Join
Learn to join two collections by key with Join.
Joining with Join
Join merges two sequences by a key, yielding only matching pairs (an inner join). It takes four parameters: the outer sequence, the inner sequence, the outer key selector, the inner key selector and the result selector.
Example: people.Join(cities, p => p.CityId, c => c.Id, (p, c) => ...). Each person arrives paired with the city whose Id matches. Items without a match do not appear in the result.
Inner join behavior
Join performs an inner join: only items with a match in both sequences appear in the result. Unmatched items are silently dropped.
The result order follows the outer sequence; for each outer item, matching inner items arrive in inner-sequence order. On large sequences Join builds a hash table and runs in O(n+m) time.
Exercise Join people with cities
Join the people and cities lists below on CityId/Id using Join. For each person print person name - city name with Console.WriteLine.
Show example solution
Merge with people.Join(cities, p => p.CityId, c => c.Id, (p, c) => p.Name + " - " + c.Name); then iterate the result with foreach.
Example solution:
using System;
using System.Collections.Generic;
using System.Linq;
class Person
{
public string Name { get; set; }
public int CityId { get; set; }
}
class City
{
public int Id { get; set; }
public string Name { get; set; }
}
class Program
{
static void Main()
{
var people = new List<Person>
{
new Person { Name = "Ayşe", CityId = 1 },
new Person { Name = "Ali", CityId = 2 }
};
var cities = new List<City>
{
new City { Id = 1, Name = "Istanbul" },
new City { Id = 2, Name = "Ankara" }
};
var result = people.Join(cities, p => p.CityId, c => c.Id, (p, c) => p.Name + " - " + c.Name);
foreach (var r in result)
{
Console.WriteLine(r);
}
}
}Test yourself
-
Complete the Join call
var result = people.____(cities, p => p.CityId, c => c.Id, (p, c) => p.Name);Show answer
var result = people.Join(cities, p => p.CityId, c => c.Id, (p, c) => p.Name); -
What does Join produce?
- Only the outer sequence
- Matching pairs
- All outer and inner items
- Groups
Show answer
Matching pairs — Correct. Join yields only matching pairs (inner join).
-
Match the parameter to its role
- Outer seq
- Left source
- Inner seq
- Right source
- Outer key
- Key of outer item
- Result selector
- What to produce from a pair