Article
11/17/2011

Queryable.Where<TSource> Method (IQueryable<TSource>, Expression<Func<TSource, Int32, Boolean>>)

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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 IQueryable(Of TSource), _
    predicate As Expression(Of Func(Of TSource, Integer, Boolean)) _
) As IQueryable(Of TSource)

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

Type Parameters

TSource
The type of the elements of source.

Parameters

source
Type: System.Linq.IQueryable<TSource>
An IQueryable<T> to filter.

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

Return Value

Type: System.Linq.IQueryable<TSource>
An IQueryable<T> that contains elements from the input sequence that satisfy the condition specified by predicate.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable<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 has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T, TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The Where<TSource>(IQueryable<TSource>, Expression<Func<TSource, Int32, Boolean>>) method generates a MethodCallExpression that represents calling Where<TSource>(IQueryable<TSource>, Expression<Func<TSource, Int32, Boolean>>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling Where<TSource>(IQueryable<TSource>, Expression<Func<TSource, Int32, Boolean>>) depends on the implementation of the type of the source parameter. The expected behavior is that it returns the elements from source that satisfy the condition specified by predicate. The index of each source element is provided as the second argument to predicate.

Examples

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

      Dim numbers() As Integer = {0, 30, 20, 15, 90, 85, 40, 75}

      ' Get all the numbers that are less than or equal to
      ' the product of their index in the array and 10.
      Dim query = numbers.AsQueryable() _
          .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 };

         // Get all the numbers that are less than or equal to
         // the product of their index in the array and 10.
         IEnumerable<int> query =
             numbers.AsQueryable()
             .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

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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