UPDATE: I am using this question as the subject of my blog today. Thanks for the great question. Please see the blog for future additions, updates, comments, and so on.
It is not entirely clear to me what exactly you are asking. “Why” questions are difficult to answer. But I’ll take a shot at it.
First, code which has an implicit conversion from char to int (note: this is not an “implicit cast”, this is an “implicit conversion”) is legal because the C# specification clearly states that there is an implicit conversion from char to int, and the compiler is, in this respect, a correct implementation of the specification.
Now, you might sensibly point out that the question has been thoroughly begged. Why is there an implicit conversion from char to int? Why did the designers of the language believe that this was a sensible rule to add to the language?
Well, first off, the obvious things which would prevent this from being a rule of the language do not apply. A char is implemented as an unsigned 16 bit integer that represents a character in a UTF-16 encoding, so it can be converted to a ushort without loss of precision, or, for that matter, without change of representation. The runtime simply goes from treating this bit pattern as a char to treating the same bit pattern as a ushort.
It is therefore possible to allow a conversion from char to ushort. Now, just because something is possible does not mean it is a good idea. Clearly the designers of the language thought that implicitly converting char to ushort was a good idea, but implicitly converting ushort to char is not. (And since char to ushort is a good idea, it seems reasonable that char-to-anything-that-ushort-goes-to is also reasonable, hence, char to int. Also, I hope that it is clear why allowing explicit casting of ushort to char is sensible; your question is about implicit conversions.)
So we actually have two related questions here: First, why is it a bad idea to allow implicit conversions from ushort/short/byte/sbyte to char? and second,
why is it a good idea to allow implicit conversions from char to ushort?
Unlike you, I have the original notes from the language design team at my disposal. Digging through those, we discover some interesting facts.
The first question is covered in the notes from April 14th, 1999, where the question of whether it should be legal to convert from byte to char arises. In the original pre-release version of C#, this was legal for a brief time. I’ve lightly edited the notes to make them clear without an understanding of 1999-era pre-release Microsoft code names. I’ve also added emphasis on important points:
[The language design committee] has chosen to provide
an implicit conversion from bytes to
chars, since the domain of one is
completely contained by the other.
Right now, however, [the runtime
library] only provide Write methods
which take chars and ints, which means
that bytes print out as characters
since that ends up being the best
method. We can solve this either by
providing more methods on the Writer
class or by removing the implicit
conversion.There is an argument for why the
latter is the correct thing to do.
After all, bytes really aren’t
characters. True, there may be a
useful mapping from bytes to chars, but ultimately, 23 does not denote the
same thing as the character with ascii
value 23, in the same way that 23B
denotes the same thing as 23L. Asking
[the library authors] to provide this
additional method simply because of
how a quirk in our type system works
out seems rather weak. So I would
suggest that we make the conversion
from byte to char explicit.
The notes then conclude with the decision that byte-to-char should be an explicit conversion, and integer-literal-in-range-of-char should also be an explicit conversion.
Note that the language design notes do not call out why ushort-to-char was also made illegal at the same time, but you can see that the same logic applies. When calling a method overloaded as M(int) and M(char), when you pass it a ushort, odds are good that you want to treat the ushort as a number, not as a character. And a ushort is NOT a character representation in the same way that a ushort is a numeric representation, so it seems reasonable to make that conversion illegal as well.
The decision to make char go to ushort was made on the 17th of September, 1999; the design notes from that day on this topic simply state “char to ushort is also a legal implicit conversion”, and that’s it. No further exposition of what was going on in the language designer’s heads that day is evident in the notes.
However, we can make educated guesses as to why implicit char-to-ushort was considered a good idea. The key idea here is that the conversion from number to character is a “possibly dodgy” conversion. It’s taking something that you do not KNOW is intended to be a character, and choosing to treat it as one. That seems like the sort of thing you want to call out that you are doing explicitly, rather than accidentally allowing it. But the reverse is much less dodgy. There is a long tradition in C programming of treating characters as integers — to obtain their underlying values, or to do mathematics on them.
In short: it seems reasonable that using a number as a character could be an accident and a bug, but it also seems reasonable that using a character as a number is deliberate and desirable. This asymmetry is therefore reflected in the rules of the language.
Does that answer your question?