Queryable.Where(Of TSource) Method (IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean)))
[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]
Filters a sequence of values based on a predicate.
Assembly: System.Core (in System.Core.dll)
'Declaration <ExtensionAttribute> _ Public Shared Function Where(Of TSource) ( _ source As IQueryable(Of TSource), _ predicate As Expression(Of Func(Of TSource, Boolean)) _ ) As IQueryable(Of TSource)
Type Parameters
- TSource
The type of the elements of source.
Parameters
- source
- Type: System.Linq.IQueryable(Of TSource)
An IQueryable(Of T) to filter.
- predicate
- Type: System.Linq.Expressions.Expression(Of Func(Of TSource, Boolean))
A function to test each element for a condition.
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.
Usage Note
In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable(Of TSource). When you use instance method syntax to call this method, omit the first parameter.| Exception | Condition |
|---|---|
| ArgumentNullException | source or predicate is Nothing. |
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, Boolean))) method generates a MethodCallExpression that represents calling Where(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, 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, 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 following code example demonstrates how to use Where(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean))) to filter a sequence.
Dim fruits As New List(Of String)(New String() _
{"apple", "passionfruit", "banana", "mango", _
"orange", "blueberry", "grape", "strawberry"})
' Get all strings whose length is less than 6.
Dim query = fruits.AsQueryable().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