This document summarizes Chapter 23 of the book "Murach's C#" on using LINQ (Language-Integrated Query). It provides objectives for applying and understanding LINQ, describes LINQ features and syntax like from, where, orderby, and join clauses. It provides examples of LINQ queries on arrays and generic lists to retrieve, filter, sort, and join data. The examples demonstrate the three stages of a LINQ query and use of anonymous types.

1. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 1
Chapter 23
How to use LINQ
(Language-Integrated
Query)

2. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 2
Objectives
Applied
1. Use any of the LINQ features presented in this chapter to query
an in-memory data structure such as an array, sorted list, or
generic list.

3. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 3
Objectives (continued)
Knowledge
1. In general terms, explain how LINQ is implemented.
2. Name the interface that a data source must implement to use
LINQ with that data source.
3. Describe the three stages of a query operation.
4. Describe how the type of a query variable is determined.
5. Describe how deferred execution works.
6. Describe when a query operation results in a projection.
7. Describe when a query operation results in an anonymous type.
8. Describe the purpose of each of the following LINQ clauses:
from, where, orderby, select, join.

4. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 4
C# keywords for working with LINQ
• from
• where
• orderby
• select
• join

5. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 5
LINQ features
• Query language is integrated with C#
• Provides IntelliSense, compile-time syntax checking, and debugging
support
• Same basic syntax for each type of query
• Provides designer tools for object-relational mappings

6. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 6
The three stages of a query operation
• Get the data source.
• Define the query expression.
• Execute the query to return the results.

7. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 7
A LINQ query that retrieves data from an array
Code that defines the array
int[] numbers = new int[6];
for (int i = 0; i < numbers.Length; i++)
numbers[i] = i;
A statement that defines the query expression
var numberList = from number in numbers
where number % 2 == 0
orderby number descending
select number;
Code that executes the query
string numberDisplay = "";
foreach (var number in numberList)
numberDisplay += "tt" + number + "n";
MessageBox.Show(numberDisplay, "Sorted Even Numbers");

8. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 8
The resulting dialog box

9. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 9
The syntax of the from clause
from [type] elementName in collectionName
An example that uses an array of decimals
A statement that gets the data source
decimal[] salesTotals =
new decimal[4] {1286.45m, 2433.49m, 2893.85m,
2094.53m};
A statement that defines the query expression
var salesList = from sales in salesTotals
select sales;
Code that executes the query
decimal sum = 0;
foreach (var sales in salesList)
sum += sales;

10. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 10
An example that uses a generic list of invoices
The Invoice class
public class Invoice
{
public int InvoiceID { get; set; }
public int CustomerID { get; set; }
public DateTime InvoiceDate { get; set; }
public decimal ProductTotal { get; set; }
public decimal SalesTax { get; set; }
public decimal Shipping { get; set; }
public decimal InvoiceTotal { get; set; }
}
A statement that gets the data source
List<Invoice> invoiceList = InvoiceDB.GetInvoices();

11. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 11
A statement that defines the query expression
var invoices = from invoice in invoiceList
select invoice;
Code that executes the query
decimal sum = 0;
foreach (var invoice in invoices)
sum += invoice.InvoiceTotal;

12. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 12
The syntax of the where clause
where condition
An example that filters the salesTotals array
A query expression that returns sales greater than $2000
var salesList = from sales in salesTotals
where sales > 2000
select sales;
Code that executes the query
string salesDisplay = "";
foreach (var sales in salesList)
salesDisplay += "t" + sales.ToString("c") + "n";
MessageBox.Show(salesDisplay, "Sales Over $2000");

13. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 13
The resulting dialog box

14. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 14
An example that filters the generic list of invoices
A query expression that returns totals over $150
var invoices = from invoice in invoiceList
where invoice.InvoiceTotal > 150
select invoice;
Code that executes the query
string invoiceDisplay = "";
foreach (var invoice in invoices)
invoiceDisplay += "tt" +
invoice.InvoiceTotal.ToString("c") + "n";
MessageBox.Show(invoiceDisplay, "Invoices Over $150");

15. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 15
The resulting dialog box

16. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 16
The syntax of the orderby clause
orderby expression1 [ascending|descending]
[, expression2 [ascending|descending]]...
An example that sorts the salesTotals array
A query expression that sorts the sales
var salesList = from sales in salesTotals
where sales > 2000
orderby sales
select sales;
Code that executes the query
string salesDisplay = "";
foreach (var sales in salesList)
salesDisplay += "tt" + sales.ToString("c") + "n";
MessageBox.Show(salesDisplay, "Sorted Sales Over $2000");

17. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 17
The resulting dialog box

18. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 18
An example that sorts the generic list of invoices
A query expression that sorts the invoices
var invoices = from invoice in invoiceList
where invoice.InvoiceTotal > 150
orderby invoice.CustomerID,
invoice.InvoiceTotal descending
select invoice;
Code that executes the query
string invoiceDisplay = "Cust IDtInvoice amountn";
foreach (var invoice in invoices)
invoiceDisplay += invoice.CustomerID + "t"
+ invoice.InvoiceTotal.ToString("c")
+ "n";
MessageBox.Show(invoiceDisplay,
"Sorted Invoices Over $150");

19. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 19
The resulting dialog box

20. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 20
Two ways to code the select clause
select columnExpression
select new [type] { [PropertyName1 =] columnExpression1
[, [PropertyName2 =] columnExpression2]... }

21. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 21
An example that selects key values from a list
The employee sales sorted list
SortedList<string, decimal> employeeSales =
new SortedList<string,decimal>();
employeeSales.Add("Anderson", 1286.45m);
employeeSales.Add("Menendez", 2433.49m);
employeeSales.Add("Thompson", 2893.85m);
employeeSales.Add("Wilkinson", 2094.53m);
A query expression that selects employee names
var employeeList = from sales in employeeSales
where sales.Value > 2000
orderby sales.Value descending
select sales.Key;
Code that executes the query
string employeeDisplay = "";
foreach (var employee in employeeList)
employeeDisplay += "ttt" + employee + "n";
MessageBox.Show(employeeDisplay,
"Sorted Employees With Sales Over $2000");

22. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 22
The resulting dialog box

23. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 23
A query expression that creates an anonymous
type from the list of invoices
var invoices = from invoice in invoiceList
where invoice.InvoiceTotal > 150
orderby invoice.CustomerID,
invoice.InvoiceTotal descending
select new { invoice.CustomerID,
invoice.InvoiceTotal };

24. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 24
The basic syntax of the join clause
join elementName in collectionName
on keyName1 equals keyName2
An example that joins data from two generic lists
The Customer class
public class Customer
{
public int CustomerID { get; set; }
public string Name { get; set; }
}
Code that gets the two data sources
List<Invoice> invoiceList = InvoiceDB.GetInvoices();
List<Customer> customerList = CustomerDB.GetCustomers();

25. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 25
A query expression that joins data
var invoices =
from invoice in invoiceList
join customer in customerList
on invoice.CustomerID equals customer.CustomerID
where invoice.InvoiceTotal > 150
orderby customer.Name,
invoice.InvoiceTotal descending
select new { customer.Name, invoice.InvoiceTotal };

26. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 26
Code that executes the query
string invoiceDisplay =
"Customer NamettInvoice amountn";
foreach (var invoice in invoices)
{
invoiceDisplay += invoice.Name + "tt";
invoiceDisplay +=
invoice.InvoiceTotal.ToString("c") + "n";
}
MessageBox.Show(invoiceDisplay,
"Joined Customer and Invoice Data");

27. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 27
The resulting dialog box

28. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 28
The Customer Invoice form

29. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 29
Customer Invoice form with generic lists
private void Form1_Load(object sender, EventArgs e)
{
List<Customer> customerList = CustomerDB.GetCustomers();
List<Invoice> invoiceList = InvoiceDB.GetInvoices();
var invoices = from invoice in invoiceList
join customer in customerList
on invoice.CustomerID equals customer.CustomerID
orderby customer.Name, invoice.InvoiceTotal
descending
select new { customer.Name,
invoice.InvoiceID,
invoice.InvoiceDate,
invoice.InvoiceTotal };
string customerName = "";
int i = 0;

30. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 30
Customer Invoice form with generic lists (cont.)
foreach (var invoice in invoices)
{
if (invoice.Name != customerName)
{
lvInvoices.Items.Add(invoice.Name);
customerName = invoice.Name;
}
else
{
lvInvoices.Items.Add("");
}
lvInvoices.Items[i].SubItems.Add(
invoice.InvoiceID.ToString());
lvInvoices.Items[i].SubItems.Add(
Convert.ToDateTime(
invoice.InvoiceDate).ToShortDateString());
lvInvoices.Items[i].SubItems.Add(
invoice.InvoiceTotal.ToString("c"));
i += 1;
}
}

31. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 31
The dataset schema

32. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 32
Customer Invoice form with typed dataset
MMABooksDataSet mmaBooksDataSet = new MMABooksDataSet();
InvoicesTableAdapter invoicesTableAdapter =
new InvoicesTableAdapter();
CustomersTableAdapter customersTableAdapter =
new CustomersTableAdapter();

33. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 33
Customer Invoice form with typed dataset (cont.)
private void Form1_Load(object sender, EventArgs e)
{
invoicesTableAdapter.Fill(mmaBooksDataSet.Invoices);
customersTableAdapter.Fill(mmaBooksDataSet.Customers);
var invoices = from invoice in mmaBooksDataSet.Invoices
join customer in mmaBooksDataSet.Customers
on invoice.CustomerID equals customer.CustomerID
orderby customer.Name, invoice.InvoiceTotal
descending
select new { customer.Name,
invoice.InvoiceID,
invoice.InvoiceDate,
invoice.InvoiceTotal };
string customerName = "";
int i = 0;

34. Murach’s C#
2010, C23 © 2010, Mike Murach & Associates, Inc.Slide 34
Customer Invoice form with typed dataset (cont.)
foreach (var invoice in invoices)
{
if (invoice.Name != customerName)
{
lvInvoices.Items.Add(invoice.Name);
customerName = invoice.Name;
}
else
{
lvInvoices.Items.Add("");
}
lvInvoices.Items[i].SubItems.Add(
invoice.InvoiceID.ToString());
lvInvoices.Items[i].SubItems.Add(
Convert.ToDateTime(
invoice.InvoiceDate).ToShortDateString());
lvInvoices.Items[i].SubItems.Add(
invoice.InvoiceTotal.ToString("c"));
i += 1;
}
}