William Christopher says he doesn't care if his long run as Father Mulcahy on television's "M*A*S*H" and "AfterMASH" has limited the type of roles he is now offered.

"I don't worry about typecasting," said Christopher, who stars in George Axelrod's comedy "The Seven Year Itch," opening tonight at the Hayloft Dinner Theatre in Manassas. "I don't want to think about anything negative.

"I can be perfectly comfortable playing serious stuff," said Christopher, but he added that other than a recent appearance on "Murder, She Wrote," he hasn't been called on recently to do so.

"One thing about me, I have to admit, is that I'm a character," Christopher said. "My face . . . it's a character face. And my voice . . . it's a character voice. That works well for comedy," said the veteran actor, whose TV credits include "Gomer Pyle, U.S.M.C.," "Hogan's Heroes" and "The Love Boat."

Christopher said he keeps in touch with cast members from "M*A*S*H." "You don't really say goodbye at the end of a series. They're still actors and you're still an actor . . . and it's wonderful to think you might come together and work again."

When he's not acting, Christopher donates much of his free time to the Devereux Foundation, a nonprofit organization with residential treatment centers around the country for emotionally disturbed and retarded children and youth. His particular interest is helping to develop better programs for the autistic. "One does what's close to one's heart," said the actor. His son suffers from autism and resides at Devereux's Santa Barbara, Calif., campus. Tap Dance Competition

Attention all tap dancers who have been waiting for that big break. The Warner Theatre, WKYS radio and Triples Nightclub are sponsoring a tap dance competition, the winner of which will perform on stage in a production number from "The Tap Dance Kid," at the Warner Feb. 25 through March 2. Dancers will be selected from auditions at Triples the next two Sundays to participate in competitions on Feb. 17 and 24, with finalists advancing to a showdown on the 24th. To schedule an audition call (301) 423-5993. Horizons' 'New Voices' Series

Horizons Theatre is seeking original one-act plays by local women playwrights. The three plays chosen will be presented during Horizons' "New Voices" series June 12-29. Scripts should be sent by March 15 to the literary manager, Horizons Theatre, 1041 Wisconsin Ave. NW, Washington, D.C. 20007. Discounts for Students

The Studio Theatre has announced a new discount ticket policy for students. Half-price tickets for remaining seats will be available 30 minutes before curtain time to students with a valid ID for all future Wednesday, Thursday and Sunday performances. The "student rush" policy begins March 12 with the opening of Harold Pinter's "The Birthday Party." Odds and Ends Horizons

Theatre will present a reading of "Outside Waco," a play by local author Patricia Browning Griffith, tomorrow at 8 p.m. And at Friday's performance of "Automatic Pilot," the feminist-oriented theater will salute Valentine's Day with a drawing for door prizes -- four $20 gift certificates from the Pleasure Chest, a Georgetown boutique specializing in the chicquest of marital aids . . . Open University will offer a one-night seminar by American National Theater artistic director Peter Sellars -- "Success and the Theater Business: How to Create Your Own Act" -- March 12; call 966-9606 . . . The Interact Company will be featured in "Come Love With Me," a blend of poetry, prose and song, Friday at 8 p.m. in the Baird Auditorium of the National Museum of Natural History.

Opening this week: tonight, "42nd Street" at Baltimore's Mechanic Theatre; Friday, "Batman vs. The League of Doom" at Source's Warehouse Rep; Saturday, "Sojourner" at the Kennedy Center's Theater Lab; Monday, "The Aunts" at Source's Main Stage.