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.

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.

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.