Manipulating Data Using LINQ Chapter 7 Any application that displays dynamic information has to retrieve data from various data sources, such as a flat file, an XML file, or a database. To access data from these data sources, programmers need to implement different techniques. This complicates the task of programmers because they need to master all the techniques. However, the task of programmers can be simplified by implementing a standard technique to access data from various data sources. Such a standard technique is provided by Language-Integrated Query (LINQ). This chapter provides an overview of LINQ. In addition, it discusses how to access data from disparate data sources. In this chapter, you will learn to:  Identify the basics of LINQ  Access data from disparate data sources by using LINQ Objectives NIIT Manipulating Data Using LINQ 7.3 When creating a dynamic Web application, you need to retrieve data from various data sources, such as a flat file, an XML file, or a database. To access data from these disparate data sources, you need to learn and implement different techniques. For example, to access data from files, you need to implement input/output streams and to access data from a database, you need to implement SQL. This complicates your task as a programmer. This task can be simplified by using LINQ. LINQ provides a standard way to access data from disparate data sources. Using LINQ, you can access data from disparate data sources by writing a standard query, LINQ query. This eases your task because you need not learn different technologies to access data. In addition, LINQ provides various facilities such as stronger typing, compile-time checking, and improved debugger support. The following figure shows a model of .NET LINQ. Model of .NET LINQ Identifying the Basics of LINQ 7.4 Manipulating Data Using LINQ NIIT The .NET Language-Integrated Query model consists of all the languages in which you can create your Web application. It includes the components such as LINQ to ADO.NET, LINQ to XML, and LINQ to objects, using which you can access the data from data sources such as database, an arraylist, and an XML. To retrieve data from data sources, you need to write a LINQ query. Therefore, you need to know the different components of a LINQ query. A LINQ query is an expression that retrieves data from a data source. It enables you to write a standard query expression to retrieve data form disparate data sources. It also allows you to filter, sort, group, and join the results of the query by using additional clauses in the query. Accessing Data by Using a Simple LINQ Query To access data by using LINQ, you need to perform the following steps: 1. Specify the data source. 2. Create the query. 3. Execute the query. Specifying the Data Source In a LINQ query operation, the first step is to specify the data source. The data source in a LINQ query can be an XML file, an array, a collection, or a database. If the data source that you want to use is an array, you can specify the data source by using the following statement: string[] customers = new string[2] {John, Mary}; In the preceding statement, the array, customers, is the data source. Similarly, if the data source that you want to use is an XML file, you can specify the data source by using the following statement: XElement customers = XElement.Load(@"c:\myCustomerList.xml"); In the preceding statement, the XElement object, customers, is the data source. After specifying the data source, you need to create a query expression to retrieve or manipulate data from the specified data source. Understanding the LINQ Query NIIT Manipulating Data Using LINQ 7.5 N ot e N ot e Creating the Query A query describes what information is to be retrieved from the specified data source. It must include the from and select clauses to retrieve data. The from clause specifies the data source from where the data is to be retrieved and the select clause specifies the type of the returned elements. Consider the following query: var query = from <type of the value returned> cus in customers select cus; The preceding query contains the from and select clauses. The from clause specifies the range variable cus and the data source Customers, and the select clause specifies the type of the values that will be returned by the query. The query is stored in a query variable, query. It is initialized with a query expression. The range variable acts as an iteration variable in a foreach loop. When the query is executed, the range variable is served as a reference of each item stored in the data source, such as a database. The var keyword is a new keyword that can infer its data type from the resulting set. A var type variable needs to be initialized at the time of declaration. The var keyword cannot be used when declaring the member variables of a class. Consider the following examples: var one = 1; //infers an int var two = "2"; // infers a string var five = null; // This results in a compilation error because //the data type is not specified in the Right hand side. Executing the Query In LINQ, the query variable only stores the query. The actual execution of the query is deferred until the query variable is iterated by using the foreach loop. The foreach loop is the place where the query results are retrieved. Consider the following code snippet: //Query execution foreach (var cus in query) { Response.Write(cus); } 7.6 Manipulating Data Using LINQ NIIT In the preceding code snippet, the iteration variable cus holds each value, one at a time, in the same sequence as returned by the query. The query variable only stores the query that is to be executed. Therefore, you can execute the query variable as many times as you want. It is useful in situations where the database is constantly updated. In such a situation, you can create a query variable and execute it at some interval to retrieve the updated data. The execution of all the queries is not deferred till query variable is iterated by using the foreach loop. The queries that perform aggregate functions such as Max and Count over a range of source elements must first iterate over those elements. Such queries execute without explicitly using the foreach loop. This is because such queries itself use the foreach loop in order to return the result. These queries return a single value. Consider the following example: var evenNumbers = from num in numbers where (num % 2) == 0 select num; int Count = evenNumbers.Count(); In the preceding example, the foreach loop has not been used to execute the query. It gets executed automatically when the Count function is used. Consider the following example in which the LINQ query is used to retrieve data from an ArrayList object: public partial class Default : System.Web.UI.Page { public class customer { public string Name { get; set; } public string City { get; set; } } protected void Page_Load(object sender, EventArgs e) { ArrayList arrList = new ArrayList(); arrList.Add( new customer { Name = "Svetlana", City = "London" } ); arrList.Add( new customer { Name = "Claire", City="Paris" } ); arrList.Add( new customer { NIIT Manipulating Data Using LINQ 7.7 Name = "Cesar", City="New York" } ); var query = from customer cus in arrList select cus; foreach (var cus in query) { Response.Write (cus.Name + " lives in " + cus.City + "<br>"); } } } In the preceding example, a class customer with two properties, Name and City, is created. After creating the class, an object of the ArrayList class is created and three objects of the type customer are added to the ArrayList object. The ArrayList object is the data source from which the data is retrieved by using the LINQ query. The LINQ query retrieves all the data from the data source. Using LINQ Queries to Filter, Sort, Group, and Join Data In addition to selecting data, a query can perform the following operations on data:  Filtering  Ordering  Grouping  Joining Filtering To retrieve data depending on certain criteria or condition, you need to filter the data. Filtering causes the query to return only the data that matches with the specified condition. The condition to retrieve data can be specified by using the where clause. For example, the following code snippet returns the data of the customers who live in London: var query = from <type of the value returned> cus in customers where cus.City == "London" select cus; You can also use the logical operators in the where clause to apply two or more conditions. For example, the following code snippet returns the data of the customers who live in London and have last name as Darcy: var query = from <type of the value returned> cus in customers where cus.City == "London" && cust.Name == "Svetlana" select cus; 7.8 Manipulating Data Using LINQ NIIT N ot e Ordering LINQ queries can also be used to retrieve data and display it in a sorted manner. This can be done by using the orderby clause. The orderby clause enables a user to sort the retrieved data in ascending or descending order. For example, the following code snippet returns the data for the customers who live in London. The data is sorted by customer names in alphabetical order: var query = from <type of the value returned> cus in customers where cus.City == "London" orderby cus.Name descending select cus; The default value for the orderby clause is ascending. Grouping You can also retrieve data and group it by using the group clause. For example, the following code snippet returns the data for all the customers, grouped on the basis of the cities they live in: var query = from <type of the value returned> cus in customers group cus by cus.City; When you end a query with a group clause, the results take the form of a list of lists. Each element in the list is an object that has a Key member and a list of elements that are grouped under that key. When you iterate over a query that produces a sequence of groups, you must use a nested foreach loop. The outer loop iterates over each group, and the inner loop iterates over each member of the group. For example, to execute the preceding query and to display the results, you need to use nested foreach loops as shown in the following code snippet: foreach (var cus in query) { foreach (var i in cus) { Response.Write(i.Name + " lives in " + i.City + "<br>"); } } NIIT Manipulating Data Using LINQ 7.9 Joining It is possible that the data you need to retrieve exists in more than one table/object. For example, you need to retrieve information regarding the orders placed by a customer. The information regarding the customers is stored in the customers table/object and the information regarding the orders is stored in the orders table/object. Both the tables/objects must have a common field, which is required to join the tables/objects. To retrieve the required information, you need to join the data from both the tables/objects. This can be done by using the join clause, as shown in the following code snippet: var query = from <type of the value returned> cus in customers join <type of the value returned> ord in Order on cus.order_id equals ord.order_id Select new {CustomerName=cus.Name, OrderAmount=Order.Amount}; In the preceding code snippet, two ArrayList objects customers and Order are the data sources. These ArrayList objects contain information about the cutomers and the orders placed by them. Both the objects have a common field order_id, which is required to join data from these objects. The data from the preceding query can be retrieved by using the following foreach loop: foreach (var cus in query) { Response.Write(cus.CustomerName + " has the order amount " + cus.OrderAmount + "<br>"); } 7.10 Manipulating Data Using LINQ NIIT In Web applications, the data that is to be displayed on the Web pages may exist in disparate data sources. These data sources can be objects, databases, or XML files. You can access data from these data sources either by using the LINQ queries or by using the LINQ data source control. LINQ provides a standard query for retrieving data from disparate data sources. These queries can be used to access data from:  Objects  Databases  XML files Accessing Data from Objects Data from objects can be accessed by using LINQ to Objects. LINQ to Objects provides an easy way to create a query for accessing data from an object collection such as an ArrayList by writing foreach loops. To use LINQ to query an object collection, you need to declare a range variable. The type of the range variable should match the type of the objects in the collection. For example, if you create an ArrayList that stores objects of type customers, the range variable should also be of the customers type, as shown in the following code snippet: var query = from customers cust in arrList select cust; In the preceding code snippet, a range variable cust is of the type customers. By declaring a range variable, each item in the ArrayList is typecasted to a customers object. The following example describes how to access data from a string array that contains the names of students in a class: string[] arr = new string[] { "Peter", "Sam" , "Philip"}; var query = from string person in arr select person; foreach (var p in query) { Response.Write(p + "<BR>"); } Accessing Data from Disparate Data Sources A ccessing Data by Using the LINQ Queries [...]... can use LINQ to ADO.NET LINQ to ADO.NET includes the following technologies to access data: LINQ to dataset LINQ to SQL LINQ to Dataset A Dataset is a collection of cached data presented in a tabular form The data from a Dataset can be retrieved by using LINQ LINQ to Dataset is basically used in situations where you have used ADO.NET for accessing data from a database Now, you want to implement LINQ queries... Objects c LINQ to dataset d LINQ to XML 4 Which of the following types of LINQ provides a run-time infrastructure for managing relational data as objects? a LINQ to SQL b LINQ to dataset c LINQ to Objects d LINQ to XML 5 In which of the following operations, the results take the form of a list of lists? a Filtering b Grouping c Joining d Ordering NIIT Manipulating Data Using LINQ 7.15 Summary In this... Data Using LINQ NIIT Practice Questions 1 Which of the following loops is used to execute the LINQ query? a While loop b Foreach loop c Do While loop d Do loop 2 The _ keyword is a new keyword that can infer its data type from the resulting set a from b where c var d select 3 Which of the following types of LINQ is used to retrieve data from a collection? a LINQ to SQL b LINQ to Objects c LINQ to... during the update operation Update Performs the insert operation Properties of the LinqDataSource Control You can retrieve data by using the LinqDataSource control, as shown in the following example: In the preceding example, the ContextTypeName attribute sets the name... learned that: LINQ provides a standard way to access data from disparate data sources When accessing data by using LINQ, you need to perform the following steps: Specify the data source Create the query Execute the query LINQ provides a standard query for retrieving data from disparate data sources LINQ queries can be used to access data from: Objects Databases XML files You can also use the LINQ data... In the preceding example, LINQ to Dataset is used to retrieve data from the Dataset and display it on the Web page Notice the use of the AsEnumerable() method It changes the compile time type of a query to IEnumerable This means that the type can be used in a foreach loop LINQ to SQL LINQ to SQL provides a run-time infrastructure for managing relational data as objects In LINQ to SQL, the data model... Designer (O/R designer), which provides a visual designer surface for creating LINQ to SQL entity classes and relationships that are based on objects in a database Activity 7.1: Using LINQ to Retrieve and Store Data LINQ to SQL entity classes are also used by the ASP.NET Dynamic Data to determine the user interface that is based on the LINQ to SQL data model ASP.NET Dynamic Data is a framework that enables... using LINQ, you need to write the following code snippet: XDocument xmlDoc = XDocument.Load("D:\\Website7\\XMLFile2.xml"); var q = from c in xmlDoc.Descendants("Customer") select (string)c.Element("CustomerID") + "-" + (string)c.Element("Name")+"-" + (string)c.Element("City")+""; foreach (string name in q) { Response.Write(name); } Accessing Data by Using the LINQ Data Source Control The LinqDataSource... a database, data-source classes, and in-memory collections The LinqDataSource control can be connected to any kind of data collection that is stored in a public field or property The declarative markup and code to perform data operations in a LinqDataSource control are the same for all data sources You can bind data-bound controls to a LinqDataSource control by using the DataSourceID property of the... relational database is mapped to an object model NIIT Manipulating Data Using LINQ 7.11 created by the developer While querying data by using LINQ to SQL, it translates the language-integrated queries in the object model defined by the developer and sends the data to the database for execution The database returns the result, and LINQ to SQL translates the result back into the form of objects that can be . following types of LINQ is used to retrieve data from a collection? a. LINQ to SQL b. LINQ to Objects c. LINQ to dataset d. LINQ to XML 4. Which of the following types of LINQ provides a run-time. data from a database, you can use LINQ to ADO.NET. LINQ to ADO.NET includes the following technologies to access data:  LINQ to dataset  LINQ to SQL LINQ to Dataset A Dataset is a collection. Properties of the LinqDataSource Control You can retrieve data by using the LinqDataSource control, as shown in the following example: <asp:LinqDataSource ID="LinqDataSource1"