类型定义和类型引用之间有什么区别？

Question

I am reading the book CLR via C# by Jeffrey Richter .

In Chapter 23(23.3.2) , it says "A System.Type object represents a type reference(as opposed to a type definition)."

What do the type reference and type definition mean here?

Answer 1

The author is contrasting System.Type with System.TypeInfo . From the documentation :

A TypeInfo object represents the type definition itself , whereas a Type object represents a reference to the type definition .

The System.TypeInfo for a class can only be instantiated if the assembly that implements the class (the type definition) is loaded or loadable. To instantiate a System.Type , the only requirement is that the current program contains a reference to the implementing assembly.

See also What's the difference between Type and TypeInfo?

Answer 2

A type definition happens when you define a new type through a class, a struct, an interface or an enum.

A type reference is a collection of information about a type, eg its name or its members.

The System.Type class is able to represent a type, but not define a type. When you call typeof(TypeName) or object.GetType() , you will deal with representations of types with objects of System.Type .

You can not directly instantiate an object of a type by just using a System.Type object. It only describes the type, but not is the type. That's why it is called a reference.

Answer 3

A type definition implies the actual implementation of the type, such as, for example if it is a class, it means the definition of the class (with all its properties and fields).

A type reference , simply means that System.Type contains the details of a particular type, that is, it is the meta-data about the Type.