{TOC}
| Namespace: | System.Linq |
| Assembly: | System.Core.dll |
| Extends: | IEnumerable<T> |
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Standard Query Operator IndexEditIntroduction
The GroupJoin operator performs a grouped join of two sequences based on matching keys extracted from the elements.
EditMethod Signatures
// 1 - Group Join
public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
this IEnumerable<TOuter> outer,
IEnumerable<TInner> inner,
Func<TOuter, TKey> outerKeySelector,
Func<TInner, TKey> innerKeySelector,
Func<TOuter, IEnumerable<TInner>, TResult> resultSelector)
// New in January 2007 CTP
// 2 - Group Join using Custom Comparer
public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
this IEnumerable<TOuter> outer,
IEnumerable<TInner> inner,
Func<TOuter, TKey> outerKeySelector,
Func<TInner, TKey> innerKeySelector,
Func<TOuter, IEnumerable<TInner>, TResult> resultSelector,
IEqualityComparer<TKey> comparer)EditExceptions
Throws an ArgumentNullException if
outer,
inner,
outerKeySelector,
innerKeySelector or
resultCollector is null.
EditPseudo-code
TODO: Needs pseudo-code explanation.
EditLoop count
1. A complete loop of the inner sequence builds key groups. Groups returned by iteration pattern. This operator implements the standard
deferred execution iterator pattern. This means, no looping will occur until the result is iterated over.
EditCode Samples
In this example,
var q use the Group Join Query Expression syntax (notice the
into, a cross-join doesn't have this), and
var q1 implements the same code using a sub-query in the select projection. They return identical results.
public static void GroupJoinSimpleExample()
{
var customers = new List<Customer>() {
new Customer {Key = 1, Name = "Gottshall" },
new Customer {Key = 2, Name = "Valdes" },
new Customer {Key = 3, Name = "Gauwain" },
new Customer {Key = 4, Name = "Deane" },
new Customer {Key = 5, Name = "Zeeman" }
};
var orders = new List<Order>() {
new Order {Key = 1, OrderNumber = "Order 1" },
new Order {Key = 1, OrderNumber = "Order 2" },
new Order {Key = 4, OrderNumber = "Order 3" },
new Order {Key = 4, OrderNumber = "Order 4" },
new Order {Key = 5, OrderNumber = "Order 5" }
};
var q = from c in customers
join o in orders on c.Key equals o.Key into g
select new {CustomerName = c.Name, Orders = g};
var q1 = from c in customers
select new { CustomerName = c.Name,
Orders = from o in orders
where c.Key == o.Key
select o };
foreach (var group in q1) {
Console.WriteLine("Customer: {0}", group.CustomerName);
foreach (var order in group.Orders)
Console.WriteLine(" - {0}", order.OrderNumber);
}
Console.ReadLine();
}
public class Customer
{
public int Key;
public string Name;
}
public class Order
{
public int Key;
public string OrderNumber;
}
Customer: Gottshall
- Order 1
- Order 2
Customer: Valdes
Customer: Gauwain
Customer: Deane
- Order 3
- Order 4
Customer: Zeeman
- Order 5
EditVB Sample
In VB Group Join operator performs a grouped join of two collections based on matching keys extracted from the elements.
Public Sub Linq()
Dim procNames() As String = {"svc", "devenv", "winlogon", "explorer"}
Dim query = From process In procNames _
Group Join procs In System.Diagnostics.Process.GetProcesses() _
On procs.ProcessName Equals process _
Into Count()
Console.WriteLine("Result:")
For Each c In query
Console.WriteLine(c.Count)
Next
End SubResult may vary,
Result:
0
2
2
13
1