Combine sugar, cornstarch, flour and salt in saucepan.
Add milk gradually, blending in well. Cook over low heat, stirring until thick.
Stir a little of the hot mixture into egg yolks, then return to saucepan.
Continue cooking 2 minutes. Remove from heat; blend in margarine and vanilla.
Cover and cool to lukewarm, stirring occasionally. Pour into baked pastry shell.
Top with meringue or whipped cream. Yield: one 9-inch pie for 6 serving.