Why are .NET value types sealed?

The reason is that most inheritance techniques relate to runtime polymorphism (virtual functions) and those don’t work on value types: for runtime polymorphism to have any meaning, objects need to be treated as references – this isn’t specific to .NET either, it’s simply a technical detail of how virtual functions are implemented.

Value types form an exception to .NET’s rule, precisely to allow lightweight objects that don’t require indirection via references. So runtime polymorphism doesn’t work for them and most aspects of inheritance become meaningless.

(There’s an exception: a value type object can be boxed, which allows for virtual methods inherited from System.Object to be called.)

To address one of your points:

• You could cast from a derived struct to the base, since they would overlap the same memory.

No, this would not be possible – casting a value type would copy its value. We’re not dealing with references here, so no overlap in memory. Casting a value type to its base type is therefore meaningless (again, unless we’re talking about conversion to object which actually performs boxing under the hood, and also operates on a copy of the value).

Still not clear? Let’s look at an example.

Let’s say we’ve got the hypothetical struct Shape and, inheriting from it, the struct Circle. Shape defines a virtual Draw method (which accepts a Graphics object). Now, let’s say we want to draw a shape on a canvas. This, of course, works perfectly well:

var circle = new Circle(new Point(10, 10), 20);
circle.Draw(e.Graphics); // e.Graphics = graphics object of our form.

– But here we don’t actually use inheritance at all. To make use of inheritance, imagine instead the following DrawObject helper method:

void DrawObject(Shape shape, Graphics g) {
    // Do some preparation on g.
    shape.Draw(g);
}

And we call it elsewhere with a Circle:

var circle = new Circle(new Point(10, 10), 20);
DrawObject(circle, e.Graphics);

– And, ka-blam – this code doesn’t draw a circle. Why? Because when we pass the circle to the DrawObject method, we do two things:

• We copy it.
• We slice it, i.e. the object shape object is really no longer a Circle – neither the original one nor a copy. Instead, its Circle portion was “sliced” away during copying and only the Shape portion remains. shape.Draw now calls the Draw method of Shape, not of Circle.

In C++, you can actually cause this behaviour. For that reason, OOP in C++ only works on pointers and references, not on value types directly. And for that same reason, .NET only allows inheritance of reference types because you couldn’t use it for value types anyway.

Notice that the above code does work in .NET if Shape is an interface. In other words, a reference type. Now the situation is different: your circle object will still be copied but it will also be boxed into a reference.

Now, .NET could theoretically allow you to inherit a struct from a class. Then the above code would work just as well as if Shape were an interface. But then, the whole advantage of having a struct in the first place vanishes: for all intents and purposes (except for local variables which never get passed to another method, hence no utility of inheritance) your struct would behave as an immutable reference type instead of a value type.