What Does the C# `null!` Operator Do and When Should You Use It?

The meaning of "null!" statement in C#

Understand the "!" operator

C# 8.0 introduces nullable reference types, allowing fine-grained control over the nullability of variables and reference types. When applied to a type, the "!" operator (also known as the "null ignore operator") changes the nullability status of the type from nullable to non-nullable.

Usage of "null!"

The "null!" statement applies the "!" operator to a null literal, indicating that null values ​​should be treated as non-nullable. This overrides the default nullability of the null literal and forces the compiler to assume that the value is not null.

Typical usage

Consider the following code:

public class Person
{
    public string? MiddleName; // 可空
    public string LastName = null!; // 不可空
}

In this example, the "MiddleName" property is nullable, meaning it can hold null values. However, the "LastName" property is marked as non-nullable using "null!". This means that the compiler will enforce that the "LastName" property will never hold null, even if it was initially assigned a value of null.

Technical explanation

Starting in C# 8.0, all reference types are non-nullable by default. The "!" operator allows you to override this setting, making nullable types non-nullable. In contrast, the "?" operator makes a non-nullable type nullable.

Use with caution

The "null!" operator should be used with caution. It overrides the compiler's null safety checks, which may mask potential null reference exceptions at runtime. You should only use it if you are sure that a nullable value will never be null.

Alternatives

In most cases it is better to use an alternative to avoid using the "null!" operator. For example, you can use nullable types and the null coalescing operator, or explicitly check for null values ​​before accessing them.

When to use "null!"

The "null!" operator is useful in certain situations:

• When the compiler cannot detect that a nullable value is actually non-nullable.
• When migrating old code that relies on nullable types.
• In some cases you want to ignore nullability checks and handle null values ​​explicitly.
• Simulate null scenarios in test cases without causing compile-time errors.

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