getting error of illegal assignment

trigger CampaignmemberControl on CampaignMember (before insert) {
    If(trigger.isInsert && trigger.isbefore){
        Set<String> CampaignMember_parentid=new Set<String>();
        for (CampaignMember cc:trigger.new){
            If(cc.Email !=null){
                CampaignMember_parentid.add(cc.CampaignId);
            }
            List <Campaign> listCampaign= [SELECT Id, Name,(SELECT Id FROM CampaignMembers) FROM Campaign WHERE Id IN:CampaignMember_parentid];
            for(CampaignMember cnew:trigger.new){
                if(listCampaign.size()>0)
                {
                    cnew.addError('Not Allowed');
                }
            }update listCampaign;
        }
    }
}