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Enumerable.Union(Of TSource) Method (IEnumerable(Of TSource), IEnumerable(Of TSource))

Produces the set union of two sequences by using the default equality comparer.

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

'Declaration
<ExtensionAttribute> _
Public Shared Function Union(Of TSource) ( _
	first As IEnumerable(Of TSource), _
	second As IEnumerable(Of TSource) _
) As IEnumerable(Of TSource)

Type Parameters

TSource

The type of the elements of the input sequences.

Parameters

first
Type: System.Collections.Generic.IEnumerable(Of TSource)
An IEnumerable(Of T) whose distinct elements form the first set for the union.
second
Type: System.Collections.Generic.IEnumerable(Of TSource)
An IEnumerable(Of T) whose distinct elements form the second set for the union.

Return Value

Type: System.Collections.Generic.IEnumerable(Of TSource)
An IEnumerable(Of T) that contains the elements from both input sequences, excluding duplicates.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable(Of TSource). When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

ExceptionCondition
ArgumentNullException

first or second is Nothing.

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.

This method excludes duplicates from the return set. This is different behavior to the Concat(Of TSource) method, which returns all the elements in the input sequences including duplicates.

The default equality comparer, Default, is used to compare values of the types that implement the IEqualityComparer(Of T) generic interface. To compare a custom data type, you need to implement this interface and provide your own GetHashCode and Equals methods for the type.

When the object returned by this method is enumerated, Union enumerates first and second in that order and yields each element that has not already been yielded.

The following code example demonstrates how to use Union(Of TSource)(IEnumerable(Of TSource), IEnumerable(Of TSource)) to obtain the union of two sequences of integers.


        ' Create two arrays of integer values.
        Dim ints1() As Integer = {5, 3, 9, 7, 5, 9, 3, 7}
        Dim ints2() As Integer = {8, 3, 6, 4, 4, 9, 1, 0}

        ' Get the set union of the two arrays.
        Dim union As IEnumerable(Of Integer) = ints1.Union(ints2)

        ' Display the resulting set's values.
        Dim output As New System.Text.StringBuilder
        For Each num As Integer In union
            output.AppendLine(num & " ")
        Next
        MsgBox(output.ToString())

        ' This code produces the following output:
        '
        ' 5 
        ' 3 
        ' 9 
        ' 7 
        ' 8 
        ' 6 
        ' 4 
        ' 1 
        ' 0 



If you want to compare sequences of objects of a custom data type, you have to implement the IEqualityComparer(Of T) generic interface in the class. The following code example shows how to implement this interface in a custom data type and provide GetHashCode and Equals methods.


Public Class Product
    Implements IEquatable(Of Product)

    Public Property Name As String
    Public Property Code As Integer

    Public Function Equals1(
        ByVal other As Product
        ) As Boolean Implements IEquatable(Of Product).Equals

        ' Check whether the compared object is null.
        If other Is Nothing Then Return False

        ' Check whether the compared object references the same data.
        If Me Is Other Then Return True

        ' Check whether the products' properties are equal.
        Return Code.Equals(other.Code) AndAlso Name.Equals(other.Name)
    End Function

    Public Overrides Function GetHashCode() As Integer

        ' Get hash code for the Name field if it is not null.
        Dim hashProductName = If(Name Is Nothing, 0, Name.GetHashCode())

        ' Get hash code for the Code field.
        Dim hashProductCode = Code.GetHashCode()

        ' Calculate the hash code for the product.
        Return hashProductName Xor hashProductCode
    End Function
End Class



After you implement this interface, you can use sequences of Product objects in the Union(Of TSource)(IEnumerable(Of TSource), IEnumerable(Of TSource)) method, as shown in the following example.


Dim store1() As Product = 
    {New Product With {.Name = "apple", .Code = 9}, 
     New Product With {.Name = "orange", .Code = 4}}

Dim store2() As Product = 
    {New Product With {.Name = "apple", .Code = 9}, 
     New Product With {.Name = "lemon", .Code = 12}}


...


' Get the products from the both arrays
' excluding duplicates.

Dim union = store1.Union(store2)

For Each product In union
    Console.WriteLine(product.Name & " " & product.Code)
Next

' This code produces the following output:
'
' apple 9
' orange 4
' lemon 12
' 


.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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