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

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

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

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) _
) As IEnumerable(Of TSource)
public static IEnumerable<TSource> Union<TSource>(
    this IEnumerable<TSource> first,
    IEnumerable<TSource> second

Type Parameters

  • TSource
    The type of the elements of the input sequences.


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.


Exception Condition

first or second is nullNothingnullptra null reference (Nothing in Visual Basic).


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 & " ")
      outputBlock.Text &= output.ToString() & vbCrLf

      ' This code produces the following output:
      ' 5 
      ' 3 
      ' 9 
      ' 7 
      ' 8 
      ' 6 
      ' 4 
      ' 1 
      ' 0 
      int[] ints1 = { 5, 3, 9, 7, 5, 9, 3, 7 };
      int[] ints2 = { 8, 3, 6, 4, 4, 9, 1, 0 };

      IEnumerable<int> union = ints1.Union(ints2);

      foreach (int num in union)
         outputBlock.Text += String.Format("{0} ", num);

       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.

Version Information

Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0


Windows Phone

See Also


Enumerable Class

Union Overload

System.Linq Namespace