barcode generator project in vb.net INTRODUCTION TO LINQ in C#

Paint ECC200 in C# INTRODUCTION TO LINQ

The where clause eliminates items from further consideration if they don t meet the specified condition. The syntax of the where clause is the following: where BooleanExpression Important things to know about the where clause are the following: A query expression can have any number of where clauses, as long as they are in the from...let...where section. An item must satisfy all the where clauses to avoid elimination from further consideration.

The following code shows an example of a query expression that contains two where clauses. The where clauses eliminate each set of integers from the two arrays where the sum of the two is not greater than or equal to 11, and the element from groupA is not the value 4. Each set of elements selected must satisfy the conditions of both where clauses. static void Main() { var groupA = new[] { 3, 4, 5, 6 }; var groupB = new[] { 6, 7, 8, 9 }; var someInts = from int a in groupA from int b in groupB let sum = a + b where sum >= 11 where a == 4 select new {a, b, sum}; foreach (var a in someInts) Console.WriteLine(a); } This code produces the following output: { a = 4, b = 7, sum = 11 } { a = 4, b = 8, sum = 12 } { a = 4, b = 9, sum = 13 }

The orderby clause takes an expression and returns the result items in order according to the expression. Figure 21-9 shows the syntax of the orderby clause. The optional keywords ascending and descending set the direction of the order. Expression is generally a field of the items. The default ordering of an orderby clause is ascending. You can, however, explicitly set the ordering of the elements to either ascending or descending, using the ascending and descending keywords. There can be any number of orderby clauses, and they must be separated by commas.

Figure 21-9. The syntax of the orderby clause The following code shows an example of student records ordered by the ages of the students. Notice that the array of student information is stored in an array of anonymous types. static void Main( ) { var students = new [] // Array of objects of an anonymous type { new { LName="Jones", FName="Mary", Age=19, Major="History" }, new { LName="Smith", FName="Bob", Age=20, Major="CompSci" }, new { LName="Fleming", FName="Carol", Age=21, Major="History" } }; var query = from student in students orderby student.Age Order by Age. select student; foreach (var s in query) { Console.WriteLine("{0}, {1}: {2} - {3}", s.LName, s.FName, s.Age, s.Major); } } This code produces the following output: Jones, Mary: 19 - History Smith, Bob: 20 - CompSci Fleming, Carol: 21 - History

There are two types of clauses that make up the select...group section the select clause and the group...by clause. While the clauses that precede the select...group section specify the data sources and which objects to choose, the select...group section does the following: The select clause specifies which parts of the chosen objects should be selected. It can specify any of the following: The entire data item A field from the data item A new object comprising several fields from the data item (or any other value, for that matter).

The group...by clause is optional and specifies how the chosen items should be grouped. We ll cover the group...by clause later in the chapter. Figure 21-10 shows the syntax for the select...group clause.

The following code shows an example of using the select clause to select the entire data item. First, the program creates an array of objects of an anonymous type. The query expression then uses the select statement to select each item in the array. using System; using System.Linq; class Program { static void Main() { var students = new[] // Array of objects of an anonymous type { new { LName="Jones", FName="Mary", Age=19, Major="History" }, new { LName="Smith", FName="Bob", Age=20, Major="CompSci" }, new { LName="Fleming", FName="Carol", Age=21, Major="History" } }; var query = from s in students select s; foreach (var q in query) Console.WriteLine("{0}, {1}: Age {2}, {3}", q.LName, q.FName, q.Age, q.Major); } } This code produces the following output: Jones, Mary: Age 19, History Smith, Bob: Age 20, CompSci Fleming, Carol: Age 21, History You can also use the select clause to choose only particular fields of the object. For example, the select clause in the following code selects only the last name of the student. var query = from s in students select s.LName; foreach (var q in query) Console.WriteLine(q); When you substitute these two statements for the corresponding two statements in the preceding full example, the program produces the following output: Jones Smith Fleming