Artigo
11/18/2011

Enumerable.Where<TSource> Method (IEnumerable<TSource>, Func<TSource, Int32, Boolean>)

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

Filters a sequence of values based on a predicate. Each element's index is used in the logic of the predicate function.

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, Integer, Boolean) _
) As IEnumerable(Of TSource)

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

Type Parameters

TSource
The type of the elements of source.

Parameters

source
Type: System.Collections.Generic.IEnumerable<TSource>
An IEnumerable<T> to filter.

predicate
Type: System.Func<TSource, Int32, Boolean>
A function to test each source element for a condition; the second parameter of the function represents the index of the source element.

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.

The first argument of predicate represents the element to test. The second argument represents the zero-based index of the element within source.

Examples

The following code example demonstrates how to use Where<TSource>(IEnumerable<TSource>, Func<TSource, Int32, Boolean>) to filter a sequence based on a predicate that involves the index of each element.

      ' Create an array of integers.
      Dim numbers() As Integer = {0, 30, 20, 15, 90, 85, 40, 75}

      ' Restrict the results to those numbers whose
      ' values are less than or equal to their index times 10.
      Dim query As IEnumerable(Of Integer) = _
          numbers.Where(Function(number, index) number <= index * 10)

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

      ' This code produces the following output:
      '
      ' 0
      ' 20
      ' 15
      ' 40

      int[] numbers = { 0, 30, 20, 15, 90, 85, 40, 75 };

      IEnumerable<int> query =
          numbers.Where((number, index) => number <= index * 10);

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

       0
       20
       15
       40
      */

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.

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