Beeliner said, (quotes Mooh) " "Everything is something. C#, E, G is a C# diminished." (end quote)

Right, which is why I put 'nothing' in quotes.

Now, if a piece in D required a C major chord, which is certainly possible, then of course a natural symbol would be put in front of the C, negating the sharp."

Sorry. To clarify, I was responding to the OP.

Beeliner again, "So if a piece is signed D (two sharps), it's true that C#, E and G are 'nothing', and that's why you are not likely to encounter that combination in that key."

Oh, I don't know about that. All 3 notes are in the key. A diminished chord on the 7th degree (in this case C#) often wants to resolve to the 1 chord (D). I'm sure you know this, but I'm struggling to understand something here. What am I missing?

Peace, Mooh.