you can't eat your cake and have it (too)

You can't have or do two things that are contradictory or impossible to have or do simultaneously. Because "have" can also mean "eat," this expression may seem redundant. However, it is based on the meaning of "have" as "to possess," i.e., to maintain possession of one's cake while still eating it, an obvious impossibility. You're never going to save enough money to buy a house if you keep buying expensive appliances and cars. You can't eat your cake and have it, too. You can't eat your cake and have it. If you demand all sorts of social benefits from the government, you need to be willing to pay the taxes to fund them.