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Enumerable.Concat<TSource> Method (IEnumerable<TSource>, IEnumerable<TSource>)

 

Concatenates two sequences.

Namespace:   System.Linq
Assembly:  System.Core (in System.Core.dll)

<ExtensionAttribute>
Public Shared Function Concat(Of TSource) (
	first As IEnumerable(Of TSource),
	second As IEnumerable(Of TSource)
) As IEnumerable(Of TSource)

Parameters

first
Type: System.Collections.Generic.IEnumerable<TSource>

The first sequence to concatenate.

second
Type: System.Collections.Generic.IEnumerable<TSource>

The sequence to concatenate to the first sequence.

Return Value

Type: System.Collections.Generic.IEnumerable<TSource>

An IEnumerable<T> that contains the concatenated elements of the two input sequences.

Type Parameters

TSource

The type of the elements of the input sequences.

Exception Condition
ArgumentNullException

first or second is null.

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

The Concat<TSource>(IEnumerable<TSource>, IEnumerable<TSource>) method differs from the Union<TSource> method because the Concat<TSource>(IEnumerable<TSource>, IEnumerable<TSource>) method returns all the original elements in the input sequences. The Union<TSource> method returns only unique elements.

The following code example demonstrates how to use Concat<TSource>(IEnumerable<TSource>, IEnumerable<TSource>) to concatenate two sequences.

Structure Pet
    Public Name As String
    Public Age As Integer
End Structure

' Returns an array of Pet objects.
Function GetCats() As Pet()
    Dim cats() As Pet = {New Pet With {.Name = "Barley", .Age = 8}, _
                         New Pet With {.Name = "Boots", .Age = 4}, _
                         New Pet With {.Name = "Whiskers", .Age = 1}}

    Return cats
End Function

' Returns an array of Pet objects.
Function GetDogs() As Pet()
    Dim dogs() As Pet = {New Pet With {.Name = "Bounder", .Age = 3}, _
                         New Pet With {.Name = "Snoopy", .Age = 14}, _
                         New Pet With {.Name = "Fido", .Age = 9}}
    Return dogs
End Function

Sub ConcatEx1()
    ' Create two arrays of Pet objects.
    Dim cats() As Pet = GetCats()
    Dim dogs() As Pet = GetDogs()

    ' Project the Name of each cat and concatenate
    ' the collection of cat name strings with a collection
    ' of dog name strings.
    Dim query As IEnumerable(Of String) = _
        cats _
        .Select(Function(cat) cat.Name) _
        .Concat(dogs.Select(Function(dog) dog.Name))

    Dim output As New System.Text.StringBuilder
    For Each name As String In query
        output.AppendLine(name)
    Next

    ' Display the output.
    MsgBox(output.ToString())
End Sub

' This code produces the following output:
'
' Barley
' Boots
' Whiskers
' Bounder
' Snoopy
' Fido

An alternative way of concatenating two sequences is to construct a collection, for example an array, of sequences and then apply the SelectMany<TSource, TResult> method, passing it the identity selector function. The following example demonstrates this use of SelectMany<TSource, TResult>.

' Create two arrays of Pet objects.
Dim cats() As Pet = GetCats()
Dim dogs() As Pet = GetDogs()

' Create an IEnumerable collection that contains two elements.
' Each element is an array of Pet objects.
Dim animals() As IEnumerable(Of Pet) = {cats, dogs}

Dim query As IEnumerable(Of String) = _
    (animals.SelectMany(Function(pets) _
                            pets.Select(Function(pet) pet.Name)))

Dim output As New System.Text.StringBuilder
For Each name As String In query
    output.AppendLine(name)
Next

' Display the output.
MsgBox(output.ToString())

' This code produces the following output:
'
' Barley
' Boots
' Whiskers
' Bounder
' Snoopy
' Fido

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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