Enumerable.Select Method
Definition
Important
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Projects each element of a sequence into a new form.
Overloads
| Select<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,TResult>) |
Projects each element of a sequence into a new form by incorporating the element's index. |
| Select<TSource,TResult>(IEnumerable<TSource>, Func<TSource,TResult>) |
Projects each element of a sequence into a new form. |
Select<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,TResult>)
- Source:
- Select.cs
- Source:
- Select.cs
- Source:
- Select.cs
Projects each element of a sequence into a new form by incorporating the element's index.
public:
generic <typename TSource, typename TResult>
[System::Runtime::CompilerServices::Extension]
static System::Collections::Generic::IEnumerable<TResult> ^ Select(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, int, TResult> ^ selector);public static System.Collections.Generic.IEnumerable<TResult> Select<TSource,TResult> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,int,TResult> selector);static member Select : seq<'Source> * Func<'Source, int, 'Result> -> seq<'Result><Extension()>
Public Function Select(Of TSource, TResult) (source As IEnumerable(Of TSource), selector As Func(Of TSource, Integer, TResult)) As IEnumerable(Of TResult)Type Parameters
- TSource
The type of the elements of source.
- TResult
The type of the value returned by selector.
Parameters
- source
- IEnumerable<TSource>
A sequence of values to invoke a transform function on.
A transform function to apply to each source element; the second parameter of the function represents the index of the source element.
Returns
An IEnumerable<T> whose elements are the result of invoking the transform function on each element of source.
Exceptions
source or selector is null.
Examples
The following code example demonstrates how to use Select<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,TResult>) to project over a sequence of values and use the index of each element.
string[] fruits = { "apple", "banana", "mango", "orange",
"passionfruit", "grape" };
var query =
fruits.Select((fruit, index) =>
new { index, str = fruit.Substring(0, index) });
foreach (var obj in query)
{
Console.WriteLine("{0}", obj);
}
/*
This code produces the following output:
{ index = 0, str = }
{ index = 1, str = b }
{ index = 2, str = ma }
{ index = 3, str = ora }
{ index = 4, str = pass }
{ index = 5, str = grape }
*/
' Create an array of strings.
Dim fruits() As String =
{"apple", "banana", "mango", "orange", "passionfruit", "grape"}
' Project each item in the array to an anonymous type
' that stores the item's index in the array and
' a substring of each item whose length is equal
' to the index position in the original array.
Dim query =
fruits.Select(Function(fruit, index) _
New With {index, .Str = fruit.Substring(0, index)})
Dim output As New System.Text.StringBuilder
For Each obj In query
output.AppendLine(obj.ToString())
Next
' Display the output.
Console.WriteLine(output.ToString())
' This code produces the following output:
'
' { index = 0, Str = }
' { index = 1, Str = b }
' { index = 2, Str = ma }
' { index = 3, Str = ora }
' { index = 4, Str = pass }
' { index = 5, Str = grape }
Remarks
This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in C# or For Each in Visual Basic.
The first argument to selector represents the element to process. The second argument to selector represents the zero-based index of that element in the source sequence. This can be useful if the elements are in a known order and you want to do something with an element at a particular index, for example. It can also be useful if you want to retrieve the index of one or more elements.
This projection method requires the transform function, selector, to produce one value for each value in the source sequence, source. If selector returns a value that is itself a collection, it is up to the consumer to traverse the subsequences manually. In such a situation, it might be better for your query to return a single coalesced sequence of values. To achieve this, use the SelectMany method instead of Select. Although SelectMany works similarly to Select, it differs in that the transform function returns a collection that is then expanded by SelectMany before it is returned.
Applies to
Select<TSource,TResult>(IEnumerable<TSource>, Func<TSource,TResult>)
- Source:
- Select.cs
- Source:
- Select.cs
- Source:
- Select.cs
Projects each element of a sequence into a new form.
public:
generic <typename TSource, typename TResult>
[System::Runtime::CompilerServices::Extension]
static System::Collections::Generic::IEnumerable<TResult> ^ Select(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, TResult> ^ selector);public static System.Collections.Generic.IEnumerable<TResult> Select<TSource,TResult> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TResult> selector);static member Select : seq<'Source> * Func<'Source, 'Result> -> seq<'Result><Extension()>
Public Function Select(Of TSource, TResult) (source As IEnumerable(Of TSource), selector As Func(Of TSource, TResult)) As IEnumerable(Of TResult)Type Parameters
- TSource
The type of the elements of source.
- TResult
The type of the value returned by selector.
Parameters
- source
- IEnumerable<TSource>
A sequence of values to invoke a transform function on.
- selector
- Func<TSource,TResult>
A transform function to apply to each element.
Returns
An IEnumerable<T> whose elements are the result of invoking the transform function on each element of source.
Exceptions
source or selector is null.
Examples
The following code example demonstrates how to use Select<TSource,TResult>(IEnumerable<TSource>, Func<TSource,TResult>) to project over a sequence of values.
IEnumerable<int> squares =
Enumerable.Range(1, 10).Select(x => x * x);
foreach (int num in squares)
{
Console.WriteLine(num);
}
/*
This code produces the following output:
1
4
9
16
25
36
49
64
81
100
*/
' Create a collection of sequential integers
' from 1 to 10 and project their squares.
Dim squares As IEnumerable(Of Integer) =
Enumerable.Range(1, 10).Select(Function(x) x * x)
Dim output As New System.Text.StringBuilder
For Each num As Integer In squares
output.AppendLine(num)
Next
' Display the output.
Console.WriteLine(output.ToString())
' This code produces the following output:
'
' 1
' 4
' 9
' 16
' 25
' 36
' 49
' 64
' 81
' 100
Remarks
This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in C# or For Each in Visual Basic.
This projection method requires the transform function, selector, to produce one value for each value in the source sequence, source. If selector returns a value that is itself a collection, it is up to the consumer to traverse the subsequences manually. In such a situation, it might be better for your query to return a single coalesced sequence of values. To achieve this, use the SelectMany method instead of Select. Although SelectMany works similarly to Select, it differs in that the transform function returns a collection that is then expanded by SelectMany before it is returned.
In query expression syntax, a select (C#) or Select (Visual Basic) clause translates to an invocation of Select.
