GroupJoin operator in C# using LINQ

GroupJoin operator works on two sequences or collections.It joins the sequences based on a key and returns group for the given key.

Following is the signature of groupjoin operator:

GroupJoin<Outer,Inner, Key, Result>(
this IEnumerable<Outer> outer,
IEnumerable<Inner> inner,
Func<TOuter, Key> outerKeySelector,
Func<TInner, Key> innerKeySelector,
Func<TOuter, Enumerable<TInner>, TResult> resultSelector
)

Following are the type parameters of the GroupJoin() method:

1. outer 1st collection
2. inner 2nd collection
3. outerKeySelector key of 1st collection
4. innerKeySelector key of 2nd collection
5. resultSelector result.The result of the GroupJoin method is of type IEnumerable<TResult>

The method is not executed until the result enumeration is iterated in a foreach collection.

In the following example we have 2 classes Manager and Employees.Each manager can have multiple employees reporting to him.We are grouping the Manager and his reportees based on the ManagerId key.

Class Manager

public class Manager
{
public int ManagerId { get; set; }
public string Name { get; set; }
}

Class Employees

public class Employees
{
public int ManagerId { get; set; }
public int Id { get; set; }
public string Name { get; set; }
}

Using outerJoin to group the 2 collections based on Manager Id:

class Program
{
static void Main(string[] args)
{
IList<Manager> managerlist = new List<Manager>() {
new Manager() {ManagerId=1,Name="Mnager1" },
new Manager() {ManagerId=2,Name="Mnager2" }
};

IList<Employees> employeelist = new List<Employees>() {
new Employees(){ Id = 1, Name="Standard 1",ManagerId=1},
new Employees(){ Id = 2, Name="Standard 2",ManagerId=1}
};

var groupJoin = managerlist.GroupJoin(employeelist, //inner sequence
manager => manager.ManagerId, //outerKey
employee => employee.ManagerId, //innerKey
(manager, employeeGroup) => new // result
{
employees = employeeGroup,
manager = manager.Name
});

foreach (var item in groupJoin)
{
Console.WriteLine(item.manager);

foreach (var emp in item.employees)
Console.WriteLine(emp.Name);
}

Console.ReadLine();

}
}

 

Follow on: