CapyKit/Documentation/Help/M_CapyKit_Extensions_LINQExtensions_LeftOuterJoin__4.md

# LeftOuterJoin<T, U, TKey, R>(IEnumerable<T>, IEnumerable<U>, Func<T, TKey>, Func<U, TKey>, Func<T, IEnumerable<U>, R>, Func<T, U>) Method


An IEnumable<T> extension method that left outer join.



## Definition
**Namespace:** <a href="N_CapyKit_Extensions">CapyKit.Extensions</a>  
**Assembly:** CapyKit (in CapyKit.dll) Version: 1.0.0+6cdd805be49c3b769a116584ea6904955ecd820d

**C#**
``` C#
public static IEnumerable<R> LeftOuterJoin<T, U, TKey, R>(
	this IEnumerable<T> source,
	IEnumerable<U> inner,
	Func<T, TKey> outerSelector,
	Func<U, TKey> innerSelector,
	Func<T, IEnumerable<U>, R> resultSelector,
	Func<T, U> defaultGenerator = null
)

```
**VB**
``` VB
<ExtensionAttribute>
Public Shared Function LeftOuterJoin(Of T, U, TKey, R) ( 
	source As IEnumerable(Of T),
	inner As IEnumerable(Of U),
	outerSelector As Func(Of T, TKey),
	innerSelector As Func(Of U, TKey),
	resultSelector As Func(Of T, IEnumerable(Of U), R),
	Optional defaultGenerator As Func(Of T, U) = Nothing
) As IEnumerable(Of R)
```
**C++**
``` C++
public:
[ExtensionAttribute]
generic<typename T, typename U, typename TKey, typename R>
static IEnumerable<R>^ LeftOuterJoin(
	IEnumerable<T>^ source, 
	IEnumerable<U>^ inner, 
	Func<T, TKey>^ outerSelector, 
	Func<U, TKey>^ innerSelector, 
	Func<T, IEnumerable<U>^, R>^ resultSelector, 
	Func<T, U>^ defaultGenerator = nullptr
)
```
**F#**
``` F#
[<ExtensionAttribute>]
static member LeftOuterJoin : 
        source : IEnumerable<'T> * 
        inner : IEnumerable<'U> * 
        outerSelector : Func<'T, 'TKey> * 
        innerSelector : Func<'U, 'TKey> * 
        resultSelector : Func<'T, IEnumerable<'U>, 'R> * 
        ?defaultGenerator : Func<'T, 'U> 
(* Defaults:
        let _defaultGenerator = defaultArg defaultGenerator null
*)
-> IEnumerable<'R> 
```



#### Parameters
<dl><dt>  <a href="https://learn.microsoft.com/dotnet/api/system.collections.generic.ienumerable-1" target="_blank" rel="noopener noreferrer">IEnumerable</a>(T)</dt><dd>The source to act on.</dd><dt>  <a href="https://learn.microsoft.com/dotnet/api/system.collections.generic.ienumerable-1" target="_blank" rel="noopener noreferrer">IEnumerable</a>(U)</dt><dd>The inner.</dd><dt>  <a href="https://learn.microsoft.com/dotnet/api/system.func-2" target="_blank" rel="noopener noreferrer">Func</a>(T, TKey)</dt><dd>The outer selector.</dd><dt>  <a href="https://learn.microsoft.com/dotnet/api/system.func-2" target="_blank" rel="noopener noreferrer">Func</a>(U, TKey)</dt><dd>The inner selector.</dd><dt>  <a href="https://learn.microsoft.com/dotnet/api/system.func-3" target="_blank" rel="noopener noreferrer">Func</a>(T, <a href="https://learn.microsoft.com/dotnet/api/system.collections.generic.ienumerable-1" target="_blank" rel="noopener noreferrer">IEnumerable</a>(U), R)</dt><dd>The result selector.</dd><dt>  <a href="https://learn.microsoft.com/dotnet/api/system.func-2" target="_blank" rel="noopener noreferrer">Func</a>(T, U)  (Optional)</dt><dd>(Optional) The default generator.</dd></dl>

#### Type Parameters
<dl><dt /><dd>Generic type parameter.</dd><dt /><dd>Generic type parameter.</dd><dt /><dd>Type of the key.</dd><dt /><dd>Type of the r.</dd></dl>

#### Return Value
<a href="https://learn.microsoft.com/dotnet/api/system.collections.generic.ienumerable-1" target="_blank" rel="noopener noreferrer">IEnumerable</a>(R)  
An enumerator that allows foreach to be used to process left outter join in this collection.

#### Usage Note
In Visual Basic and C#, you can call this method as an instance method on any object of type <a href="https://learn.microsoft.com/dotnet/api/system.collections.generic.ienumerable-1" target="_blank" rel="noopener noreferrer">IEnumerable</a>(T). When you use instance method syntax to call this method, omit the first parameter. For more information, see <a href="https://docs.microsoft.com/dotnet/visual-basic/programming-guide/language-features/procedures/extension-methods" target="_blank" rel="noopener noreferrer">

Extension Methods (Visual Basic)</a> or <a href="https://docs.microsoft.com/dotnet/csharp/programming-guide/classes-and-structs/extension-methods" target="_blank" rel="noopener noreferrer">

Extension Methods (C# Programming Guide)</a>.

## See Also


#### Reference
<a href="T_CapyKit_Extensions_LINQExtensions">LINQExtensions Class</a>  
<a href="Overload_CapyKit_Extensions_LINQExtensions_LeftOuterJoin">LeftOuterJoin Overload</a>  
<a href="N_CapyKit_Extensions">CapyKit.Extensions Namespace</a>