Article
11/18/2011

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

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Returns elements from a sequence as long as a specified condition is true. The 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 TakeWhile(Of TSource) ( _
    source As IQueryable(Of TSource), _
    predicate As Expression(Of Func(Of TSource, Integer, Boolean)) _
) As IQueryable(Of TSource)

public static IQueryable<TSource> TakeWhile<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>
The sequence to return elements from.

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 occurring before the element at which the test specified by predicate no longer passes.

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 TakeWhile<TSource>(IQueryable<TSource>, Expression<Func<TSource, Int32, Boolean>>) method generates a MethodCallExpression that represents calling TakeWhile<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 TakeWhile<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 applies predicate to each element in source until it finds an element for which predicate returns false. It returns all the elements up until that point. The index of each source element is provided as the second argument to predicate.

Examples

The following code example demonstrates how to use TakeWhile<TSource>(IQueryable<TSource>, Expression<Func<TSource, Int32, Boolean>>) to return elements from the start of a sequence as long as a condition that uses the index of the element is true.

      Dim fruits() As String = _
          {"apple", "passionfruit", "banana", "mango", _
           "orange", "blueberry", "grape", "strawberry"}

      ' Take strings from the array until a string whose length
      ' is less than its index in the array is found.
      Dim query = fruits.AsQueryable() _
          .TakeWhile(Function(fruit, index) fruit.Length >= index)

      ' 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
      ' passionfruit
      ' banana
      ' mango
      ' orange
      ' blueberry

         string[] fruits = { "apple", "passionfruit", "banana", "mango", 
                                  "orange", "blueberry", "grape", "strawberry" };

         // Take strings from the array until a string whose length
         // is less than its index in the array is found.
         IEnumerable<string> query =
             fruits.AsQueryable()
             .TakeWhile((fruit, index) => fruit.Length >= index);

         foreach (string fruit in query)
            outputBlock.Text += fruit + "\n";

         /*
             This code produces the following output:

             apple
             passionfruit
             banana
             mango
             orange
             blueberry
         */

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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