What does null! statement mean?

Question

I've recently seen the following code:

public class Person
{
    //line 1
    public string FirstName { get; }
    //line 2
    public string LastName { get; } = null!;
    //assign null is possible
    public string? MiddleName {get; } = null;

    public Person(string firstName, string lastName, string middleName)
    {
        FirstName = firstName;
        LastName = lastName;
        MiddleName = middleName;
    }

    public Person(string firstName, string lastName)
    {
        FirstName = firstName;
        LastName = lastName;
        MiddleName = null;
    }
}

Basically I try to dig into new c# 8 features. One of them is NullableReferenceTypes. Actually there're a lot of articles and information about it already. E.g. this article is quite good. But I din't find any information about this new statement null! Can someone provide to me explanation for it ? Why I need to use this ? And what is difference between line1 and line2 ?

Answer 1

What is the ! operator when used on a type?

The ! operator, when used on a type, is called the Null Forgiving Operator [docs]. It was introduced in C# 8.0

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Technical Explanation

Typical usage

Assuming this definition:

class Person
{
  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.

Inner workings

Using 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.

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

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

Since C# 8.0 all reference types are Non-nullable by default.

The "nullability" can be modified by these 2 new type-operators:

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

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

? Operator usage.

1. 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.

2. Non-Nullable string y;

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

! Operator usage.

string x;
string? y = null;

1. x = y

   • Illegal! - Warning: "y" may be null
   • The left side of the assignment is non-nullable but the right side is nullable.

2. x = y!

   • Legal!
   • The right and left side of the assignment is non-nullable.
   • Works since y! Applies the ! operator to y which makes it non-nullable.

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

This is an Anti-Pattern.

You should try to avoid using the ! Null-Forgiving-Operator.

There are valid use-cases ( outlined in detail below ) like unit-tests where this operator is appropriate to use. In 99% of the cases though, you are better off with an alternative solution. Please do not slap dozens of !'s in your code, just to silence the warnings. Think if your cause really warrants the use.

Use - but with care. If there is no concrete purpose / use-case prefer not to use it.

It negates the effects of null-safety you get guaranteed by the compiler.

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?

• 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.

---

Answering your question specifically.

So what does null! mean?

It tells the compiler that null is not a null 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.

Picking apart what is happening.

public string LastName { get; } = null!;

This line defines a non-nullable class property named LastNameof type string. Since it is non-nullable you can technically not assign null to it - obviously.

But you do just that by using the ! operator. Because null! is not null - as far as the compiler is concerned about null-safety.

Answer 2

When the "nullable reference types" feature is turned on, the compiler tracks which values in your code it thinks may be null or not. There are times where the compiler could have insufficient knowledge.

For example, you may be using a delayed initialization pattern, where the constructor doesn't initialize all the fields with actual (non-null) values, but you always call an initialization method which guarantees the fields are non-null. In such case, you face a trade-off:

• if you mark the field as nullable, the compiler is happy, but you have to un-necessarily check for null when you use the field,
• if you leave the field as non-nullable, the compiler will complain that it is not initialized by the constructors (you can suppress that with null!), then the field can be used without null check.

Note that by using the ! suppression operator, you are taking on some risk. Imagine that you are not actually initializing all the fields as consistently as you thought. Then the use of null! to initialize a field covers up the fact that a null is slipping in. Some unsuspecting code can receive a null and therefore fail.

More generally, you may have some domain knowledge: "if I checked a certain method, then I know that some value isn't null":

if (CheckEverythingIsReady())
{
   // you know that `field` is non-null, but the compiler doesn't. The suppression can help
   UseNonNullValueFromField(this.field!);
}

Again, you must be confident of your code's invariant to do this ("I know better").