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

 

Produces the set union of two sequences by using a specified IEqualityComparer(Of T).

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

<ExtensionAttribute>
Public Shared Function Union(Of TSource) (
	first As IEnumerable(Of TSource),
	second As IEnumerable(Of TSource),
	comparer As IEqualityComparer(Of TSource)
) As IEnumerable(Of TSource)

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.

comparer
Type: System.Collections.Generic.IEqualityComparer(Of TSource)

The IEqualityComparer(Of T) to compare values.

Return Value

Type: System.Collections.Generic.IEnumerable(Of TSource)

An IEnumerable(Of T) that contains the elements from both input sequences, excluding duplicates.

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.

If comparer is null, the default equality comparer, Default, is used to compare values.

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

The Concat(Of TSource) method differs from the Union(Of TSource) method because the Concat(Of TSource) method returns all the elements in the input sequences including duplicates, whereas Union(Of TSource) returns only unique values.

The following example shows how to implement an equality comparer that can be used in the Union(Of TSource) method.

Public Class Product
    Public Property Name As String
    Public Property Code As Integer
End Class

' Custom comparer for the Product class
Public Class ProductComparer
    Implements IEqualityComparer(Of Product)

    Public Function Equals1(
        ByVal x As Product, 
        ByVal y As Product
        ) As Boolean Implements IEqualityComparer(Of Product).Equals

        ' Check whether the compared objects reference the same data.
        If x Is y Then Return True

        'Check whether any of the compared objects is null.
        If x Is Nothing OrElse y Is Nothing Then Return False

        ' Check whether the products' properties are equal.
        Return (x.Code = y.Code) AndAlso (x.Name = y.Name)
    End Function

    Public Function GetHashCode1(
        ByVal product As Product
        ) As Integer Implements IEqualityComparer(Of Product).GetHashCode

        ' Check whether the object is null.
        If product Is Nothing Then Return 0

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

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

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

After you implement this comparer, you can use sequences of Product objects in the Union(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, New ProductComparer())

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

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

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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