Inheritable only inside assembly in C#

I think Eric Lippert gets the defining quotes here:

• Preventing third-party derivation part one
• Preventing third-party derivation part two

The language itself doesn't have anything that makes this easy, but you should be able to do this by making your constructors internal. If you do this, however, you won't be able to new it up from external assemblies, so you'll have to add a factory method.

public class Foo
{
    internal Foo()
    {
    }

    public Foo Create()
    {
        return new Foo();
    }
}

Here's an alternative that lets you new up the class in external assemblies

public sealed class Foo : FooBase
{

}

public class FooBase
{
    internal FooBase() { }
}

One question you might ask yourself, however, is exactly why you want the class to be sealed. Sometimes it's inevitable, but I have seen the sealed keyword get abused so often that I thought I'd bring it up. Often, developers will seal classes because they are overprotective of their code. Most of the time, if a class is well designed, it doesn't need to be sealed.

If your class is abstract, there's a very simple way to do it:

public abstract class Foo {
    internal abstract void DoNothing();
    // ... other members
}

Now although your class and most members are public, the internal abstract member makes it impossible to derive from the class outside your assembly.