LINQ TO OBJECTS in Visual C#.NET

Printer Data Matrix ECC200 in Visual C#.NET LINQ TO OBJECTS

Listing 27-9. Creating a More Complex Condition in a where Clause using System; using System.Collections.Generic; using System.Linq; class Listing 09 { static void Main(string[] args) { List<string> myFruitList = new List<string>() { "apple", "plum", "cherry", "grape", "banana", "pear", "mango" , "persimmon", "lemon", "lime", "coconut", "pineapple", "orange"}; IEnumerable<string> results = from e in myFruitList where e[0] == 'p' && e[1] == 'e' && e.Length > 4 select e; foreach (string str in results) { Console.WriteLine("Result item: {0}", str); } // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } The query in Listing 27-9 filters the data elements, allowing only those whose first letter is p, whose second letter is e, and whose Length is greater than four characters. Compiling and running Listing 27-9 produces the following results: Result item: persimmon Press enter to finish

Projecting data, also known as selecting data, is performed by the select clause of a LINQ query. When you project data, you select what will be placed into the results for each item that is being processed. The projected data can be the same as the source data, values from one or more members of the source data, or different data entirely. You have already seen how to project the same data as in the results (Listing 27-9, for example). Almost all LINQ queries end with a select clause. In the following sections, I ll show you how to do some other kinds of projection.

You can project the value of a single member of a data type into the results by selecting that value in the select clause of your LINQ query. We already saw this when we selected the first character of a string value using an indexer, but Listing 27-10 provides another example. Listing 27-10. Projecting a Single Member using System; using System.Collections.Generic; using System.Linq; class Listing 10 { static void Main(string[] args) { List<string> myFruitList = new List<string>() { "apple", "plum", "cherry", "grape", "banana", "pear", "mango" , "persimmon", "lemon", "lime", "coconut", "pineapple", "orange"}; IEnumerable<int> results = from e in myFruitList where e[0] == 'p' select e.Length; foreach (int i in results) { Console.WriteLine("Result item: {0}", i); } // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } The generic type of the data source is string in Listing 27-10, and the select clause in projects the value of the Length member, which is an int. Notice that the generic type of the results has changed to int as well. The result from this query will be a sequence of int values representing the number of characters in the fruit names. The LINQ query in Listing 27-10 has a where clause as well. I have included this to demonstrate that you can combine the various LINQ operations to create more complex queries. In this query, the where clause filters the data items so only those that start with the letter p are passed to the select clause, which then projects the Length value. Compiling and running Listing 27-10 produces the following results: Result item: 4 Result item: 4 Result item: 9 Result item: 9 Press enter to finish

You can select more than one member from a data type by projecting an anonymous type (anonymous types are explained in 15). Listing 27-11 provides an example. Listing 27-11. Projecting an Anonymous Type using System; using System.Collections.Generic; using System.Linq; class Listing 11 { static void Main(string[] args) { List<string> myFruitList = new List<string>() { "apple", "plum", "cherry", "grape", "banana", "pear", "mango" , "persimmon", "lemon", "lime", "coconut", "pineapple", "orange"}; var results = from e in myFruitList select new { Length = e.Length, FirstChar = e[0] }; foreach (var item in results) { Console.WriteLine("Result item - Length: {0}, FirstChar: {1}", item.Length, item.FirstChar); } // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } The anonymous type in Listing 27-11 contains two properties. The Length property contains the number of characters in the fruit name, and the FirstChar property contains the first character in the name. You must use the var keyword for the result generic type when you project an anonymous type and when you enumerate the results, as shown in the listing. Compiling and running Listing 27-11 produces the following results: Result Result Result Result Result Result Result item item item item item item item Length: Length: Length: Length: Length: Length: Length: 5, 4, 6, 5, 6, 4, 5, FirstChar: FirstChar: FirstChar: FirstChar: FirstChar: FirstChar: FirstChar: a p c g b p m