使用LINQ缩小结果集

Question

Say I have a method called GetCatsByColor which takes a color as a string, a method GetCatsByName which takes a name as a string, and GetCatsByBirthDate which takes two DateTime s acting as a range of time.

Now say I have a CatFilter class which holds a List of names, List of colors, and two DateTime s, representing the "from" date and the "to" date of a timespan. What I am trying to do is create a GetFilteredCats method that takes one of these Filter objects and returns a collection of Cats that meet the specifications of the given Filter .

I'm having a hard time coming up with an efficient way of getting the desired results, ideally using LINQ/lambda expressions.

What is the best way to go about doing this sort of join? What extension methods should I be looking at? Modifying a collection in a foreach loop generally isn't advisable/possible, so what should my strategy be?

Answer 1

What i normally do is to put a check in the where clause which checks if the filter is needed before doing the actual filter. When the runtime needs to evaluate the filter, it is skipped entirely if it isn't needed.

public class CatFilter
{
    public List<string> Names = new List<string>();
    public List<string> Colors = new List<string>();
    public DateTime? BirthDateStartRange = null;
    public DateTime? BirthDateEndRange = null;
}

public List<Cat> GetFilteredCats(CatFilter filter)
{
    List<Cat> result = new List<Cat>();

    var query = cats
        .Where(a => !filter.Names.Any() || filter.Names.Contains(a.Name))
        .Where(a => !filter.Colors.Any() || filter.Colors.Contains(a.Color))
        .Where(a => filter.BirthDateStartRange == null || a.DateOfBirth >= filter.BirthDateStartRange)
        .Where(a => filter.BirthDateEndRange == null || a.DateOfBirth <= filter.BirthDateEndRange);

    result.AddRange(query);
    return result;
}

And then calling it like this

cats.Add(new Cat("Felix", "Black", DateTime.Today.AddDays(-1)));
cats.Add(new Cat("Garfield", "Orange", DateTime.Today.AddDays(-10)));

CatFilter filter = new CatFilter();
filter.Names.Add("Garfield");

List<Cat> result = GetFilteredCats(filter);

Answer 2

Correct way to do this is to make method GetFilteredCats , accept your filter and return correct cats throught LINQ composition:

IEnumerable<Cat> cats = //.. get all cats here

if (filter.FilterByColor)
   cats = cats.Where(c=>c.Color = filter.Color);

if (filter.FilterByName)
   cats = cats.Where(c=>c.Name = filter.Name);

if (filter.FilterByDate)
   cats = cats.Where(c=>c.Date > filter.FromDate && c.Date < filter.ToDate)

return cats.ToList(); // finally filter data and return them.

In case of performance. I don't think this can logicaly be done using different aproach. But this will become problem when you start hitting milions of cats. And at this point, database should be used instead. Those have clever indexing and clustering for your convenience.

Answer 3

Something like this should work, please note it's not tested

List<string> names = new List<string>();
            List<Color> colors = new List<Color>();
            List<DateTime> dobs = new List<DateTime>();

            List<cat> cats = new List<cat>();


            var filtered = from c in cats
                           join n in names on c.name equals n
                           join cl in colors on c.color equals cl
                           join db in dobs on c.dob equals db

                           select c;

You can also have some list with two dates, in which case you'll need to put a where condition, where c.dob <= date1 && c.dob >= date2, or something similar. Hope this helps.

Answer 4

You could use Expression trees. When a CatFilter object is passed to your GetFilteredCats method, based on the properties that are set on this object, you generate Expressions (ie seen in pseudo-code below) which you concatenate and use to build out a full LINQ query.

Something like:

Expression catFilter =
from cat in Cats
    where <Expression> and <Expression> and ...
select cat

Then simply compile (Expression.Compile) and execute.