Freigeben über

  • Artikel

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

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

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

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

Type Parameters

  • TSource
    The type of the elements of source.

Parameters

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, Boolean>>) method generates a MethodCallExpression that represents calling Where<TSource>(IQueryable<TSource>, Expression<Func<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<TSource>(IQueryable<TSource>, Expression<Func<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.

Examples

The following code example demonstrates how to use Where<TSource>(IQueryable<TSource>, Expression<Func<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


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

         // Get all strings whose length is less than 6.
         IEnumerable<string> query =
             fruits.AsQueryable().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

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.