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

Returns elements from a sequence as long as a specified condition is true.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

'Declaration
<ExtensionAttribute> _
Public Shared Function TakeWhile(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)
The sequence to return elements from.
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 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(Of TSource). When you use instance method syntax to call this method, omit the first parameter.

ExceptionCondition
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 TakeWhile(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean))) method generates a MethodCallExpression that represents calling TakeWhile(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 TakeWhile(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 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 following code example demonstrates how to use TakeWhile(Of TSource)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Boolean))) to return elements from the start of a sequence as long as a condition is true.


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

      ' Take strings from the array until a string
      ' that is equal to "orange" is found.
      Dim query = fruits.AsQueryable() _
          .TakeWhile(Function(fruit) String.Compare("orange", fruit, True) <> 0)

      Dim output As New System.Text.StringBuilder
      For Each fruit As String In query
         output.AppendLine(fruit)
      Next

      ' Display the output.
      outputBlock.Text &= output.ToString() & vbCrLf

      'This code produces the following output:

      'apple
      'banana
      'mango



Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

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

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