Question

What are all the difference between an abstract class, and a class with only protected constructor(s)? They seem to be pretty similar to me, in that you can’t instantiate either one.

EDIT:

How would you create an instance in a derived class, with a base class with a protected constructor? For instance:

public class ProtectedConstructor
{
    protected ProtectedConstructor()
    {

    }

    public static ProtectedConstructor GetInstance()
    {
        return new ProtectedConstructor(); // this is fine
    }
}

public class DerivedClass : ProtectedConstructor
{

    public void createInstance()
    {
        ProtectedConstructor p = new ProtectedConstructor(); // doesn't compile
    }

    public static ProtectedConstructor getInstance()
    {
        return new ProtectedConstructor(); // doesn't compile

    }
}

Answer 1

You can instantiate a class with protected constructors from within the class itself – in a static constructor or static method. This can be used to implement a singleton, or a factory-type thing.

An abstract class cannot be instantiated at all – the intent is that one or more child classes will complete the implementation, and those classes will get instantiated

Edit:

if you call ProtectedConstructor.GetInstance(); instead of new ProtectedConstructor();, it works. Maybe protected constructors can’t be called this way? But protected methods certainly can.

Here is an interesting article on the topic.