LINQ Line by Line (Part 2): the select clause

LINQ Line by Line (Part 2): the select clause

The Basics

The select clause ends a LINQ query expression. It determines what the evaluation of the query will give back when it is evaluated. The select clause is highlighted in yellow in the example below.

    1 var query =

    2     from person in GetPeople()

    3     select person;

The Basic Select Clause

The select clause consists of the select keyword followed by an expresion.

The Expression

I have a simple way of thinking about an expression that holds true most of the time. An expression is anything that can be assigned to a variable. The candidates for this part of the select clause can be any expression: a number, string, new instance of a class, new instance of an anonymous class, an existing instance, a mathmatical expression, a boolean expression, and more. The word expression is a little confusing because a LINQ query itself is (by default) an _“Expression”_: an abstract representation of a query.

Example: Selecting a Member of a Class

    1 var nameQuery =

    2     from person in GetPeople()

    3     select person.Name;

    4 

    5 foreach (var name in nameQuery)

    6 {

    7     Console.WriteLine(“The name is {0}”, name);

    8 }

Example: Selecting an Unrelated Expression

    1 int n = 0;

    2 var accQuery = from person in GetPeople()

    3                select n++;

    4 

    5 foreach (int acc in accQuery)

    6 {

    7     Console.WriteLine(“The number is {0}”, n);

    8 }

The output of this little snippet is:

The number is 1
The number is 2
The number is 3
The number is 4
The number is 5

The interesting point of this example is that n doesn’t change values until the query is evaluated. Until then, the query only lives as an expression tree. The act of asking for the enumerator evaluates the query and n starts to be incremented each time MoveNext is called.

Transfiguration from Person to Rapper

var rapQuery = 

    from person in GetPeople()

    select new

    {

        FirstName = person.FirstName,

        RapName =

            person.FirstName[0] +

            person.LastName.Substring(0, 3)

                .ToLowerInvariant()

    };

foreach (var rapper in rapQuery)

{

    Console.WriteLine(

        ”{0}, your rap name is: {1}”, 

            rapper.FirstName, rapper.RapName);

}

The output of this little snippet is:

John, your rap name is: Jdoe
Jane, your rap name is: Jfon
Jonah, your rap name is: Jwha
Jim, your rap name is: Jdea
Jasper, your rap name is: Junk

The Extras to be Used in Dinner Conversation

The simple example of a query expression, one that selects all elements in its sequence; without filtering, ordering, or grouping has a fancy name: a degenerate query. Basically, when the expression tree is evaluated it can be replaced by the original sequence. This comes in handy in a more complicated query expression as the degenerate query can be prunned from the tree.

Link to Linq

Please make sure you check out: 101 Linq Examples.

Tags: LINQ, C#