What Does the `!` (Null-Forgiving) Operator Mean in C# 8.0?

null! Meaning of operator

In C# 8.0 and its nullable reference types feature, the ! operator has a new purpose, called the "null-tolerant operator". When applied to a type, this operator overrides the nullability of the value, making it non-nullable, meaning that although nullable by default, null is treated as a "non-null" type.

Practical example

Consider a class where LastName properties are nullable and MiddleName properties are non-nullable:

public class Person
{
    public string? LastName { get; set; }
    public string MiddleName { get; set; } = null!;
}

Difference between the first and second lines

• First line: The LastName property is nullable, allowing it to hold a null value, represented by the ? operator.
• Second line: MiddleName attributes are non-nullable, represented by !. This means it cannot hold null values. However, the null! expression explicitly overrides this non-nullability and treats null as a non-null value.

Technical explanation

C# 8.0 introduced "null safety", where all reference types are non-nullable by default. To represent a nullable type, the ? operator must be used. Conversely, the ! operator can be applied to nullable types to indicate non-nullability.

Notes

• Avoid overuse: The null-tolerant operator should be used with caution, as it defeats the purpose of null-safety.
• Runtime Exceptions: The ! operator only disables compiler-level checks; if the value is null, runtime exceptions may still occur.
• Valid use cases: Despite its limitations, the null-tolerant operator is used when the compiler cannot infer the nullability of a value, when migrating legacy code, or when testing the behavior of code when handling null Might be helpful.

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