Chained user-defined explicit conversions in C#, Part Two

Reader Larry Lard asks a follow-up question regarding the subject of Monday’s blog entry. Why is it that the compiler knows that (int)(new Base()) will always fail, and therefore makes the conversion illegal, but does not know that (Derived)(new Base()) will always fail, and make that conversion illegal too?

There are two answers, a general answer and a specific answer.

The general answer is that in most cases, the C# type system operates solely on types. There are some exceptions where we peer closer at an expression when doing a conversion – lambdas are the obvious huge exception to that rule, since the convertibility of a lambda to a particular type depends on the structure of the type and the contents of the lambda parameters and body. There are smaller exceptions as well – literal zero is convertible to any enum, for example. But in general, we cleave to the principle that the type system mostly makes decisions based on the types of expressions, not the content of the expressions themselves.

Thus, in the examples above, we only see that the argument of the cast operator is something of type Base. The fact that this thing can be no more derived than Base is lost; it’s something of type Base, and therefore might be something that can be converted to Derived.

The more specific answer is that in fact, this doesn’t always fail. Betcha didn’t know that!

This is one of the most obscure, bizarre (and frankly, also one of the most poorly specified and documented) parts of the C# implementation. I myself just happened to learn about it recently. But it is true -- there is a situation where there is a non-user-defined explicit conversion from Base to Derived such that (Derived)(new Base()) succeeds at runtime.

Via a similar mechanism, there is also a situation where (Base)(new Base()) fails at runtime!

Actually, it gets even worse than that. Last time, I mentioned that there were two times when the compiler inserts an "explicit" cast on your behalf. The case I am referring to here introduces a third that I didn't know about. This means that it is possible for Base b = new Base(); to compile but fail at runtime!

So, another challenge to my readers: does anyone know the extremely obscure way that this can happen?

I’ll give you a hint: the Base will typically start with the letter I, not B...

And in related news, I've also recently learned of a fourth situation in which the compiler inserts an explicit cast. I'll blog more about that later this week probably.