You forgot to instantiate a reference type. In the following example, names is declared but never instantiated (the affected line is commented out in the C# example since it doesn't compile):

using System.Collections.Generic;

public class UseBeforeAssignExample
{
    public static void Main(string[] args)
    {
        int value = int.Parse(args[0]);
        List<string> names;
        if (value > 0)
            names = [];

        //names.Add("Major Major Major");
    }
}

// Compilation displays a warning like the following:
//    warning BC42104: Variable //names// is used before it
//    has been assigned a value. A null reference exception could result
//    at runtime.
//
//          names.Add("Major Major Major")
//          ~~~~~
// The example displays output like the following output:
//    Unhandled Exception: System.NullReferenceException: Object reference
//    not set to an instance of an object.
//       at UseBeforeAssignExample.Main()

open System

[<EntryPoint>]
let main args =
    let value = Int32.Parse args[0]
    // Set names to null, don't initialize it. 
    let mutable names = Unchecked.defaultof<ResizeArray<string>>
    if value > 0 then
        names <- ResizeArray()
    names.Add "Major Major Major"
    0
// Compilation does not display a warning as this is an extremely rare occurance in F#.
// Creating a value without initalizing either requires using 'null' (not possible
// on types defined in F# without [<AllowNullLiteral>]) or Unchecked.defaultof.
//
// The example displays output like the following output:
//    Unhandled Exception: System.NullReferenceException: Object reference
//    not set to an instance of an object.
//       at Example.main()

Imports System.Collections.Generic

Module Example
   Public Sub Main()
      Dim names As List(Of String)
      names.Add("Major Major Major")       
   End Sub
End Module
' Compilation displays a warning like the following:
'    Example1.vb(10) : warning BC42104: Variable 'names' is used before it 
'    has been assigned a value. A null reference exception could result 
'    at runtime.
'    
'          names.Add("Major Major Major")
'          ~~~~~
' The example displays output like the following output:
'    Unhandled Exception: System.NullReferenceException: Object reference 
'    not set to an instance of an object.
'       at Example.Main()

Some compilers issue a warning when they compile this code. Others issue an error, and the compilation fails. To address this problem, instantiate the object so that its value is no longer null. The following example does this by calling a type's class constructor.

using System.Collections.Generic;

public class AnotherExample
{
    public static void Main()
    {
        List<string> names = ["Major Major Major"];
    }
}

let names = ResizeArray()
names.Add "Major Major Major"

Imports System.Collections.Generic

Module Example
   Public Sub Main()
      Dim names As New List(Of String)()
      names.Add("Major Major Major")       
   End Sub
End Module

You forgot to dimension an array before initializing it. In the following example, values is declared to be an integer array, but the number of elements that it contains is never specified. The attempt to initialize its values therefore throws a NullReferenceException exception.

int[] values = null;
for (int ctr = 0; ctr <= 9; ctr++)
    values[ctr] = ctr * 2;

foreach (int value in values)
    Console.WriteLine(value);

// The example displays the following output:
//    Unhandled Exception:
//       System.NullReferenceException: Object reference not set to an instance of an object.
//       at Array3Example.Main()

let values: int[] = null
for i = 0 to 9 do
    values[i] <- i * 2

for value in values do
    printfn $"{value}"
// The example displays the following output:
//    Unhandled Exception:
//       System.NullReferenceException: Object reference not set to an instance of an object.
//       at <StartupCode$fs>.main()

Module Example
   Public Sub Main()
       Dim values() As Integer
       For ctr As Integer = 0 To 9
          values(ctr) = ctr * 2
       Next
          
       For Each value In values
          Console.WriteLine(value)
       Next      
   End Sub
End Module
' The example displays the following output:
'    Unhandled Exception: 
'       System.NullReferenceException: Object reference not set to an instance of an object.
'       at Example.Main()

You can eliminate the exception by declaring the number of elements in the array before initializing it, as the following example does.

int[] values = new int[10];
for (int ctr = 0; ctr <= 9; ctr++)
    values[ctr] = ctr * 2;

foreach (int value in values)
    Console.WriteLine(value);

// The example displays the following output:
//    0
//    2
//    4
//    6
//    8
//    10
//    12
//    14
//    16
//    18

let values = Array.zeroCreate<int> 10
for i = 0 to 9 do
    values[i] <- i * 2

for value in values do
    printfn $"{value}"
// The example displays the following output:
//    0
//    2
//    4
//    6
//    8
//    10
//    12
//    14
//    16
//    18

Module Example
   Public Sub Main()
       Dim values(9) As Integer
       For ctr As Integer = 0 To 9
          values(ctr) = ctr * 2
       Next
          
       For Each value In values
          Console.WriteLine(value)
       Next      
   End Sub
End Module
' The example displays the following output:
'    0
'    2
'    4
'    6
'    8
'    10
'    12
'    14
'    16
'    18

For more information on declaring and initializing arrays, see Arrays and Arrays.

You get a null return value from a method, and then call a method on the returned type. This sometimes is the result of a documentation error; the documentation fails to note that a method call can return null. In other cases, your code erroneously assumes that the method will always return a non-null value.

The code in the following example assumes that the Array.Find method always returns Person object whose FirstName field matches a search string. Because there is no match, the runtime throws a NullReferenceException exception.

public static void NoCheckExample()
{
    Person[] persons = Person.AddRange([ "Abigail", "Abra",
                                      "Abraham", "Adrian", "Ariella",
                                      "Arnold", "Aston", "Astor" ]);
    string nameToFind = "Robert";
    Person found = Array.Find(persons, p => p.FirstName == nameToFind);
    Console.WriteLine(found.FirstName);
}

// The example displays the following output:
//       Unhandled Exception: System.NullReferenceException:
//       Object reference not set to an instance of an object.

open System

type Person(firstName) =
    member _.FirstName = firstName

    static member AddRange(firstNames) =
        Array.map Person firstNames

let persons = 
    [| "Abigail"; "Abra"; "Abraham"; "Adrian"
       "Ariella"; "Arnold"; "Aston"; "Astor" |]
    |> Person.AddRange

let nameToFind = "Robert"
let found = Array.Find(persons, fun p -> p.FirstName = nameToFind)

printfn $"{found.FirstName}"

// The example displays the following output:
//       Unhandled Exception: System.NullReferenceException:
//       Object reference not set to an instance of an object.
//          at <StartupCode$fs>.main()

Module Example
   Public Sub Main()
      Dim persons() As Person = Person.AddRange( { "Abigail", "Abra",
                                                   "Abraham", "Adrian",
                                                   "Ariella", "Arnold", 
                                                   "Aston", "Astor" } )    
      Dim nameToFind As String = "Robert"
      Dim found As Person = Array.Find(persons, Function(p) p.FirstName = nameToFind)
      Console.WriteLine(found.FirstName)
   End Sub
End Module

Public Class Person
   Public Shared Function AddRange(firstNames() As String) As Person()
      Dim p(firstNames.Length - 1) As Person
      For ctr As Integer = 0 To firstNames.Length - 1
         p(ctr) = New Person(firstNames(ctr))
      Next   
      Return p
   End Function
   
   Public Sub New(firstName As String)
      Me.FirstName = firstName
   End Sub 
   
   Public FirstName As String
End Class
' The example displays the following output:
'       Unhandled Exception: System.NullReferenceException: 
'       Object reference not set to an instance of an object.
'          at Example.Main()

To address this problem, test the method's return value to ensure that it's not null before calling any of its members, as the following example does.

public static void ExampleWithNullCheck()
{
    Person[] persons = Person.AddRange([ "Abigail", "Abra",
                                      "Abraham", "Adrian", "Ariella",
                                      "Arnold", "Aston", "Astor" ]);
    string nameToFind = "Robert";
    Person found = Array.Find(persons, p => p.FirstName == nameToFind);
    if (found != null)
        Console.WriteLine(found.FirstName);
    else
        Console.WriteLine($"'{nameToFind}' not found.");
}

// The example displays the following output:
//        'Robert' not found

open System

[<AllowNullLiteral>]
type Person(firstName) =
    member _.FirstName = firstName

    static member AddRange(firstNames) =
        Array.map Person firstNames

let persons = 
    [| "Abigail"; "Abra"; "Abraham"; "Adrian"
       "Ariella"; "Arnold"; "Aston"; "Astor" |]
    |> Person.AddRange

let nameToFind = "Robert"
let found = Array.Find(persons, fun p -> p.FirstName = nameToFind)

if found <> null then
    printfn $"{found.FirstName}"
else 
    printfn $"{nameToFind} not found."

// Using F#'s Array.tryFind function
// This does not require a null check or [<AllowNullLiteral>]
let found2 = 
    persons |> Array.tryFind (fun p -> p.FirstName = nameToFind)

match found2 with
| Some firstName ->
    printfn $"{firstName}"
| None ->
    printfn $"{nameToFind} not found."

// The example displays the following output:
//        Robert not found.
//        Robert not found.

Module Example
   Public Sub Main()
      Dim persons() As Person = Person.AddRange( { "Abigail", "Abra",
                                                   "Abraham", "Adrian",
                                                   "Ariella", "Arnold", 
                                                   "Aston", "Astor" } )    
      Dim nameToFind As String = "Robert"
      Dim found As Person = Array.Find(persons, Function(p) p.FirstName = nameToFind)
      If found IsNot Nothing Then
         Console.WriteLine(found.FirstName)
      Else
         Console.WriteLine("{0} not found.", nameToFind)
      End If   
   End Sub
End Module

Public Class Person
   Public Shared Function AddRange(firstNames() As String) As Person()
      Dim p(firstNames.Length - 1) As Person
      For ctr As Integer = 0 To firstNames.Length - 1
         p(ctr) = New Person(firstNames(ctr))
      Next   
      Return p
   End Function
   
   Public Sub New(firstName As String)
      Me.FirstName = firstName
   End Sub 
   
   Public FirstName As String
End Class
' The example displays the following output:
'       Robert not found

You're using an expression (for example, you chained a list of methods or properties together) to retrieve a value and, although you're checking whether the value is null, the runtime still throws a NullReferenceException exception. This occurs because one of the intermediate values in the expression returns null. As a result, your test for null is never evaluated.

The following example defines a Pages object that caches information about web pages, which are presented by Page objects. The Example.Main method checks whether the current web page has a non-null title and, if it does, displays the title. Despite this check, however, the method throws a NullReferenceException exception.

public class Chain1Example
{
    public static void Main()
    {
        var pages = new Pages();
        if (!string.IsNullOrEmpty(pages.CurrentPage.Title))
        {
            string title = pages.CurrentPage.Title;
            Console.WriteLine($"Current title: '{title}'");
        }
    }
}

public class Pages
{
    readonly Page[] _page = new Page[10];
    int _ctr = 0;

    public Page CurrentPage
    {
        get { return _page[_ctr]; }
        set
        {
            // Move all the page objects down to accommodate the new one.
            if (_ctr > _page.GetUpperBound(0))
            {
                for (int ndx = 1; ndx <= _page.GetUpperBound(0); ndx++)
                    _page[ndx - 1] = _page[ndx];
            }
            _page[_ctr] = value;
            if (_ctr < _page.GetUpperBound(0))
                _ctr++;
        }
    }

    public Page PreviousPage
    {
        get
        {
            if (_ctr == 0)
            {
                if (_page[0] is null)
                    return null;
                else
                    return _page[0];
            }
            else
            {
                _ctr--;
                return _page[_ctr + 1];
            }
        }
    }
}

public class Page
{
    public Uri URL;
    public string Title;
}

// The example displays the following output:
//    Unhandled Exception:
//       System.NullReferenceException: Object reference not set to an instance of an object.
//       at Chain1Example.Main()

open System

type Page() =
    [<DefaultValue>]
    val mutable public URL: Uri
    [<DefaultValue>]
    val mutable public Title: string

type Pages() =
    let pages = Array.zeroCreate<Page> 10
    let mutable i = 0

    member _.CurrentPage
        with get () = pages[i]
        and set (value) =
            // Move all the page objects down to accommodate the new one.
            if i > pages.GetUpperBound 0 then
                for ndx = 1 to pages.GetUpperBound 0 do
                    pages[ndx - 1] <- pages[ndx]

            pages[i] <- value
            if i < pages.GetUpperBound 0 then
                i <- i + 1

    member _.PreviousPage =
        if i = 0 then
            if box pages[0] = null then
                Unchecked.defaultof<Page>
            else
                pages[0]
        else
            i <- i - 1
            pages[i + 1]

let pages = Pages()
if String.IsNullOrEmpty pages.CurrentPage.Title |> not then
    let title = pages.CurrentPage.Title
    printfn $"Current title: '{title}'"


// The example displays the following output:
//    Unhandled Exception:
//       System.NullReferenceException: Object reference not set to an instance of an object.
//       at <StartupCode$fs>.main()

Module Example
   Public Sub Main()
      Dim pages As New Pages()
      Dim title As String = pages.CurrentPage.Title
   End Sub
End Module

Public Class Pages 
   Dim page(9) As Page
   Dim ctr As Integer = 0
   
   Public Property CurrentPage As Page
      Get
         Return page(ctr)
      End Get
      Set
         ' Move all the page objects down to accommodate the new one.
         If ctr > page.GetUpperBound(0) Then
            For ndx As Integer = 1 To page.GetUpperBound(0)
               page(ndx - 1) = page(ndx)
            Next
         End If    
         page(ctr) = value
         If ctr < page.GetUpperBound(0) Then ctr += 1 
      End Set
   End Property
   
   Public ReadOnly Property PreviousPage As Page
      Get
         If ctr = 0 Then 
            If page(0) Is Nothing Then
               Return Nothing
            Else
               Return page(0)
            End If   
         Else
            ctr -= 1
            Return page(ctr + 1)
         End If
      End Get
   End Property         
End Class

Public Class Page
   Public URL As Uri
   Public Title As String
End Class
' The example displays the following output:
'    Unhandled Exception: 
'       System.NullReferenceException: Object reference not set to an instance of an object.
'       at Example.Main()

The exception is thrown because pages.CurrentPage returns null if no page information is stored in the cache. This exception can be corrected by testing the value of the CurrentPage property before retrieving the current Page object's Title property, as the following example does:

var pages = new Pages();
Page current = pages.CurrentPage;
if (current != null)
{
    string title = current.Title;
    Console.WriteLine($"Current title: '{title}'");
}
else
{
    Console.WriteLine("There is no page information in the cache.");
}

// The example displays the following output:
//       There is no page information in the cache.

let pages = Pages()
let current = pages.CurrentPage
if box current <> null then
    let title = current.Title
    printfn $"Current title: '{title}'"
else
    printfn "There is no page information in the cache."
// The example displays the following output:
//       There is no page information in the cache.

Module Example
   Public Sub Main()
      Dim pages As New Pages()
      Dim current As Page = pages.CurrentPage
      If current IsNot Nothing Then 
         Dim title As String = current.Title
         Console.WriteLine("Current title: '{0}'", title)
      Else
         Console.WriteLine("There is no page information in the cache.")
      End If   
   End Sub
End Module
' The example displays the following output:
'       There is no page information in the cache.

You're enumerating the elements of an array that contains reference types, and your attempt to process one of the elements throws a NullReferenceException exception.

The following example defines a string array. A for statement enumerates the elements in the array and calls each string's Trim method before displaying the string.

string[] values = [ "one", null, "two" ];
for (int ctr = 0; ctr <= values.GetUpperBound(0); ctr++)
    Console.Write("{0}{1}", values[ctr].Trim(),
                  ctr == values.GetUpperBound(0) ? "" : ", ");
Console.WriteLine();

// The example displays the following output:
//    Unhandled Exception:
//       System.NullReferenceException: Object reference not set to an instance of an object.

open System

let values = [| "one"; null; "two" |]
for i = 0 to values.GetUpperBound 0 do
    printfn $"""{values[i].Trim()}{if i = values.GetUpperBound 0 then "" else ", "}"""
printfn ""
// The example displays the following output:
//    Unhandled Exception:
//       System.NullReferenceException: Object reference not set to an instance of an object.
//       at <StartupCode$fs>.main()

Module Example
   Public Sub Main()
      Dim values() As String = { "one", Nothing, "two" }
      For ctr As Integer = 0 To values.GetUpperBound(0)
         Console.Write("{0}{1}", values(ctr).Trim(), 
                       If(ctr = values.GetUpperBound(0), "", ", ")) 
      Next
      Console.WriteLine()
   End Sub
End Module
' The example displays the following output:
'    Unhandled Exception: System.NullReferenceException: 
'       Object reference not set to an instance of an object.
'       at Example.Main()

This exception occurs if you assume that each element of the array must contain a non-null value, and the value of the array element is in fact null. The exception can be eliminated by testing whether the element is null before performing any operation on that element, as the following example shows.

string[] values = [ "one", null, "two" ];
for (int ctr = 0; ctr <= values.GetUpperBound(0); ctr++)
    Console.Write("{0}{1}",
                  values[ctr] != null ? values[ctr].Trim() : "",
                  ctr == values.GetUpperBound(0) ? "" : ", ");
Console.WriteLine();

// The example displays the following output:
//       one, , two

open System

let values = [| "one"; null; "two" |]
for i = 0 to values.GetUpperBound 0 do
    printf $"""{if values[i] <> null then values[i].Trim() else ""}{if i = values.GetUpperBound 0 then "" else ", "}"""
Console.WriteLine()
// The example displays the following output:
//       one, , two

Module Example
   Public Sub Main()
      Dim values() As String = { "one", Nothing, "two" }
      For ctr As Integer = 0 To values.GetUpperBound(0)
         Console.Write("{0}{1}", 
                       If(values(ctr) IsNot Nothing, values(ctr).Trim(), ""), 
                       If(ctr = values.GetUpperBound(0), "", ", ")) 
      Next
      Console.WriteLine()
   End Sub
End Module
' The example displays the following output:
'       one, , two

A method when it accesses a member of one of its arguments, but that argument is null. The PopulateNames method in the following example throws the exception at the line names.Add(arrName);.

using System.Collections.Generic;

public class NRE2Example
{
    public static void Main()
    {
        List<string> names = GetData();
        PopulateNames(names);
    }

    private static void PopulateNames(List<string> names)
    {
        string[] arrNames = [ "Dakota", "Samuel", "Nikita",
                            "Koani", "Saya", "Yiska", "Yumaevsky" ];
        foreach (string arrName in arrNames)
            names.Add(arrName);
    }

    private static List<string> GetData()
    {
        return null;
    }
}

// The example displays output like the following:
//    Unhandled Exception: System.NullReferenceException: Object reference
//    not set to an instance of an object.
//       at NRE2Example.PopulateNames(List`1 names)
//       at NRE2Example.Main()

let populateNames (names: ResizeArray<string>) =
    let arrNames =
        [ "Dakota"; "Samuel"; "Nikita"
          "Koani"; "Saya"; "Yiska"; "Yumaevsky" ]
    for arrName in arrNames do
        names.Add arrName

let getData () : ResizeArray<string> =
    null

let names = getData ()
populateNames names

// The example displays output like the following:
//    Unhandled Exception: System.NullReferenceException: Object reference
//    not set to an instance of an object.
//       at Example.PopulateNames(List`1 names)
//       at <StartupCode$fs>.main()

Imports System.Collections.Generic

Module Example
   Public Sub Main()
      Dim names As List(Of String) = GetData()
      PopulateNames(names)
   End Sub
   
   Private Sub PopulateNames(names As List(Of String))
      Dim arrNames() As String = { "Dakota", "Samuel", "Nikita",
                                   "Koani", "Saya", "Yiska", "Yumaevsky" }
      For Each arrName In arrNames
         names.Add(arrName)
      Next
   End Sub
   
   Private Function GetData() As List(Of String)
      Return Nothing   
   End Function
End Module
' The example displays output like the following:
'    Unhandled Exception: System.NullReferenceException: Object reference 
'    not set to an instance of an object.
'       at Example.PopulateNames(List`1 names)
'       at Example.Main()

To address this issue, make sure that the argument passed to the method is not null, or handle the thrown exception in a try…catch…finally block. For more information, see Exceptions.

A list is created without knowing the type, and the list was not initialized. The GetList method in the following example throws the exception at the line emptyList.Add(value).

using System;
using System.Collections.Generic;
using System.Collections;
using System.Runtime.Serialization;

public class NullReferenceExample
{
    public static void Main()
    {
        var listType = GetListType();
        _ = GetList(listType);
    }

    private static Type GetListType()
    {
        return typeof(List<int>);
    }

    private static IList GetList(Type type)
    {
        var emptyList = (IList)FormatterServices.GetUninitializedObject(type); // Does not call list constructor
        var value = 1;
        emptyList.Add(value);
        return emptyList;
    }
}
// The example displays output like the following:
//    Unhandled Exception: System.NullReferenceException: 'Object reference
//    not set to an instance of an object.'
//    at System.Collections.Generic.List`1.System.Collections.IList.Add(Object item)
//    at NullReferenceExample.GetList(Type type): line 24

To address this issue, make sure that the list is initialized (one way to do this is to call Activator.CreateInstance instead of FormatterServices.GetUninitializedObject), or handle the thrown exception in a try…catch…finally block. For more information, see Exceptions.