Článek
11/18/2011

Enumerable.Select<TSource, TResult> Method (IEnumerable<TSource>, Func<TSource, Int32, TResult>)

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Projects each element of a sequence into a new form by incorporating the element's index.

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

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function Select(Of TSource, TResult) ( _
    source As IEnumerable(Of TSource), _
    selector As Func(Of TSource, Integer, TResult) _
) As IEnumerable(Of TResult)

public static IEnumerable<TResult> Select<TSource, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, int, TResult> selector
)

Type Parameters

TSource
The type of the elements of source.

TResult
The type of the value returned by selector.

Parameters

source
Type: System.Collections.Generic.IEnumerable<TSource>
A sequence of values to invoke a transform function on.

selector
Type: System.Func<TSource, Int32, TResult>
A transform function to apply to each source element; the second parameter of the function represents the index of the source element.

Return Value

Type: System.Collections.Generic.IEnumerable<TResult>
An IEnumerable<T> whose elements are the result of invoking the transform function on each element of source.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<TSource>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception	Condition
ArgumentNullException	source or selector is nulla null reference (Nothing in Visual Basic).

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 Visual 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.

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.

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

      ' 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 }

      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)
      {
         outputBlock.Text += String.Format("{0}", obj) + "\n";
      }

      /*
       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}
      */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

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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