The SwissFEL main linac consists of more than hundred constant gradient C-band accelerating structures which boost the beam energy from 410 MeV at the injector to the fina nominal energy of 5.8 GeV. With a repetition rate of 100 Hz, two bunches per pulse can be accelerated with a spacing of 28 ns to feed simultaneously two different FEL arms. Rising of the long range wakefields both longitudinal and transverse, could affect this multibunch operation, causing degenerative effects on the quality of the second bunch. A direct computation of the longitudinal and transverse wakes by means of time domain simulations is compared with a model based on the computation of the dispersion curves of the wake modes by frequency domain simulations. A good agreement is obtained for both the synchronous frequency and impedance of all the main modes contributing to the wakefields Moreover, the total longitudinal wake at 28 ns is below the tighter tolerances required by the beam dynamics, so that neither Higher Order Modes (HOMs) nor beam loading require compensation. The effects on the beam of the long range transverse wakefield are also negligeable.