ROA CTLR ADVISES C172 PLT OF UNAUTHORIED CLASS C AIRSPACE ENTRY.

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I DEPARTED VA TECH ARPT (BCB) AND CONTACTED ROANOKE APCH TO REQUEST VFR FLT FOLLOWING TO LYNCHBURG; VA (LYH) AT 5500 FT. WAS ASSIGNED A SQUAWK AND IDENTED AS 'RADAR CONTACT;' BUT TOLD THAT APCH WAS NOT RECEIVING MY MODE C. I CONFIRMED CURRENT ALT AND OFFERED TO CYCLE XPONDER PWR TO RESTORE MODE C. AFTER SEVERAL MINS; APCH RPTED THEY STILL COULDN'T READ MODE C; BUT TO 'JUST LET ME KNOW WHEN YOU'RE LEVEL 5500 FT.' I RPTED LEVEL AT THAT TIME; AND WAS ISSUED A TFC WARNING. AT THIS POINT; I WAS 300 FT ABOVE; AND APPROX 2 MI W OF THE ROANOKE CLASS C AIRSPACE. THE NEXT XMISSION I RECEIVED FROM APCH; AFTER SEVERAL MINS AND AFTER I WAS WELL WITHIN THE LATERAL LIMITS OF THE AIRSPACE; WAS (ESSENTIALLY) 'SINCE YOU'RE TRANSITIONING CLASS C WITH NO MODE C I'M GOING TO HAVE TO RPT YOU. PLEASE GIVE ME YOUR NAME AND HOME BASE.' I WAS ABSOLUTELY SHOCKED; AND MY FIRST RESPONSE WAS A KNEE-JERK SELF DEFENSE REACTION; CLAIMING I WAS ACTUALLY ABOVE CLASS C. OF COURSE I KNOW MODE C IS STILL REQUIRED THERE; AND THE CTLR REMINDED ME OF THIS. WANTING TO COOPERATE; I PROVIDED THE REQUESTED INFO; BUT ALSO EXPRESSED MY CONCERN THAT ATC DID NOT PROVIDE ANY INSTRUCTION TO REMAIN CLR; ALTER COURSE; ETC; AND THAT IT WAS MY BELIEF HE WAS GRANTING AN EXCLUSION; AS ATC HAS THE AUTH TO DO. HE REPLIED; 'IT'S OK; WE CAN ALLOW IT AND OFTEN DO. WE JUST WANT YOUR INFO IN CASE YOU TRY TO COME BACK OFTEN.' I STILL HAVE NO CLR IDEA AS TO WHETHER OR NOT THIS CTLR CONSIDERED ME IN VIOLATION OR NOT. FROM MY PERSPECTIVE; THE CTLR ISSUED A SQUAWK; ACKNOWLEDGED RADAR CONTACT; WAS PROVIDING TFC CALLOUTS; ACCEPTED MY ROUTING AND ALT; AND OFFERED MEANS TO COMPLY WITH INTENT OF MODE C -- 'JUST TELL ME WHEN YOU'RE LEVEL AT 5500 FT; AND TELL ME OF ANY CLBS OR DSCNTS.' NO VECTORS WERE PROVIDED. NO REQUEST TO REMAIN CLR. IT WAS; IN EFFECT; TACIT ISSUANCE OF AN EXCLUSION TO THE MODE C REQUIREMENT. THERE IS NO SPECIFIC LANGUAGE REQUIREMENT FOR GRANTING EXCLUSIONS OF THIS TYPE. IN THIS CASE; HAVING ESTABLISHED COM AND RADAR CONTACT WELL IN ADVANCE; I FELT THE CTLR; WITH CLR UNDERSTANDING OF MY INTENDED RTE; HAD AMPLE OPPORTUNITY TO ASK ME TO DIVERT BECAUSE HE COULDN'T ACCEPT ME IN HIS AIRSPACE WITH A SUDDENLY-BROKEN MODE C. INSTEAD; HE ALLOWED ME TO CONTINUE AND ASKED FOR ALT CONFIRMATION IN AN ATTEMPT TO CIRCUMVENT THE PROB. ATC EXCLUSIONARY AUTH IS POWERFUL; BUT APPARENTLY THERE IS NO STRICT GUIDELINE TO ARTICULATING THIS AUTH. IN MY CASE; IT WAS VERY EASY TO ASSUME THAT SINCE I WAS 'IN THE SYS' AND COMMUNICATING WITH ATC; THAT ATC WOULD ARTICULATE THEIR REQUESTS. INSTEAD; I FEEL AS THOUGH I WAS INVITED INTO AN 'ENTRAPMENT' SIT; WHERE ATC; INSTEAD OF BEING HELPFUL; BEGAN POLICING. ALL THIS MAY BE A RESULT OF HEIGHTENED SUSPICION RESULTING FROM RECENT TERRORIST ACTIONS. HOWEVER; IF THIS IS THE CASE; I THINK THE ROLE OF ATC IS BEING UNDERMINED BY THAT SUSPICION. AS AN ASIDE -- THE EASTERN ROANOKE APCH CTLR WAS VERY HELPFUL IN TRYING TO DIAGNOSE THE PROB WITH MY MODE C; TRYING MULTIPLE SQUAWKS; ETC. ACTUALLY TURNED OUT MODE C WAS RESPONDING INTERMITTENTLY. THE PLANE IS UNDERGOING DIAGNOSTIC MAINT AS I WRITE THIS. I HAD COMPLETED A TRIP THE DAY BEFORE THROUGH SEVERAL RADAR SVCS; AND ACTUALLY TRANSITED ROANOKE'S AREA THAT VERY MORNING WITH NO TROUBLES. THIS WAS A NEW; UNEXPECTED MALFUNCTION.