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

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Filters a sequence of values based on a predicate.

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

Syntax

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

public static IEnumerable<TSource> Where<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.Collections.Generic.IEnumerable<TSource>
An IEnumerable<T> that contains elements from the input sequence that satisfy the condition.

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).

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.

In query expression syntax, a where (Visual C#) or Where (Visual Basic) clause translates to an invocation of Where<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>).

Examples

The following code example demonstrates how to use Where<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>) to filter a sequence.

      ' Create a list of strings.
      Dim fruits As New List(Of String)(New String() _
                              {"apple", "passionfruit", "banana", "mango", _
                               "orange", "blueberry", "grape", "strawberry"})

      ' Restrict the results to those strings whose 
      ' length is less than six.
      Dim query As IEnumerable(Of String) = _
          fruits.Where(Function(fruit) fruit.Length < 6)

      ' Display the results.
      Dim output As New System.Text.StringBuilder
      For Each fruit As String In query
         output.AppendLine(fruit)
      Next
      outputBlock.Text &= output.ToString() & vbCrLf

      ' This code produces the following output:
      '
      ' apple
      ' mango
      ' grape


      List<string> fruits =
          new List<string> { "apple", "passionfruit", "banana", "mango", 
                             "orange", "blueberry", "grape", "strawberry" };

      IEnumerable<string> query = fruits.Where(fruit => fruit.Length < 6);

      foreach (string fruit in query)
      {
         outputBlock.Text += fruit + "\n";
      }
      /*
       This code produces the following output:

       apple
       mango
       grape
      */

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.