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.NET Framework Class Library
EnumerableUnionTSource Method (IEnumerableTSource, IEnumerableTSource, IEqualityComparerTSource)

Produces the set union of two sequences by using a specified IEqualityComparerT.

Namespace:   System.Linq
Assembly:  System.Core (in System.Core.dll)
Syntax
<[%$TOPIC/bb358407_en-us_VS_110_2_0_0_0_0%]> _
Public Shared Function Union(Of TSource) ( _
	first As [%$TOPIC/bb358407_en-us_VS_110_2_0_0_0_1%](Of TSource), _
	second As [%$TOPIC/bb358407_en-us_VS_110_2_0_0_0_2%](Of TSource), _
	comparer As [%$TOPIC/bb358407_en-us_VS_110_2_0_0_0_3%](Of TSource) _
) As [%$TOPIC/bb358407_en-us_VS_110_2_0_0_0_4%](Of TSource)
public static [%$TOPIC/bb358407_en-us_VS_110_2_0_1_0_0%]<TSource> Union<TSource>(
	this [%$TOPIC/bb358407_en-us_VS_110_2_0_1_0_1%]<TSource> first,
	[%$TOPIC/bb358407_en-us_VS_110_2_0_1_0_2%]<TSource> second,
	[%$TOPIC/bb358407_en-us_VS_110_2_0_1_0_3%]<TSource> comparer
)
[[%$TOPIC/bb358407_en-us_VS_110_2_0_2_0_0%]]
public:
generic<typename TSource>
static [%$TOPIC/bb358407_en-us_VS_110_2_0_2_0_1%]<TSource>^ Union(
	[%$TOPIC/bb358407_en-us_VS_110_2_0_2_0_2%]<TSource>^ first, 
	[%$TOPIC/bb358407_en-us_VS_110_2_0_2_0_3%]<TSource>^ second, 
	[%$TOPIC/bb358407_en-us_VS_110_2_0_2_0_4%]<TSource>^ comparer
)
static member Union : 
        first:[%$TOPIC/bb358407_en-us_VS_110_2_0_3_0_0%]<'TSource> * 
        second:[%$TOPIC/bb358407_en-us_VS_110_2_0_3_0_1%]<'TSource> * 
        comparer:[%$TOPIC/bb358407_en-us_VS_110_2_0_3_0_2%]<'TSource> -> [%$TOPIC/bb358407_en-us_VS_110_2_0_3_0_3%]<'TSource> 

Type Parameters

TSource

The type of the elements of the input sequences.

Parameters

first
Type: System.Collections.GenericIEnumerableTSource

An IEnumerableT whose distinct elements form the first set for the union.

second
Type: System.Collections.GenericIEnumerableTSource

An IEnumerableT whose distinct elements form the second set for the union.

comparer
Type: System.Collections.GenericIEqualityComparerTSource

The IEqualityComparerT to compare values.

Return Value

Type: System.Collections.GenericIEnumerableTSource
An IEnumerableT 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 IEnumerableTSource. 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).
Exceptions
ExceptionCondition
ArgumentNullException

first or second is .

Remarks

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

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 ConcatTSource method differs from the Union method because the ConcatTSource method returns all the elements in the input sequences including duplicates, whereas Union returns only unique values.

Examples

The following example shows how to implement an equality comparer that can be used in the Union 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
public class Product
{
    public string Name { get; set; }
    public int Code { get; set; }
}

// Custom comparer for the Product class 
class ProductComparer : IEqualityComparer<Product>
{
    // Products are equal if their names and product numbers are equal. 
    public bool Equals(Product x, Product y)
    {

        //Check whether the compared objects reference the same data. 
        if (Object.ReferenceEquals(x, y)) return true;

        //Check whether any of the compared objects is null. 
        if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
            return false;

        //Check whether the products' properties are equal. 
        return x.Code == y.Code && x.Name == y.Name;
    }

    // If Equals() returns true for a pair of objects  
    // then GetHashCode() must return the same value for these objects. 

    public int GetHashCode(Product product)
    {
        //Check whether the object is null 
        if (Object.ReferenceEquals(product, null)) return 0;

        //Get hash code for the Name field if it is not null. 
        int hashProductName = product.Name == null ? 0 : product.Name.GetHashCode();

        //Get hash code for the Code field. 
        int hashProductCode = product.Code.GetHashCode();

        //Calculate the hash code for the product. 
        return hashProductName ^ hashProductCode;
    }

}

After you implement this comparer, you can use sequences of Product objects in the Union 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 
' 
Product[] store1 = { new Product { Name = "apple", Code = 9 }, 
                       new Product { Name = "orange", Code = 4 } };

Product[] store2 = { new Product { Name = "apple", Code = 9 }, 
                       new Product { Name = "lemon", Code = 12 } };


...


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

IEnumerable<Product> union =
  store1.Union(store2, new ProductComparer());

foreach (var product in union)
    Console.WriteLine(product.Name + " " + product.Code);

/*
    This code produces the following output:

    apple 9
    orange 4
    lemon 12
*/
Version Information

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1
Platforms

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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