We focus on the strategyproofness of voting systems where voters must choose a number of options among several possibilities. These systems include those that are used for Participatory Budgeting, where we organize an election to determine the allocation of a community's budget (city, region, etc.) dedicated to the financing of projects.

We present a model for studying voting mechanisms and the Constrained Change Property (CCP), which will be used to design voting mechanisms that are always strategyproof. We also define a new notion of social choice function and use it to design a new class of utilitarian voting mechanisms that we call score voting. We prove that the mechanisms designed with core voting with a neutral score function are equivalent to knapsack voting on the same instance and that any score voting designed with a total score function is strategyproof if and only if its score function satisfies CCP.