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Enumerable.Count<TSource> Method (IEnumerable<TSource>, Func<TSource, Boolean>)

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Returns a number that represents how many elements in the specified sequence satisfy a condition.

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

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function Count(Of TSource) ( _
    source As IEnumerable(Of TSource), _
    predicate As Func(Of TSource, Boolean) _
) As Integer

public static int Count<TSource>(
    this IEnumerable<TSource> source,
    Func<TSource, bool> predicate
)

Type Parameters

  • TSource
    The type of the elements of source.

Parameters

  • predicate
    Type: System.Func<TSource, Boolean>
    A function to test each element for a condition.

Return Value

Type: System.Int32
A number that represents how many elements in the sequence satisfy the condition in the predicate function.

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

source or predicate is nulla null reference (Nothing in Visual Basic).
OverflowException

The number of elements in source is larger than MaxValue.

Remarks

If the type of source implements ICollection<T>, that implementation is used to obtain the count of elements. Otherwise, this method determines the count.

You should use the LongCount method when you expect and want to allow the result to be greater than MaxValue.

In Visual Basic query expression syntax, an Aggregate Into Count() clause translates to an invocation of Count.

Examples

The following code example demonstrates how to use Count<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>) to count the elements in an array that satisfy a condition.

   Structure Pet
      Public Name As String
      Public Vaccinated As Boolean
   End Structure

   Sub CountEx2()
      ' Create an array of Pet objects.
      Dim pets() As Pet = {New Pet With {.Name = "Barley", .Vaccinated = True}, _
                           New Pet With {.Name = "Boots", .Vaccinated = False}, _
                           New Pet With {.Name = "Whiskers", .Vaccinated = False}}

      Try
         ' Count the number of Pets in the array where the Vaccinated property is False.
         Dim numberUnvaccinated As Integer = _
             pets.Count(Function(p) p.Vaccinated = False)
         ' Display the output.
         outputBlock.Text &= "There are " & numberUnvaccinated & " unvaccinated animals." & vbCrLf
      Catch e As OverflowException
         outputBlock.Text &= "The count is too large to store as an Int32. Try using LongCount() instead." & vbCrLf
      End Try

   End Sub

   ' This code produces the following output:
   '
   ' There are 2 unvaccinated animals.


      class Pet
      {
         public string Name { get; set; }
         public bool Vaccinated { get; set; }
      }

      public static void CountEx2(System.Windows.Controls.TextBlock outputBlock)
      {
         Pet[] pets = { new Pet { Name="Barley", Vaccinated=true },
                            new Pet { Name="Boots", Vaccinated=false },
                            new Pet { Name="Whiskers", Vaccinated=false } };

         try
         {
            int numberUnvaccinated = pets.Count(p => p.Vaccinated == false);
            outputBlock.Text += String.Format("There are {0} unvaccinated animals.", numberUnvaccinated) + "\n";
         }
         catch (OverflowException)
         {
            outputBlock.Text += "The count is too large to store as an Int32." + "\n";
            outputBlock.Text += "Try using the LongCount() method instead." + "\n";
         }
      }

      // This code produces the following output:
      //
      // There are 2 unvaccinated animals.

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.