select new Example
This page was last reviewed on Jun 22, 2021.
Dot Net Perls
Select new. A C# query expression can create a new data structure. With the select new clause, we create new objects of an anonymous type as the result of a query.
C# notes. The "select new" keywords are contextual—the "new" keyword has other uses in the C# language. We use these keywords in a query expression (System.Linq is required).
First example. The select new clause creates an anonymous type instance. In this program, we set the Value and Id in each element—each new object will have a Value and Id.
Then You can loop over the AnonymousType instances in the query expression result. We use var to simplify the syntax.
Tip This query expression transformed the string array into a collection of objects with a Value and Id property.
using System; using System.Linq; class Program { static void Main() { string[] array = { "one", "two", "three", "four", "", "seven" }; // Use Select New. var result = from element in array select new { Value = element, Id = Id(element) }; foreach (var anonymous in result) { string value = anonymous.Value; int id = anonymous.Id; Console.WriteLine(anonymous); } } static int _value; static int Id(string element) { // Get Id. return string.IsNullOrEmpty(element) ? -1 : _value++; } }
{ Value = one, Id = 0 } { Value = two, Id = 1 } { Value = three, Id = 2 } { Value = four, Id = 3 } { Value = , Id = -1 } { Value = seven, Id = 4 }
HTML. This example constructs HTML strings from a string array. In each anonymous type instance, an Html property and a Length property are present.
Tip For ideal performance when combining many strings together, consider a StringBuilder or a char array.
char Array
using System; using System.Linq; class Program { static void Main() { string[] items = { "cat", "dog", "mouse" }; // Project items into HTML strings. var html = from item in items select new { Html = "<p>" + item + "</p>", Length = item.Length }; // Display all projected values. foreach (var value in html) { Console.WriteLine(value.Html); Console.WriteLine(value.Length); } } }
<p>cat</p> 3 <p>dog</p> 3 <p>mouse</p> 5
A summary. With select new, we project elements into anonymous types. This syntax allows us to construct anonymous data structures. These are created as they are evaluated (lazily).
LINQ tips. You can avoid declaring classes to store this data entirely if you wish to. Classes clarify and improve programs, and anonymous types lack these positive features.
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Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Jun 22, 2021 (rewrite).
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