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

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)

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.

ExceptionCondition
ArgumentNullException

source or selector is null.

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.

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)
      {
         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}
      */



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

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

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