C# 0 The Complete Reference

Main( )

, an array of 

ContactInfo

 is created. Then, a query is declared in which the 

data source is an array of 

ContactInfo

, but the sequence returned contains 

EmailAddress

objects. Thus, the type of the sequence returned by 

select

 is not 

ContactInfo

, but rather 

EmailAddress

, and these objects are created during the execution of the query.

// Use a query to obtain a sequence of EmailAddresses

// from a list of ContactInfo.

using System;

using System.Linq;

class ContactInfo {

  public string Name { get; set; }

  public string Email { get; set; }

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  public string Phone { get; set; }

  public ContactInfo(string n, string a, string p) {

    Name = n;

    Email = a;

    Phone = p;

  }

}

class EmailAddress {

  public string Name { get; set; }

  public string Address { get; set; }

  public EmailAddress(string n, string a) {

    Name = n;

    Address = a;

  }

}

class SelectDemo3 {

  static void Main() {

    ContactInfo[] contacts = {

         new ContactInfo("Herb", "[e-mail]", "555-1010"),

         new ContactInfo("Tom", "[e-mail]", "555-1101"),

         new ContactInfo("Sara", "[e-mail]", "555-0110")

    };

    // Create a query that creates a list of EmailAddress objects.

    var emailList = from entry in contacts

                    select new EmailAddress(entry.Name, entry.Email);

    Console.WriteLine("The e-mail list is");

    // Execute the query and display the results.

    foreach(EmailAddress e in emailList)

      Console.WriteLine("  {0}: {1}", e.Name, e.Address );

  }

}

The output is shown here:

The e-mail list is

  Herb: [e-mail]

  Tom: [e-mail]

  Sara: [e-mail]

In the query, pay special attention to the 

select

 clause :

select new EmailAddress(entry.Name, entry.Email);

It creates a new 

EmailAddress

 object that contains the name and e-mail address obtained 

from a 

ContactInfo

 object in the 

contacts

 array. The key point is that new 

EmailAddress

objects are created by the query in its 

select

 clause, during the query’s execution.

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Use Nested from Clauses

A query can contain more than one 

from

 clause. Thus, a query can contain nested 

from

clauses. One common use of a nested 

from

 clause is found when a query needs to obtain 

data from two different sources. Here is a simple example. It uses two 

from

 clauses to 

iterate over two different character arrays. It produces a sequence that contains all possible 

combinations of the two sets of characters.

// Use two from clauses to create a list of all

// possible combinations of the letters A, B, and C

// with the letters X, Y, and Z.

using System;

using System.Linq;

// This class holds the result of the query.

class ChrPair {

  public char First;

  public char Second;

  public ChrPair(char c, char c2) {

    First = c;

    Second = c2;

  }

}

class MultipleFroms {

  static void Main() {

    char[] chrs = { 'A', 'B', 'C' };

    char[] chrs2 = { 'X', 'Y', 'Z' };

    // Notice that the first from iterates over chrs and

    // the second from iterates over chrs2.

    var pairs = from ch1 in chrs

                  from ch2 in chrs2

                  select new ChrPair(ch1, ch2);

    Console.WriteLine("All combinations of ABC with XYZ: ");

    foreach(var p in pairs)

      Console.WriteLine("{0} {1}", p.First, p.Second);

  }

}

The output is shown here:

All combinations of ABC with XYZ:

A X

A Y

A Z

B X

B Y

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