Queryable.TakeWhile<TSource> Method (IQueryable<TSource>, Expression<Func<TSource, Boolean>>)
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Returns elements from a sequence as long as a specified condition is true.
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, Boolean)) _
) As IQueryable(Of TSource)
public static IQueryable<TSource> TakeWhile<TSource>(
this IQueryable<TSource> source,
Expression<Func<TSource, 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, Boolean>>
A function to test each element for a condition.
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, Boolean>>) method generates a MethodCallExpression that represents calling TakeWhile<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 TakeWhile<TSource>(IQueryable<TSource>, Expression<Func<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.
Examples
The following code example demonstrates how to use TakeWhile<TSource>(IQueryable<TSource>, Expression<Func<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
string[] fruits = { "apple", "banana", "mango", "orange",
"passionfruit", "grape" };
// Take strings from the array until a string
// that is equal to "orange" is found.
IEnumerable<string> query =
fruits.AsQueryable()
.TakeWhile(fruit => String.Compare("orange", fruit, StringComparison.CurrentCultureIgnoreCase) != 0);
foreach (string fruit in query)
outputBlock.Text += fruit + "\n";
/*
This code produces the following output:
apple
banana
mango
*/
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.