Add type parameter constraint to prevent abstract classes

Question

Is it possible to restrict a type parameter to concrete implementations of an abstract class, if those implementations don\'t have default constructors?

For example, if

Answer 1

You can add the new() constraint, which will require that the class not be abstract and have a default constructor.

Answer 2

public class ZooPen<T> where T : Animal
{
    public ZooPen()
    {
        if (typeof(T).IsAbstract)
            throw new ArgumentException(typeof(T).Name + " must be non abstract class");
    }
}

Answer 3

Here is one way to accomplish what you're asking for.

abstract class Animal
{
    readonly string Name;
    Animal() { }
    public Animal(string name) { Name = name; }
}

abstract class Animal<T> : Animal where T : Animal<T>
{
    public Animal(string name) : base(name) { }
}

class Penguin : Animal<Penguin>
{
    public Penguin() : base("Penguin") { }
}

class Chimpanzee : Animal<Chimpanzee>
{
    public Chimpanzee() : base("Chimpanzee") { }
}

class ZooPen<T> where T : Animal<T>
{
}

class Example
{
    void Usage()
    {
        var penguins = new ZooPen<Penguin>();
        var chimps = new ZooPen<Chimpanzee>();
        //this line will not compile
        //var animals = new ZooPen<Animal>();
    }
}

Anyone maintaining this code will probably be a bit confused, but it does exactly what you want.