I'm not sure i understand your question. Please post the code and the error you're recieving...

but I think you're saying one of two things
1) Decimal.Parse doesn't work from within the switch () statement
I'm not sure that you can call a fnc from insinde the switch( ) statement. If you can not, then parse to decimal on the line before, then just dump the new decimal into the switch statement

the other thing i think you might be saying is that you can not use a decimal in a switch statement, and I have never tried to do that (i'm not sure a switch statement is really a good design choice if your working with decimals.) you may want to look into a series of if statements that may work better with decimals... post back to let me know if that helped, if not, give more specifics about your question...