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Projection Operations

Projection refers to the operation of transforming an object into a new form that often consists only of those properties that will be subsequently used. By using projection, you can construct a new type that is built from each object. You can project a property and perform a mathematical function on it. You can also project the original object without changing it.

The standard query operator methods that perform projection are listed in the following section.

Method Name

Description

C# Query Expression Syntax

Visual Basic Query Expression Syntax

More Information

Select

Projects values that are based on a transform function.

select

Select

Enumerable.Select

Queryable.Select

SelectMany

Projects sequences of values that are based on a transform function and then flattens them into one sequence.

Use multiple from clauses

Use multiple From clauses

Enumerable.SelectMany

Queryable.SelectMany

Select

The following example uses the select clause in C# or Select clause in Visual Basic to project the first letter from each string in a list of strings.



            List<string> words = new List<string>() { "an", "apple", "a", "day" };

            var query = from word in words
                        select word.Substring(0, 1);

            foreach (string s in query)
                Console.WriteLine(s);

            /* This code produces the following output:

                a
                a
                a
                d
            */



SelectMany

The following example uses multiple from clauses in C# or From clauses in Visual Basic to project each word from each string in a list of strings.



            List<string> phrases = new List<string>() { "an apple a day", "the quick brown fox" };

            var query = from phrase in phrases
                        from word in phrase.Split(' ')
                        select word;

            foreach (string s in query)
                Console.WriteLine(s);

            /* This code produces the following output:

                an
                apple
                a
                day
                the
                quick
                brown
                fox
            */



The work of both Select() and SelectMany() is to produce a result value (or values) from source values. Select() produces one result value for every source value. The overall result is therefore a collection that has the same number of elements as the source collection. In contrast, SelectMany() produces a single overall result that contains concatenated sub-collections from each source value. The transform function that is passed as an argument to SelectMany() must return an enumerable sequence of values for each source value. These enumerable sequences are then concatenated by SelectMany() to create one large sequence.

The following two illustrations show the conceptual difference between the actions of these two methods. In each case, assume that the selector (transform) function selects the array of flowers from each source value.

This illustration depicts how Select() returns a collection that has the same number of elements as the source collection.

Conceptual illustration of the action of Select()

This illustration depicts how SelectMany() concatenates the intermediate sequence of arrays into one final result value that contains each value from each intermediate array.

Graphic showing the action of SelectMany().

Code Example

The following example compares the behavior of Select() and SelectMany(). The code creates a "bouquet" of flowers by taking the first two items from each list of flower names in the source collection. In this example, the "single value" that the transform function Select<TSource, TResult>(IEnumerable<TSource>, Func<TSource, TResult>) uses is itself a collection of values. This requires the extra foreach (For Each in Visual Basic) loop in order to enumerate each string in each sub-sequence.


class Bouquet
{
    public List<string> Flowers { get; set; }
}

static void SelectVsSelectMany()
{
    List<Bouquet> bouquets = new List<Bouquet>() {
        new Bouquet { Flowers = new List<string> { "sunflower", "daisy", "daffodil", "larkspur" }},
        new Bouquet{ Flowers = new List<string> { "tulip", "rose", "orchid" }},
        new Bouquet{ Flowers = new List<string> { "gladiolis", "lily", "snapdragon", "aster", "protea" }},
        new Bouquet{ Flowers = new List<string> { "larkspur", "lilac", "iris", "dahlia" }}
    };

    // *********** Select ***********            
    IEnumerable<List<string>> query1 = bouquets.Select(bq => bq.Flowers);

    // ********* SelectMany *********
    IEnumerable<string> query2 = bouquets.SelectMany(bq => bq.Flowers);

    Console.WriteLine("Results by using Select():");
    // Note the extra foreach loop here.
    foreach (IEnumerable<String> collection in query1)
        foreach (string item in collection)
            Console.WriteLine(item);

    Console.WriteLine("\nResults by using SelectMany():");
    foreach (string item in query2)
        Console.WriteLine(item);

    /* This code produces the following output:

       Results by using Select():
        sunflower
        daisy
        daffodil
        larkspur
        tulip
        rose
        orchid
        gladiolis
        lily
        snapdragon
        aster
        protea
        larkspur
        lilac
        iris
        dahlia

       Results by using SelectMany():
        sunflower
        daisy
        daffodil
        larkspur
        tulip
        rose
        orchid
        gladiolis
        lily
        snapdragon
        aster
        protea
        larkspur
        lilac
        iris
        dahlia
    */

}


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