Why must the new() constraint require a public constructor?

问题

Disclaimer: Theoretical Question

The new constraint specifies that any type argument in a generic class declaration must have a public parameterless constructor.

Source: http://msdn.microsoft.com/en-us/library/sd2w2ew5(v=vs.80).aspx

What if I wanted my generic class to have a protected parameterless constructor instead? For instance, if I want to write a Singleton class which I "attach" to other classes to make them Singletons, I don't want the derived classes to be instantiable - everything should go through the .Instance property.

internal class Singleton<T> where T : new()
{
    public static T Instance { get; private set; }

    static Singleton()
    {
        Singleton<T>.Instance = new T();
    }
}

internal class OnlyOneOfMe : Singleton<OnlyOneOfMe>
{
    protected OnlyOneOfMe()
    {
    }
}

This way, Singleton<T> is able to create the only instance of the OnlyOneOfMe class, but nothing else can (unless it is a subclass).

"What if a generic parent class could access the generic type's protected members?"

回答1:

Because that is the definition of the constraint. It's a bit like asking why does T : class require that T be a reference type. It's true by definition.

Additionally, if it weren't a public constructor, what would be the point of the constraint? The class receiving the type parameter T wouldn't be able to call the constructor if it weren't public.

回答2:

You can call a protected constructor using reflection. However this should raise warning signs that you are doing something you are not supposed to. In most cases, you should be able to avoid a singleton and use dependency injection instead. If that doesn't work either, you can use something like the ambient context pattern (see my answer here).

回答3:

If the constructor were protected, Singleton wouldn't be able to call it.

And I'd avoid implementing the singleton pattern like that anyway, even if I could. It's messy - what if you wanted a singleton class that inherits from an abstract one?

回答4:

.NET would not know that you don't want to accept

class OnlyOneOfMe : Singleton<Other>

as a valid class. Since it is actually valid it will try to make the class and needs a public Other constructor.

来源：https://stackoverflow.com/questions/5984152/why-must-the-new-constraint-require-a-public-constructor