DA20 WAS INVOLVED IN A NMAC. THE PLT WAS DISTRACTED BY THE EVENT; AND INADVERTENTLY ENTERED CLASS B AIRSPACE.

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I TOOK OFF ON A LCL VFR FLT FROM BFI RWY 13L AND HEADED 15 MI NE VIA THE MERCER DEP TO PRACTICE MANEUVERS. CEILING WAS 16000 FT BROKEN; WINDS LIGHT AND VARIABLE AND GREATER THAN 10 MI VISIBILITY. I EVENTUALLY MADE MY WAY NW TO PAE; PERFORMED 3 TOUCH-AND-GO LNDGS ON RWY 34R THEN HEADED SW TOWARD VASHON ISLAND TO SET UP FOR THE VASHON ARR INTO BFI. AS I FLEW OVER BAINBRIDGE ISLAND; I DECIDED TO TURN E TO SET UP FOR A STRAIGHT-IN APCH TO RWY 13L INSTEAD OF FLYING FURTHER S FOR THE VASHON ARR. ALL THE WHILE I HAD BEEN MINDFUL TO MONITOR MY ALT AND POS SO AS TO STAY OUT OF THE SEATTLE TCA. WHILE TRANSITING EBOUND ACROSS PUGET SOUND TOWARDS THE W POINT RPTING POINT; I RECEIVED A TCAS ALERT FOR TFC AT MY ALT AT LESS THAN 1 MI S OF MY POS HEADING IN A NORTHERLY DIRECTION. I LOOKED OUT TO MY R AND THEY HAD THE RIGHT-OF-WAY AND ALSO TO PUT VERT SEPARATION BTWN US. WHEN I WAS ABLE TO IDENT THE TFC; IT WAS A SMALL HIGH-WING ACFT; PROBABLY A C172; AND APPEARED TO BE APPROX 100-200 FT ABOVE ME AND PASSED LESS THAN 200 FT BEHIND. AT THIS POINT I WAS ALREADY OVER THE W POINT RPTING POINT AND CALLED BFI TWR AND ADVISED THEM OF MY POS AND MY INTENTIONS TO LAND. THE TWR CTLR ADVISED ME TO MAKE STRAIGHT-IN RWY 13L. AS I TURNED SW FOR THE STRAIGHT-IN APCH I WAS AT 2500 FT MSL AND STILL OUTSIDE OF SEATTLE'S CLASS B AIRSPACE. HOWEVER; I BECAME DISTR AS I FIXATED ON HOW CLOSE I HAD COME TO THE C172 AND HOW DISCONCERTING IT WAS THAT IT HAD BEEN INDISTINGUISHABLE FROM THE LIGHTS OFF IN THE DISTANCE UNTIL THE VERY LAST MOMENT. I DID NOT REALIZE THAT I WAS QUICKLY APCHING THE LATERAL BOUNDARY OF THE SEATTLE TCA EXTENDING FROM 1800 FT MSL TO 10000 FT MSL. AFTER A FEW MOMENTS; THE BFI TWR CTLR ASKED ME TO SAY MY ALT AND I RPTED MY ALT AS 2500 FT. SHE QUICKLY RESPONDED AND INSTRUCTED ME TO IMMEDIATELY DSND BELOW 1800 FT BECAUSE I HAD JUST ENTERED INTO SEATTLE'S CLASS B AIRSPACE. I BEGAN AN EMER DSCNT WHILE MAKING A TURN TO THE N. AS I NEARED THE FLOOR OF THE CLASS B SHELF THE BFI CTLR GAVE ME A TA OF AN ACFT (I BELIEVE SHE SAID IT WAS A DASH-8) AT 3700 FT. I DO NOT REMEMBER WHAT THE CTLR SAID REGARDING THE TFC'S LOCATION TO MY CURRENT POS. I INFORMED THE CTLR THAT I HAD DSNDED OUT OF CLASS B AIRSPACE AND PROCEEDED TO TURN S TO CONTINUE MY APCH TO RWY 13L AND LANDED SAFELY WITHOUT FURTHER INCIDENT. THE CTLR DID NOT REQUEST THAT I CALL THE TWR. FOCUSING MY THOUGHTS ON THE APCH RATHER THAN DWELLING ON THE ENCOUNTER WITH THE C172 OVER PUGET SOUND WOULD LIKELY HAVE PREVENTED THE INCURSION INTO THE TCA.