User-defined conversion operator from base class

Question

Introduction

I am aware that \"user-defined conversions to or from a base class are not allowed\". MSDN gives, as an explanation to this rule, \"You

Answer 1

It's not a design flaw. Here's why:

Entity entity = new Body();
Body body = (Body) entity;

If you were allowed to write your own user-defined conversion here, there would be two valid conversions: an attempt to just do a normal cast (which is a reference conversion, preserving identity) and your user-defined conversion.

Which should be used? Would you really want is so that these would do different things?

// Reference conversion: preserves identity
Object entity = new Body();
Body body = (Body) entity;

// User-defined conversion: creates new instance
Entity entity = new Body();
Body body = (Body) entity;

Yuk! That way madness lies, IMO. Don't forget that the compiler decides this at compile-time, based only on the compile-time types of the expressions involved.

Personally I'd go with solution C - and possibly even make it a virtual method. That way Body could override it to just return this, if you want it to be identity preserving where possible but creating a new object where necessary.