Queryable.TakeWhile Method

Definition

Namespace:

System.Linq

Assembly:

System.Linq.Queryable.dll

Returns elements from a sequence as long as a specified condition is true, and then skips the remaining elements.

Overloads

TakeWhile<TSource>(IQueryable<TSource>, Expression<Func<TSource,Int32,Boolean>>)	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.
TakeWhile<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)	Returns elements from a sequence as long as a specified condition is true.

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

Source:

Queryable.cs

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.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static System::Linq::IQueryable<TSource> ^ TakeWhile(System::Linq::IQueryable<TSource> ^ source, System::Linq::Expressions::Expression<Func<TSource, int, bool> ^> ^ predicate);

public static System.Linq.IQueryable<TSource> TakeWhile<TSource> (this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,int,bool>> predicate);

static member TakeWhile : System.Linq.IQueryable<'Source> * System.Linq.Expressions.Expression<Func<'Source, int, bool>> -> System.Linq.IQueryable<'Source>

<Extension()>
Public Function TakeWhile(Of TSource) (source As IQueryable(Of TSource), predicate As Expression(Of Func(Of TSource, Integer, Boolean))) As IQueryable(Of TSource)

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IQueryable<TSource>

The sequence to return elements from.

predicate

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.

Returns

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.

Exceptions

ArgumentNullException

source or predicate is null.

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.

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)
    Console.WriteLine(fruit);

/*
    This code produces the following output:

    apple
    passionfruit
    banana
    mango
    orange
    blueberry
*/

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
MsgBox(output.ToString())

' This code produces the following output:

' apple
' passionfruit
' banana
' mango
' orange
' blueberry

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.

TakeWhile<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)

Source:

Queryable.cs

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

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static System::Linq::IQueryable<TSource> ^ TakeWhile(System::Linq::IQueryable<TSource> ^ source, System::Linq::Expressions::Expression<Func<TSource, bool> ^> ^ predicate);

public static System.Linq.IQueryable<TSource> TakeWhile<TSource> (this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,bool>> predicate);

static member TakeWhile : System.Linq.IQueryable<'Source> * System.Linq.Expressions.Expression<Func<'Source, bool>> -> System.Linq.IQueryable<'Source>

<Extension()>
Public 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

IQueryable<TSource>

The sequence to return elements from.

predicate

Expression<Func<TSource,Boolean>>

A function to test each element for a condition.

Returns

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.

Exceptions

ArgumentNullException

source or predicate is null.

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.

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, true) != 0);

foreach (string fruit in query)
    Console.WriteLine(fruit);

/*
    This code produces the following output:

    apple
    banana
    mango
*/

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.
MsgBox(output.ToString())

'This code produces the following output:

'apple
'banana
'mango

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.