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 didn't find any information about this new statement null!
Can someone provide me an explanation for it?
Why do I need to use this?
And what is the difference between line1 and line2?

Accepted Answer

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"

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

Must Read

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.

What Does Null! Statement Mean?

4 Answers

Typical usage

Technical Explanation

Must Read

Null Safety

Alexander Ross