What does a public constructor on an internal clas

问题:

This question already has an answer here:

• What's the difference between a public constructor in an internal class and an internal constructor? 5 answers

I've seen some C# code that declares a class with an internal modifier, with a public constructor:

internal class SomeClass
{
    public SomeClass()
    {
    }
}

What is the point of having a public constructor, if the visibility of the entire class is internal, thus it can be seen only inside the defining assembly?

Also, does this make any sense in case SomeClass is a nested class?

回答1:

The internal class scope overrides the public MyClass() constructor scope, making the constructor internal.

Using public on the constructor makes it easier to update the class to public later, but confuses intent. I don't do it.

Edit 3: I missed part of your question. It is still fine to do that if your class is nested. The nesting can't make any difference, even if it is nested in a private class in a public class in a ... (see C# language specification - 3.5.2 Accessibility domains).

EDIT: And, if i recall, if the ctor is internal, it can't be used as a generic type where there is a constraint requiring where T : new(), this would require a public constructor (ref. C# language specification (version 4.0) - 4.4.3 Bound and unbound types).

Edit 2: Code sample demonstrating the above

class Program
{
    internal class InternalClass {
        internal InternalClass() { }
    }
    internal class InternalClassPublicCtor {
        public InternalClassPublicCtor() { }        
    }
    internal class GenericClass<T>
        where T : new() {}

    static void Main(string[] args) {
        GenericClass<InternalClass> doesNotCompile = new GenericClass<InternalClass>();
        GenericClass<InternalClassPublicCtor> doesCompile = new GenericClass<InternalClassPublicCtor>();
    }
}

回答2:

From MSDN - Access Modifiers (C# Programming Guide):

Normally, the accessibility of a member is not greater than the accessibility of the type that contains it. However, a public member of an internal class might be accessible from outside the assembly if the member implements interface methods or overrides virtual methods that are defined in a public base class.

So if, for example, you have an internal implementation of a public interface, you can still expose certain members as public.

Additionally, suppose you suddenly want your internal class to be public. It's a lot easier simply to change the access modifier on the class than all of the members.

回答3:

The internal keyword is an access modifier for types and type members. Internal members are accessible only within files in the same assembly.

Example: Microsoft internal modifier