null, nullable types and null-safe operators

What is null?

A reference-type variable, such as a string or an array, holds an arrow to an object. null means the arrow points at nothing. It is useful to say "not known yet" or "not found". The danger is using it: asking for the length of nothing makes no sense, so the program crashes.

string? middleName = null;
Console.WriteLine(middleName.Length);   // NullReferenceException at run time

Value types cannot be null... unless you add ?

Value types like int, double, decimal, bool and DateTime always have a value; an int is 0 by default, never null. Sometimes you need "no value", for example a student who has not taken the exam yet. Add ? to the type to make a nullable value type.

int? marks = null;              // not taken yet
Console.WriteLine(marks.HasValue);   // False
marks = 72;
Console.WriteLine(marks.Value);      // 72
int sure = marks ?? 0;               // 72, or 0 if marks were null

Nullable reference types: help from the compiler

Since C# 8 you can switch on nullable reference types, and new projects have it on by default (<Nullable>enable</Nullable> in the .csproj). Then string means "should never be null" and string? means "may be null". The compiler tracks this and warns you when you might use a null. It does not change how the program runs; it is an early-warning system.

string name = null;          // warning CS8600: converting null to non-nullable type
string? nick = GetNickname();
Console.WriteLine(nick.Length);   // warning CS8602: dereference of a possibly null reference
if (nick != null)
    Console.WriteLine(nick.Length);   // no warning: the compiler knows it is not null here

static string? GetNickname() => null;

The null-safe operators

string? input = Console.ReadLine();
int? len = input?.Length;                  // null if input is null
string display = input ?? "(nothing typed)";
string clean = input?.Trim() ?? "";        // combine them

string? city = null;
city ??= "Unknown";
Console.WriteLine(city);                   // Unknown

Checking for null

string? email = GetEmail();
if (email is null) Console.WriteLine("No email");
if (email is not null) Console.WriteLine(email.ToLower());
if (string.IsNullOrWhiteSpace(email)) Console.WriteLine("Missing or blank");

static string? GetEmail() => "  ";

Common mistakes and fixes

• Ignoring CS8602 warnings: each one is a possible crash. Add a null check, use ?. and ??, or make sure the value is set.
• Reading .Value on an empty int?: throws. Use ?? or GetValueOrDefault() instead.
• Returning null for "no items": return an empty array or list instead, so callers can loop without checks.
• Using ! everywhere to quieten the compiler: it hides real bugs. Fix the cause.