName(person);

public static void WriteName(NotNull<Person> person)
{
    Console.WriteLine(person.Value.Name);
}

Person person = GetPerson();

public static NotNull<Person> GetPerson()
{
    return new Person { Name = "John" };
}

Person person = (NotNull<Person>)GetPerson();

public static Person GetPerson()
{
    return new Person { Name = "John" };
}

[System.Diagnostics.DebuggerNonUserCode]
public static class NotNullExtension
{
    public static T NotNull<T>(this T @this) where T : class
    {
        if (@this == null)
        {
            throw new Exception("null value not allowed");
        }

        return @this;
    }
}

var person = GetPerson().NotNull();

var address = country?.State?.County?.City;

if (i == null) {
    // Handle this
}

public class Student
{
    private string FirstName;
    private string LastName;
    public string GetFullName()
    {
        return FirstName + LastName;
    }
}

public class StudentInfo
{
    public string GetStudentName()
    {
        Student s;
        string fullname = s.GetFullName();
        return fullname;
    }
}