writing a function in RDLC on the base of predicate

Hi , i want to write a function on the base of some logic and taking sum of values on the bases of that predicate.

below can be the predicate. Kindly help me how to create and use that function in RDLC .

=IIf(StrComp(Fields!PurchaseOrSale.Value,"S",CompareMethod.Text) = 0,"Revenue",IIf(StrComp(Fields!PurchaseOrSale.Value,"P",CompareMethod.Text) = 0,"Expence","Gross Profit"))