public class ItemViewModel
{
    public int itemId { get; set; }

    public string itemName { get; set; }

    public int? categoryId { get; set; }
}

public List<ItemViewModel> GetData(int?[] categoryIds){

    var data = db.Query<ItemViewModel>("SELECT itemId, itemName, categoryId FROM items;").ToList();

    // So far so good. Getting data as List<ItemViewModel>
    // Now, if there are any categoryIds passed in to the controller, filter the data accordingly:

    if(categoryIds!= null && categoryIds.Any()){
        data = data.Where(x => x.categoryId != null && categoryIds.Contains((int)x.categoryId));
    }

    return data
}