我可以将课程内部封闭，但外部密封吗？

Question

在C＃中，是否可以创建一个可以从内部派生（未密封）的类，但是然后阻止其他人在外部引用我的库继承自我的公共类（密封）？

Answer 1

I suppose you could make the constructor of the class internal so that only other classes in your assembly could derive from them, if you still need to create instances of that class you could provide a factory method to return instances.

edit to add a sample:

public class MyFoo
{
    internal MyFoo()
    {
    }

    public static MyFoo CreateFoo()
    {
        return new MyFoo();
    }
}

Answer 2

Eric Lippert notes three ways to do this. tl;dr: don't seal your class, but include an internal abstract method in your class, or make all the constructors internal or private, or use the PermissionSet attribute to add metadata to the class.

http://blogs.msdn.com/b/ericlippert/archive/2008/09/26/preventing-third-party-derivation-part-one.aspx

http://blogs.msdn.com/b/ericlippert/archive/2008/10/06/preventing-third-party-derivation-part-two.aspx

Answer 3

You can also try doing something like this.

internal class InternalExtendible
{
    public string MyProperty { get; set; }
}

public sealed class ExternalSealedClass : InternalExtendible
{
}

Create an Internal class and create a public empty class that inherits from the internal class. When referencing the dll, only the public class will be visible to the user, but all the functionality of the internal class will be exposed.