Do interfaces derive from System.Object? C# spec says yes, Eric says no, reality says no

Question

Question is simple and asked in the title.

C# 4.0 Specification says: (§4.2.2)

The object class type is the ultimate base class of all other

Answer 1

Interface types do not inherit from Object, but storage locations of interface types hold references to class-type objects which (if non-null) are guaranteed to inherit from System.Object.

I think understanding what's going on will be easiest if one starts by examining the difference between value types and class types. Suppose I have a structure:

public struct SimplePoint {public int x,y;}

and I have two methods

public doSomethingWithPoint(SimplePoint pt) ...
public doSomethingWithObject(Object it) ...

and cal each method:

SimplePoint myPoint = ...;
doSomethingWithPoint(myPoint);
dosomethingWithObject(myPoint);

The first call does not pass a thing that derives from Object. It instead passes the contents of all of SimplePoint's public and private fields. The second call needs a thing which derives from Object, so it generates a new heap object instance of type SimplePoint which contains all the public and private fields of the value-type SimplePoint, and loads all those fields with the corresponding values from myPoint, and passes a reference to that object.

Note that the type SimplePoint actually describes two different kinds of things: a collection of fields (i.e. the value type) and a heap-object type. Which meaning is applicable depends upon the context where the type is used.

Interface types have a similar wrinkle: when used as storage-location types, they specify that the storage location will hold an object reference. When used as a generic constraint, they say nothing about how the type will be stored. Thus, a storage location of an interface type will hold a reference to a heap object that genuinely does inherit from System.Object, but a variable of a type constrained to an interface might hold either a reference or a bunch of fields.