Linq 内同一列上的多个条件，其中

Question

How can I write a linq query to match two condition on same column in the table?

Here one person can be assigned to multiple types of works and it is store in PersonWorkTypes table containing the details of persons and their worktypes.

So I need to get the list of persons who have both fulltime and freelance works.

I have tried

people.where(w => w.worktype == "freelance" && w.worktype == "fulltime")

But it returns an empty result.

Answer 1

You can try this

public class Person {
    public string Name {get;set;}
    public List<PersonWorkType> PersonWorkTypes {get;set;}
}

public class PersonWorkType {
    public string Type {get;set;}
}

public static void Main()
{
    var people = new List<Person>();
    var person = new Person { Name = "Toño", PersonWorkTypes = new List<PersonWorkType>() { new PersonWorkType { Type = "freelance" } } };
    var person2 = new Person { Name = "Aldo", PersonWorkTypes = new List<PersonWorkType>() { new PersonWorkType { Type = "freelance" }, new PersonWorkType { Type = "fulltime" } } };
    var person3 = new Person { Name = "John", PersonWorkTypes = new List<PersonWorkType>() { new PersonWorkType { Type = "freelance" }, new PersonWorkType { Type = "fulltime" } } };

    people.Add(person);
    people.Add(person2);        
    people.Add(person3);

    var filter = people.Where(p => p.PersonWorkTypes.Any(t => t.Type == "freelance") && p.PersonWorkTypes.Any(t => t.Type == "fulltime"));


    foreach(var item in filter) {
        Console.WriteLine(item.Name);
    }
}

This returns person that contains both types in PersonWorkTypes

Answer 2

w.worktype=="freelance"
w.worktype=="fulltime"

These are mutually exclusive to each other, and therefore cannot both be true to ever satisfy your AND(&&) operator.

I am inferring that you have two (or more) different rows in your table per person, one for each type of work they do. If so, the Where() method is going to check your list line-by-line individually and won't be able to check two different elements of a list to see if Alice (for example) both has en entry for "freelance" and an entry for "fulltime" as two different elements in the list. Unfortuantely, I can't think of an easy way to do this in a single query, but something like this might work:

var fulltimeWorkers = people.Where(w=>w.worktype=="fulltime");
var freelanceWorkers = people.Where(w=>w.worktype=="freelance");
List<Person> peopleWhoDoBoth = new List<Person>();
foreach (var worker in fulltimeWorkers)
{
   if (freelanceWorkers.Contains(worker)
      peopleWhoDoBoth.Add(worker);
}

This is probably not the most efficient way possible of doing it, but for small data sets, it shouldn't matter.

Answer 3

AS already said, && operator means, that BOTH conditions has to be met. So in your condition it means that you want worktype type to by freelance and fulltime at the same time, which is not possible:)

Most probably you want employees that have work type freelance OR fulltime , thus your condition should be:

people.Where(w=>w.worktype=="freelance" || w.worktype =="fulltime")

Or, if person can be set more than once in this table, then you could do:

people
  .Where(w=>w.worktype=="freelance" || w.worktype =="fulltime")
  // here I assume that you have name of a person,
  // Basically, here I group by person
  .GroupBy(p => p.Name)
  // Here we check if any person has two entries,
  // but you have to be careful here, as if person has two entries
  // with worktype freelance or two entries with fulltime, it
  // will pass condition as well.
  .Where(grp => grp.Count() == 2)
  .Select(grp => grp.FirstOrDefault());