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Queryable.Select<TSource, TResult> Method (IQueryable<TSource>, Expression<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 IQueryable<TResult> Select<TSource, TResult>(
	this IQueryable<TSource> source,
	Expression<Func<TSource, int, TResult>> selector
)

Type Parameters

TSource

The type of the elements of source.

TResult

The type of the value returned by the function represented by selector.

Parameters

source
Type: System.Linq.IQueryable<TSource>
A sequence of values to project.
selector
Type: System.Linq.Expressions.Expression<Func<TSource, Int32, TResult>>
A projection function to apply to each element.

Return Value

Type: System.Linq.IQueryable<TResult>
An IQueryable<T> whose elements are the result of invoking a projection 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 IQueryable<TSource>. When you use instance method syntax to call this method, omit the first parameter.

ExceptionCondition
ArgumentNullException

source or selector is null.

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 Select<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, Int32, TResult>>) method generates a MethodCallExpression that represents calling Select<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, Int32, TResult>>) 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 Select<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, Int32, TResult>>) depend on the implementation of the type of the source parameter. The expected behavior is that it invokes selector on each element of source to project it into a different form.

The following code example demonstrates how to use Select<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, Int32, TResult>>) to project over a sequence of values and use the index of each element in the projected form.


         string[] fruits = { "apple", "banana", "mango", "orange", 
                                  "passionfruit", "grape" };

         // Project an anonymous type that contains the
         // index of the string in the source array, and
         // a string that contains the same number of characters
         // as the string's index in the source array.
         var query =
             fruits.AsQueryable()
             .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

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

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