• Article

Enumerable.Concat<TSource> Method

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Concatenates two sequences.

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

Syntax

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

public static IEnumerable<TSource> Concat<TSource>(
    this IEnumerable<TSource> first,
    IEnumerable<TSource> second
)

Type Parameters

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

Parameters

Return Value

Type: System.Collections.Generic.IEnumerable<TSource>
An IEnumerable<T> that contains the concatenated elements of the two input sequences.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<TSource>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception Condition
ArgumentNullException

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

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.

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

Examples

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.
      outputBlock.Text &= output.ToString() & vbCrLf
   End Sub

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


      class Pet
      {
         public string Name { get; set; }
         public int Age { get; set; }
      }

      static Pet[] GetCats()
      {
         Pet[] cats = { new Pet { Name="Barley", Age=8 },
                            new Pet { Name="Boots", Age=4 },
                            new Pet { Name="Whiskers", Age=1 } };
         return cats;
      }

      static Pet[] GetDogs()
      {
         Pet[] dogs = { new Pet { Name="Bounder", Age=3 },
                            new Pet { Name="Snoopy", Age=14 },
                            new Pet { Name="Fido", Age=9 } };
         return dogs;
      }

      public static void ConcatEx1()
      {
         Pet[] cats = GetCats();
         Pet[] dogs = GetDogs();

         IEnumerable<string> query =
             cats.Select(cat => cat.Name).Concat(dogs.Select(dog => dog.Name));

         foreach (string name in query)
         {
            outputBlock.Text += name + "\n";
         }
      }

      // 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 method, passing it the identity selector function. The following example demonstrates this use of SelectMany.

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

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


         Pet[] cats = GetCats();
         Pet[] dogs = GetDogs();

         IEnumerable<string> query =
             new[] { cats.Select(cat => cat.Name), dogs.Select(dog => dog.Name) }
             .SelectMany(name => name);

         foreach (string name in query)
         {
            outputBlock.Text += name + "\n";
         }

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

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.