How do I use Angular's inject within a canActivate function?

In the following canActivate function, I need to get the number of Items in a cart. I inject the CartService for this purpose but am getting an error that inject() must be called from an injection context. How do I fix this?

const cartService = inject(CartService);
const toastr = inject(ToastrService);

let numberOfCartItems = 0;

const cartEffect = effect(() => {
numberOfCartItems = cartService.cartSignal().length;
})

export const canActivateCartGuard: CanActivateFn = (route, state) => {
if(numberOfCartItems) {
return true;
}
toastr.error("Your cart is currently empty!", "")
return false;
};

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