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

 

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)

<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)

Parameters

source
Type: System.Linq.IQueryable(Of TSource)

An IQueryable(Of T) to filter.

predicate
Type: System.Linq.Expressions.Expression(Of Func(Of 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(Of TSource)

An IQueryable(Of T) that contains elements from the input sequence that satisfy the condition specified by predicate.

Type Parameters

TSource

The type of the elements of source.

Exception Condition
ArgumentNullException

source or predicate is null.

This method has at least one parameter of type Expression(Of TDelegate) whose type argument is one of the Func(Of T, TResult) types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression(Of TDelegate).

The Where(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Int32, Boolean))) method generates a MethodCallExpression that represents calling Where(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of 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(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of 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.

The following code example demonstrates how to use Where(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of 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
MsgBox(output.ToString())

' This code produces the following output:

' 0
' 20
' 15
' 40

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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