Concat items in linq

I have the following code:

class Person
{
    public String Name { get; set; }
    public String LastName { get; set; }
    public String City { get; set; }

    public Person(String name, String lastName, String city)
    {
        Name = name;
        LastName = lastName;
        City = city;
    }
}

...

personList.Add(new Person("a", "b", "1"));
personList.Add(new Person("c", "d", "1"));
personList.Add(new Person("e", "f", "2"));
personList.Add(new Person("g", "h", "1"));
personList.Add(new Person("i", "j", "2"));
personList.Add(new Per开发者_如何学Goson("k", "l", "1"));
personList.Add(new Person("m", "n", "3"));
personList.Add(new Person("o", "p", "3"));
personList.Add(new Person("q", "r", "4"));
personList.Add(new Person("s", "t", "5"));

So then I want to group the list by Cities, and I do the following;

var result = personList.GroupBy(x => x.City);

But now what I want to do is to concatenate the items that have 1 or 3 as a City (it can be specified dynamically)

Example:

The first item on result would return an array of persons that contain cities 1, 3

Thanks!

You can just use a Where() filter and project each remaining group into an array using ToArray():

var result = personList.GroupBy(x => x.City)
                       .Where ( g => g.Key == someCity || g.Key == anotherCity)
                       .Select( g => g.ToArray());

First build your list of cities that you want to search against

List<int> citesToFind = new List<int>();
// add 1, 3, etc to this list (can be generated dyamically)

Then use .Contains() in your LINQ:

var result = from person in personList
             where citiesToFind.Contains(person.City)
             select person;

Of course you can add whatever other grouping or filtering you want, but using .Contains() is I think the essential part you're missing.

How about the following? You could stick it in an extension method if you want to make the usage neater.

var personDictionary = new Dictionary<string, List<Person>>();
foreach(var person in personList)
{
  if (personDictionary.HasKey(person.City)
  {
    personDictionary[person.City].Add(person);
  }
  else
  {
    personDictionary[person.City] = new List<Person>{person};
  }
}

You can then query the personDictionary for the people from any city of your choosing.