LINQ inner join

Question

I have two collections:

List<int> ids;
List<User> users;

Where User has id, name, etc.

I would like to inner join these two collections and return a new List<int> with ids from the first collection which are also in the second collection (User id's).

I am new to LINQ and don't know where to start.

Thanks.

Answer 1

You don't need to use join to do this:

List<int> commonIds = ids.Intersect(users.Select(u => u.Id)).ToList();

EDIT: In response to the question in the comments, you could get a list of users without using Join:

var matchingUsers = users.Where(u => ids.Contains(u.Id));

However this is quite inefficient since the Where clause has to scan the list of ids for each user. I think Join would be the best way to handle this case:

List<User> matchingUsers = users.Join(ids, u => u.Id, id => id, (user, id) => user).ToList();

Answer 2

Copied from Microsoft docs:

In relational database terms, an inner join produces a result set in which each element of the first collection appears one time for every matching element in the second collection. If an element in the first collection has no matching elements, it does not appear in the result set. The Join method, which is called by the join clause in C#, implements an inner join.

This topic shows you how to perform four variations of an inner join:

• A simple inner join that correlates elements from two data sources based on a simple key.

• An inner join that correlates elements from two data sources based on a composite key. A composite key, which is a key that consists of more than one value, enables you to correlate elements based on more than one property.

• A multiple join in which successive join operations are appended to each other.

• An inner join that is implemented by using a group join.

Example Simple Key Join Example

The following example creates two collections that contain objects of two user-defined types, Person and Pet. The query uses the join clause in C# to match Person objects with Pet objects whose Owner is that Person. The select clause in C# defines how the resulting objects will look. In this example the resulting objects are anonymous types that consist of the owner's first name and the pet's name. C#

class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

class Pet
{
    public string Name { get; set; }
    public Person Owner { get; set; }
}

/// <summary>
/// Simple inner join.
/// </summary>
public static void InnerJoinExample()
{
    Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
    Person terry = new Person { FirstName = "Terry", LastName = "Adams" };
    Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };
    Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" };
    Person rui = new Person { FirstName = "Rui", LastName = "Raposo" };

    Pet barley = new Pet { Name = "Barley", Owner = terry };
    Pet boots = new Pet { Name = "Boots", Owner = terry };
    Pet whiskers = new Pet { Name = "Whiskers", Owner = charlotte };
    Pet bluemoon = new Pet { Name = "Blue Moon", Owner = rui };
    Pet daisy = new Pet { Name = "Daisy", Owner = magnus };

    // Create two lists.
    List<Person> people = new List<Person> { magnus, terry, charlotte, arlene, rui };
    List<Pet> pets = new List<Pet> { barley, boots, whiskers, bluemoon, daisy };

    // Create a collection of person-pet pairs. Each element in the collection
    // is an anonymous type containing both the person's name and their pet's name.
    var query = from person in people
                join pet in pets on person equals pet.Owner
                select new { OwnerName = person.FirstName, PetName = pet.Name };

    foreach (var ownerAndPet in query)
    {
        Console.WriteLine("\"{0}\" is owned by {1}", ownerAndPet.PetName, ownerAndPet.OwnerName);
    }
}

// This code produces the following output:
//
// "Daisy" is owned by Magnus
// "Barley" is owned by Terry
// "Boots" is owned by Terry
// "Whiskers" is owned by Charlotte
// "Blue Moon" is owned by Rui

Note that the Person object whose LastName is "Huff" does not appear in the result set because there is no Pet object that has Pet.Owner equal to that Person. Example Composite Key Join Example

Instead of correlating elements based on just one property, you can use a composite key to compare elements based on multiple properties. To do this, specify the key selector function for each collection to return an anonymous type that consists of the properties you want to compare. If you label the properties, they must have the same label in each key's anonymous type. The properties must also appear in the same order.

The following example uses a list of Employee objects and a list of Student objects to determine which employees are also students. Both of these types have a FirstName and a LastName property of type String. The functions that create the join keys from each list's elements return an anonymous type that consists of the FirstName and LastName properties of each element. The join operation compares these composite keys for equality and returns pairs of objects from each list where both the first name and the last name match. C#

class Employee
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public int EmployeeID { get; set; }
}

class Student
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public int StudentID { get; set; }
}

/// <summary>
/// Performs a join operation using a composite key.
/// </summary>
public static void CompositeKeyJoinExample()
{
    // Create a list of employees.
    List<Employee> employees = new List<Employee> {
        new Employee { FirstName = "Terry", LastName = "Adams", EmployeeID = 522459 },
         new Employee { FirstName = "Charlotte", LastName = "Weiss", EmployeeID = 204467 },
         new Employee { FirstName = "Magnus", LastName = "Hedland", EmployeeID = 866200 },
         new Employee { FirstName = "Vernette", LastName = "Price", EmployeeID = 437139 } };

    // Create a list of students.
    List<Student> students = new List<Student> {
        new Student { FirstName = "Vernette", LastName = "Price", StudentID = 9562 },
        new Student { FirstName = "Terry", LastName = "Earls", StudentID = 9870 },
        new Student { FirstName = "Terry", LastName = "Adams", StudentID = 9913 } };

    // Join the two data sources based on a composite key consisting of first and last name,
    // to determine which employees are also students.
    IEnumerable<string> query = from employee in employees
                                join student in students
                                on new { employee.FirstName, employee.LastName }
                                equals new { student.FirstName, student.LastName }
                                select employee.FirstName + " " + employee.LastName;

    Console.WriteLine("The following people are both employees and students:");
    foreach (string name in query)
        Console.WriteLine(name);
}

// This code produces the following output:
//
// The following people are both employees and students:
// Terry Adams
// Vernette Price

Example Multiple Join Example

Any number of join operations can be appended to each other to perform a multiple join. Each join clause in C# correlates a specified data source with the results of the previous join.

The following example creates three collections: a list of Person objects, a list of Cat objects, and a list of Dog objects.

The first join clause in C# matches people and cats based on a Person object matching Cat.Owner. It returns a sequence of anonymous types that contain the Person object and Cat.Name.

The second join clause in C# correlates the anonymous types returned by the first join with Dog objects in the supplied list of dogs, based on a composite key that consists of the Owner property of type Person, and the first letter of the animal's name. It returns a sequence of anonymous types that contain the Cat.Name and Dog.Name properties from each matching pair. Because this is an inner join, only those objects from the first data source that have a match in the second data source are returned. C#

class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

class Pet
{
    public string Name { get; set; }
    public Person Owner { get; set; }
}

class Cat : Pet
{ }

class Dog : Pet
{ }

public static void MultipleJoinExample()
{
    Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
    Person terry = new Person { FirstName = "Terry", LastName = "Adams" };
    Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };
    Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" };
    Person rui = new Person { FirstName = "Rui", LastName = "Raposo" };
    Person phyllis = new Person { FirstName = "Phyllis", LastName = "Harris" };

    Cat barley = new Cat { Name = "Barley", Owner = terry };
    Cat boots = new Cat { Name = "Boots", Owner = terry };
    Cat whiskers = new Cat { Name = "Whiskers", Owner = charlotte };
    Cat bluemoon = new Cat { Name = "Blue Moon", Owner = rui };
    Cat daisy = new Cat { Name = "Daisy", Owner = magnus };

    Dog fourwheeldrive = new Dog { Name = "Four Wheel Drive", Owner = phyllis };
    Dog duke = new Dog { Name = "Duke", Owner = magnus };
    Dog denim = new Dog { Name = "Denim", Owner = terry };
    Dog wiley = new Dog { Name = "Wiley", Owner = charlotte };
    Dog snoopy = new Dog { Name = "Snoopy", Owner = rui };
    Dog snickers = new Dog { Name = "Snickers", Owner = arlene };

    // Create three lists.
    List<Person> people =
        new List<Person> { magnus, terry, charlotte, arlene, rui, phyllis };
    List<Cat> cats =
        new List<Cat> { barley, boots, whiskers, bluemoon, daisy };
    List<Dog> dogs =
        new List<Dog> { fourwheeldrive, duke, denim, wiley, snoopy, snickers };

    // The first join matches Person and Cat.Owner from the list of people and
    // cats, based on a common Person. The second join matches dogs whose names start
    // with the same letter as the cats that have the same owner.
    var query = from person in people
                join cat in cats on person equals cat.Owner
                join dog in dogs on 
                new { Owner = person, Letter = cat.Name.Substring(0, 1) }
                equals new { dog.Owner, Letter = dog.Name.Substring(0, 1) }
                select new { CatName = cat.Name, DogName = dog.Name };

    foreach (var obj in query)
    {
        Console.WriteLine(
            "The cat \"{0}\" shares a house, and the first letter of their name, with \"{1}\".", 
            obj.CatName, obj.DogName);
    }
}

// This code produces the following output:
//
// The cat "Daisy" shares a house, and the first letter of their name, with "Duke".
// The cat "Whiskers" shares a house, and the first letter of their name, with "Wiley".

Example Inner Join by using Grouped Join Example

The following example shows you how to implement an inner join by using a group join.

In query1, the list of Person objects is group-joined to the list of Pet objects based on the Person matching the Pet.Owner property. The group join creates a collection of intermediate groups, where each group consists of a Person object and a sequence of matching Pet objects.

By adding a second from clause to the query, this sequence of sequences is combined (or flattened) into one longer sequence. The type of the elements of the final sequence is specified by the select clause. In this example, that type is an anonymous type that consists of the Person.FirstName and Pet.Name properties for each matching pair.

The result of query1 is equivalent to the result set that would have been obtained by using the join clause without the into clause to perform an inner join. The query2 variable demonstrates this equivalent query. C#

class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

class Pet
{
    public string Name { get; set; }
    public Person Owner { get; set; }
}

/// <summary>
/// Performs an inner join by using GroupJoin().
/// </summary>
public static void InnerGroupJoinExample()
{
    Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
    Person terry = new Person { FirstName = "Terry", LastName = "Adams" };
    Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };
    Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" };

    Pet barley = new Pet { Name = "Barley", Owner = terry };
    Pet boots = new Pet { Name = "Boots", Owner = terry };
    Pet whiskers = new Pet { Name = "Whiskers", Owner = charlotte };
    Pet bluemoon = new Pet { Name = "Blue Moon", Owner = terry };
    Pet daisy = new Pet { Name = "Daisy", Owner = magnus };

    // Create two lists.
    List<Person> people = new List<Person> { magnus, terry, charlotte, arlene };
    List<Pet> pets = new List<Pet> { barley, boots, whiskers, bluemoon, daisy };

    var query1 = from person in people
                 join pet in pets on person equals pet.Owner into gj
                 from subpet in gj
                 select new { OwnerName = person.FirstName, PetName = subpet.Name };

    Console.WriteLine("Inner join using GroupJoin():");
    foreach (var v in query1)
    {
        Console.WriteLine("{0} - {1}", v.OwnerName, v.PetName);
    }

    var query2 = from person in people
                 join pet in pets on person equals pet.Owner
                 select new { OwnerName = person.FirstName, PetName = pet.Name };

    Console.WriteLine("\nThe equivalent operation using Join():");
    foreach (var v in query2)
        Console.WriteLine("{0} - {1}", v.OwnerName, v.PetName);
}

// This code produces the following output:
//
// Inner join using GroupJoin():
// Magnus - Daisy
// Terry - Barley
// Terry - Boots
// Terry - Blue Moon
// Charlotte - Whiskers
//
// The equivalent operation using Join():
// Magnus - Daisy
// Terry - Barley
// Terry - Boots
// Terry - Blue Moon
// Charlotte - Whiskers

Compiling the Code

• Create a new Console Application project in Visual Studio.

• Add a reference to System.Core.dll if it is not already referenced.

• Include the System.Linq namespace.

• Copy and paste the code from the example into the program.cs file, below the Main method. Add a line of code to the Main method to call the method you pasted in.

• Run the program.