Why use a public method in an internal class?

问题:

There is a lot of code in one of our projects that looks like this:

internal static class Extensions
{
    public static string AddFoo(this string s)
    {
        if (!string.IsNullOrEmpty(s)) return s + "Foo";
        return "Foo";
    }
}

Is there any explicit reason to do this other than "it is easier to make the type public later?"

I suspect it only matters in very strange edge cases (reflection in Silverlight) or not at all.

回答1:

UPDATE: This question was the subject of my blog in September 2014. Thanks for the great question!

There is considerable debate on this question even within the compiler team itself.

First off, it's wise to understand the rules. A public member of a class or struct is a member that is accessible to anything that can access the containing type. So a public member of an internal class is effectively internal.

So now, given an internal class, should its members that you wish to access in the assembly be marked as public or internal?

My opinion is: mark such members as public.

I use "public" to mean "this member is not an implementation detail". A protected member is an implementation detail; there is something about it that is going to be needed to make a derived class work. An internal member is an implementation detail; something else internal to this assembly needs the member in order to work correctly. A public member says "this member represents the key, documented functionality provided by this object."

Basically, my attitude is: suppose I decided to make this internal class into a public class. In order to do that, I want to change exactly one thing: the accessibility of the class. If turning an internal class into a public class means that I have to also turn an internal member into a public member, then that member was part of the public surface area of the class, and it should have been public in the first place.

Other people disagree. There is a contingent that says that they want to be able to glance at the declaration of a member and immediately know whether it is going to be called only from internal code.

Unfortunately, that doesn't always work out nicely; for example, an internal class that implements an internal interface still has to have the implementing members marked as public, because they are part of the public surface of the class.

回答2:

If the class is internal, it doesn't matter from an accessibility standpoint whether you mark a method internal or public. However it is still good to use the type you would use if the class were public.

While some have said that this eases transitions from internal to public. It also serves as part of the description of the method. Internal methods typically are considered unsafe for unfettered access, while public methods are considered to be (mostly) free game.

By using internal or public as you would in a public class, you ensure that you are communicating what style of access is expected, while also easing the work required to make the class public in the future.

回答3:

I often mark my methods in internal classes public instead of internal as a) it doesn't really matter and b) I use internal to indicate that the method is internal on purpose (there is some reason why I don't want to expose this method in a public class. Therefore, if I have an internal method I really have to understand the reason why it's internal before changing it to public whereas if I am dealing with a public method in an internal class I really have to think about why the class is internal as opposed to why each method is internal.

回答4:

I suspect that "it is easier to make the type public later?" is it.

The scoping rules mean that the method will only be visible as internal - so it really doesn't matter whether the methods are marked public or internal.

One possibility that comes to mind is that the class was public and was later changed to internal and the developer didn't bother to change all the method accessibility modifiers.

回答5:

In some cases, it may also be that the internal type implements a public interface which would mean that any methods defined on that interface would still need to be declared as public.

回答6:

It's the same, the public method will be really marked as internal since it's inside a internal class, but it has an advantaje(as you guested), if you want to mark the class as public, you have to change fewer code.

回答7:

internal says the member can only be accessed from within the same assembly. Other classes in that assembly can access the internal public member, but would not be able to access a private or protected member, internal or not.

回答8:

I actually struggled with this today. Until now I would have said that methods should all be marked with internal if the class was internal and would have considered anything else simply bad coding or laziness, specially in enterprise development; however, I had to sub class a public class and override one of it's methods:

internal class SslStreamEx : System.Net.Security.SslStream
{
    public override void Close()
    {
        try
        {
            // Send close_notify manually
        }
        finally
        {
            base.Close();
        }
    }
}

The method MUST be public and it got me thinking that there's really no logical point to setting methods as internal unless they really must be, as Eric Lippert said.

Until now I've never really stopped to think about it, I just accepted it, but after reading Eric's post it really got me thinking and after a lot of deliberating it makes a lot of sense.

回答9:

There does be a difference. In our project we have made a lot of classes internal, but we do unit test in another assembly and in our assembly info we used InternalsVisibleTo to allow the UnitTest assembly to call the internal classes. I've noticed if internal class has an internal constructor we are not able to create instance using Activator.CreateInstance in the unit test assembly for some reason. But if we change the constructor to public but class is still internal, it works fine. But I guess this is a very rare case (Like Eric said in the original post: Reflection).

回答10:

I think I have an additional opinion on this. At first, I was wondering about how it makes sense to declare something to public in an internal class. Then I have ended up here, reading that it could be good if you later decide to change the class to public. True. So, a pattern formed in my mind: If it does not change the current behavior, then be permissive, and allow things that does not makes sense (and does not hurt) in the current state of code, but later it would, if you change the declaration of the class.

Like this:

public sealed class MyCurrentlySealedClass
{
    protected void MyCurretlyPrivateMethod()
    {
    }
}

According to the "pattern" I have mentioned above, this should be perfectly fine. It follows the same idea. It behaves as a private method, since you can not inherit the class. But if you delete the sealed constraint, it is still valid: the inherited classes can see this method, which is absolutely what I wanted to achieve. But you get a warning: CS0628, or CA1047. Both of them is about do not declare protected members in a sealed class. Moreover, I have found full agreement, about that it is senseless: 'Protected member in sealed class' warning (a singleton class)

So after this warning and the discussion linked, I have decided to make everything internal or less, in an internal class, because it conforms more that kind of thinking, and we don't mix different "patterns".