Filter a list of objects with dynamic boolean operators

Question

I have this method:

private static List<ObjectA> ListItemsToShow(IEnumerable<ObjectA> listObjectB, 
                                             string _rarity, 
                                             string _type, 
                                             string _color,
                                             int? _quantity, 
                                             int? _sold, 
                                             string _quantitySymbol,
                                             string _soldSymbol)
{
    List<ObjectA> listToReturn = (from item in listObjectB 
                                  where _rarity == "All" || item.rarity == _rarity
                                  where _type == "All" || item.Type.Contains(_type) 
                                  where _color == "All" || item.Color.Contains(_color) 
                                  where _quantity == null || item.NbInStock == (int)_quantity
                                  where _sold == null || item.QtySold == (int)_sold
                                  select item).ToList();
    return listToReturn;
}

Up to now it does it job: based on a static List of objects, it returns what can be filtered from the list of original objects.

Now I want to add a dynamic parameter: the quantitySymbol and soldSymbol. Each would be one of those choices:

>
<
>=
<=

So that I may get, for example, all items which NbInStock is <, >, <= or >= than those retained in the original list. The same would apply for the QtySold property.

I have a bit of trouble figuring how I could do it in a linq statement, I'd need help to make this out.

Danvil · Accepted Answer · 2014-03-21 12:38:26Z

3

You could use a functional for filtering:

List<ObjectA> ItemsToShow(IEnumerable<ObjectA> listObjectB, Func<int,bool> stockFilter) {
  return (
    from item in listObjectB
    where (stockFilter == null || stockFilter(item.NbInStock)
    select item
  ).ToList();
}

and use it like this:

ItemsToShow(data, stock => (stock <= 10));
ItemsToShow(data, stock => (stock == 25));
ItemsToShow(data, stock => (stock > 3));
ItemsToShow(data, null); // does not use a stock filter

If you need to create the stock filter from a string you can use a factory function:

Func<int,bool> CreateCompareFilter(string op, int? quantity) {
   if(quantity == null) return null;
   if(op == "==") return x => (x == quantity);
   if(op == "<") return x => (x < quantity);
   ...
   return null;
}

Then you can write

ItemsToShow(data, CreateCompareFilter("==",10));

So in total this would look like this:

private static List<ObjectA> ListItemsToShow(IEnumerable<ObjectA> listObjectB, 
                                             string _rarity, 
                                             string _type, 
                                             string _color,
                                             Func<int,bool> _stockFilter,
                                             Func<int,bool> _soldFilter)
{
    return (
        from item in listObjectB 
        where _rarity == "All" || item.rarity == _rarity
        where _type == "All" || item.Type.Contains(_type) 
        where _color == "All" || item.Color.Contains(_color) 
        where _stockFilter == null || _stockFilter(item.NbInStock)
        where _soldFilter == null || _soldFilter(item.QtySold)
        select item
    ).ToList();
}

ListItemsToShow(data, rarity, type, color,
   CreateCompareFilter(_quantitySymbol,_quantity)
   CreateCompareFilter(_soldSymbol,_sold));

edited Mar 21, 2014 at 12:38

answered Mar 21, 2014 at 12:20

Danvil

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4 Comments

hsim Over a year ago

Ok, but how could I evaluate which stock filter I may get from a string? The string is obtained from a view / dropdownlist value in an Ajax call.

hsim Over a year ago

Yes, I will try it out, that's very interesting.

hsim Over a year ago

I'm trying to understand. That will create a lamdba expression, correct? So how could I mix it with my statement like I made up above?

hsim Over a year ago

Ah! Ok! The stockfilters are made before calling the method. Thanks!

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