"Null-Correctness Assisting Attributes: AllowNull"

// AllowNull indicates that a null value is permitted when setting/passing a value.

// Although the type of this property is string (and not string?) we will allow people to 'set' a null value,
// which will actually be replaced with a non-null fallback value. Hence we [AllowNull].

[AllowNull] 
public string NonNullableProperty {
	get => _nonNullableField;
	set => _nonNullableField = value ?? "fallback";
}


Code snippet taken from "Two Decades of C#: A Reference - C# 8".