Question

I’m reading book “the C# programming Language”, 4th Edition, by Anders Hejlsberg etc.

There are several definitions that are a bit twisting:

unbound generic type: A generic type declaration, by itself, denotes an unbound generic type …

constructed type: A type that includes at least one type argument is called a constructed type.

open type: An open type is a type that involves type parameters.

closed type: A closed type is a type that is not an open type.

unbound type: refers to a nongeneric type or an unbound generic type.

bound type: refers to a nongeneric type or a constructed type.
[annotate] ERIC LIPPERT: Yes, nongeneric types are considered to be both bound and unbound.

Question 1, is below what I listed correct?

int                     //non-generic, closed, unbound & bound, 
class A<T, U, V>        //generic,     open,   unbound, 
class A<int, U, V>      //generic,     open,   bound, constructed 
class A<int, int, V>    //generic,     open,   bound, constructed
class A<int, int, int>  //generic,     closed, bound, constructed

Question 2, The books says
“An unbound type refers to the entity declared by a type declaration.
An unbound generic type is not itself a type, and it cannot be used as the type of a variable, argument, or return value, or as a base type.
The only construct in which an unbound generic type can be referenced is the typeof expression (§7.6.11).”
Fine, but below is a small testing program that can compile:

public class A<W, X> { }

// Q2.1: how come unbounded generic type A<W,X> can be used as a base type?
public class B<W, X> : A<W, X> { } 

public class C<T,U,V>
{
    // Q2.2: how come unbounded generic type Dictionary<T, U> can be used as a return value?
    public Dictionary<T,U> ReturnDictionary() { return new Dictionary<T, U>(); }

    // Q2.3: how come unbounded generic type A<T, U> can be used as a return value?
    public A<T, U> ReturnB() { return new A<T, U>(); }
}

Answer 1

These are examples of unbound generic types:

• List<>
• Dictionary<,>

They can be used with typeof, i.e., the following are valid expressions:

• typeof(List<>)
• typeof(Dictionary<,>)

That should answer your question 2. With respect to question 1, note that type arguments can be constructed types or type parameters. Thus, your list should be updated as follows:

public class MyClass<T, U> {  // declares the type parameters T and U

    // all of these are
    // - generic,
    // - constructed (since two type arguments are supplied), and
    // - bound (since they are constructed):

    private Dictionary<T, U> var1;     // open (since T and U are type parameters)
    private Dictionary<T, int> var2;   // open (since T is a type parameter)
    private Dictionary<int, int> var3; // closed
}