How do I fix a NullReferenceException in C#?

#Find it

The stack trace names the file and line. On that line, one of the things before a . is null. In .NET 6+ the message usually names it directly:

Object reference not set to an instance of an object.
   at Calculator.Calculate() in Form1.cs:line 42

#The usual suspects

An uninitialised field. Reference type fields default to null:

private List<int> _scores;          // null until assigned
private List<int> _scores = new();  // fix

A lookup that found nothing:

var user = users.FirstOrDefault(u => u.Id == id);   // null when no match
Console.WriteLine(user.Name);                       // boom

A control referenced before the designer created it — in WinForms, code in the constructor that runs before InitializeComponent().

A missing config value. Configuration["Key"] returns null for an absent key.

#Guard it

if (user is not null)
    Console.WriteLine(user.Name);

Console.WriteLine(user?.Name ?? "Unknown");

#Turn on nullable reference types

In your .csproj:

<Nullable>enable</Nullable>

Now the compiler distinguishes string from string? and warns when you dereference something that might be null. This converts a runtime crash into a compile-time warning, which is the single highest-value setting in a modern C# project.

#Fail loudly at the boundary

ArgumentNullException.ThrowIfNull(user);

Better to throw at the entry point, naming the parameter, than to let null travel three layers deeper and crash somewhere unrelated.