If you need to represent unknown data in a column, you make it nullable. If you will always have data in the column, it's better to make it not nullable, as

1. Dealing with nulls can be annoying and counterintuitive
2. It saves a bit of space
3. On some database systems, null values are not indexed.

In PHP there is a function named isset - I think there's no shorter version than this.

TL;DR

The key to understanding what null! means is understanding the ! operator. You may have used it before as the "not" operator. However, since C# 8.0 and its new "nullable-reference-types" feature, the operator got a second meaning. It can be used on a type to control Nullability, it is then called the "Null Forgiving Operator".

Basically, null! applies the ! operator to the value null. This overrides the nullability of the value null to non-nullable, telling the compiler that null is a "non-null" type.

Typical usage

Assuming this definition:

class Person
{
    // Not every person has a middle name. We express "no middle name" as "null"
    public string? MiddleName;
}

The usage would be:

void LogPerson(Person person)
{
    Console.WriteLine(person.MiddleName.Length);  // WARNING: may be null
    Console.WriteLine(person.MiddleName!.Length); // No warning
}

This operator basically turns off the compiler null checks for this usage.

Technical Explanation

The groundwork that you will need to understand what null! means.

Null Safety

C# 8.0 tries to help you manage your null-values. Instead of allowing you to assign null to everything by default, they have flipped things around and now require you to explicitly mark everything you want to be able to hold a null value.

This is a super useful feature, it allows you to avoid NullReferenceExceptions by forcing you to make a decision and enforcing it.

How it works

There are 2 states a variable can be in - when talking about null-safety.

• Nullable - Can be null.
• Non-Nullable - Cannot be null.

Since C# 8.0 all reference types are non-nullable by default. Value types have been non-nullable since C# 2.0!

The "nullability" can be modified by 2 new (type-level) operators:

• ! = from Nullable to Non-Nullable
• ? = from Non-Nullable to Nullable

These operators are counterparts to one another. The Compiler uses the information that you define with these operators to ensure null-safety.

Examples

? Operator usage.

This operator tells the compiler that a variable can hold a null value. It is used when defining variables.

• Nullable string? x;

  • x is a reference type - So by default non-nullable.
  • We apply the ? operator - which makes it nullable.
  • x = null Works fine.

• Non-Nullable string y;

  • y is a reference type - So by default non-nullable.
  • y = null Generates a warning since you assign a null value to something that is not supposed to be null.

Nice to know: Using object? is basically just syntactic sugar for System.Nullable<object>

! Operator usage.

This operator tells the compiler that something that could be null, is safe to be accessed. You express the intent to "not care" about null safety in this instance. It is used when accessing variables.

string x;
string? y;

• x = y
  • Illegal! Warning: "y" may be null
  • The left side of the assignment is non-nullable but the right side is nullable.
  • So it does not work, since it is semantically incorrect
• x = y!
  • Legal!
  • y is a reference type with the ? type modifier applied so it is nullable if not proven otherwise.
  • We apply ! to y which overrides its nullability settings to make it non-nullable
  • The right and left side of the assignment are non-nullable. Which is semantically correct.

WARNING The ! operator only turns off the compiler-checks at a type-system level - At runtime, the value may still be null.

Use carefully!

You should try to avoid using the Null-Forgiving-Operator, usage may be the symptom of a design flaw in your system since it negates the effects of null-safety you get guaranteed by the compiler.

Reasoning

Using the ! operator will create very hard to find bugs. If you have a property that is marked non-nullable, you will assume you can use it safely. But at runtime, you suddenly run into a NullReferenceException and scratch your head. Since a value actually became null after bypassing the compiler-checks with !.

Why does this operator exist then?

There are valid use-cases (outlined in detail below) where usage is appropriate. However, in 99% of the cases, you are better off with an alternative solution. Please do not slap dozens of !'s in your code, just to silence the warnings.

• In some (edge) cases, the compiler is not able to detect that a nullable value is actually non-nullable.
• Easier legacy code-base migration.
• In some cases, you just don't care if something becomes null.
• When working with Unit-tests you may want to check the behavior of code when a null comes through.

Ok!? But what does null! mean?

It tells the compiler that null is not a nullable value. Sounds weird, doesn't it?

It is the same as y! from the example above. It only looks weird since you apply the operator to the null literal. But the concept is the same. In this case, the null literal is the same as any other expression/type/value/variable.

The null literal type is the only type that is nullable by default! But as we learned, the nullability of any type can be overridden with ! to non-nullable.

The type system does not care about the actual/runtime value of a variable. Only its compile-time type and in your example the variable you want to assign to LastName (null!) is non-nullable, which is valid as far as the type-system is concerned.

Consider this (invalid) piece of code.

object? null;
LastName = null!;

It's convenient for the way SQL is typically used. Consider this statements:

SELECT people.name, cars.model FROM people
INNER JOIN cars
    ON people.car_licenceplate = cars.licenceplate

If null = null, then this would return all pairs of people with no license plate with all unregistered cars in the database, a usually undesirable result.

It's particularly convenient that, even if you use any null value even in a more complex expression, you won't get a value back, even if other values may also happen to be null. In other languages you'd need to null check everything in advance to get that behavior, having it by default is very convenient for the type of things SQL is typically used for.

null in SQL is exempt from a lot of other rules too. For example they are excluded from unique constraints. All indicating it represents more the absence of a value rather than a special value.

Some other languages do also have a ThreeValueBoolean or a similar type that behaves more like a SQL null, though only for booleans. Also most every language has similar non self-equality for NaN. It's not a concept unique to SQL.

NULL is used to represent "no value" and allow that to be distinguished between 1 and 0, true or false, or an empty string versus a string with content. It's similar to nil in Ruby, null in JavaScript or NULL in PHP.

If you define a column as NOT NULL then it won't allow an INSERT without a value for that column being specified. If you have a DEFAULT then this will be applied automatically. If you use an ORM it may fill it in for you with a safe, minimal default.

Columns that can be NULL require an almost insignificant amount of additional storage per row, one bit, to hold the NULL or NOT NULL flag.

Remember that NULL in MySQL is unusual in that it is not greater than, less than, or equal to any other value. This is why IS NULL and IS NOT NULL are required for logical comparisons.

Null: The Billion Dollar Mistake. Tony Hoare:

I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years. In recent years, a number of program analysers like PREfix and PREfast in Microsoft have been used to check references, and give warnings if there is a risk they may be non-null. More recent programming languages like Spec# have introduced declarations for non-null references. This is the solution, which I rejected in 1965.

Actually, you can use a literal 0 anyplace you would use NULL.

Section 6.3.2.3p3 of the C standard states:

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

And section 7.19p3 states:

The macros are:

NULL

which expands to an implementation-defined null pointer constant

So 0 qualifies as a null pointer constant, as does (void *)0 and NULL. The use of NULL is preferred however as it makes it more evident to the reader that a null pointer is being used and not the integer value 0.