A reference type stores a reference to its data rather than containing the data directly. Classes and strings are reference types; structs and integers are value types. The distinction affects copying, method parameters, and nullability. section 2.4 section 5.2.2
Reference types matter in several practical situations:
- References resemble pointers conceptually, but C# manages them differently and automatically boxes value types when an object is required. section 2.4
- Reference types can be null. Nullable reference type annotations such as string? explicitly allow null and help the compiler identify unsafe usage. section 5.2.2
- A nonnullable property can be initialized with a default value or required through a constructor, preventing objects from being created without essential data. section 5.2.2
Requiring a nonnull reference
A Book class can require its Title reference through a constructor: public Book(string title) { Title = title; }. Creating new Book(null) then produces a compiler warning because Title is nonnullable, while new Book("Blah") satisfies the declared contract. section 5.2.2
Listing 5.7 Book constructor
public class Book
{
public int Id { get; set; } #1
public string Title { get; set; } #2
public Book(string title) #3
{
Title = title; #4
}
}
Inspect the Book constructor and its Title property to see how a reference-type invariant is enforced during object creation. section 5.2.2
