make barcode with vb.net 21: LINQ in C#.NET

Generator Code-128 in C#.NET 21: LINQ

21: LINQ
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Anonymous Types and Implicitly Typed Variables
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In Example 21-1, when you retrieve the information from the collection, you retrieve the entire Book object, but you output only the title and author. That s somewhat wasteful, because you re retrieving more information than you actually need. Since you need just the title and author, it would be preferable to be able to say something like this:
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IEnumerable<Book> resultsAuthor = from testBook in bookList where testBook.Author == "Jesse Liberty" select testBook.Title, testBook.Author;
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That construction will cause an error, though, because your query can return only one type of object. You could define a new class say, bookTitleAuthor to hold just the two bits of information you need, but that would also be wasteful, because the class would get used in only one spot in your program, right here when you retrieve and then output the data. Instead, you can just define a new class on the fly, like this:
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IEnumerable<Book> resultsAuthor = from testBook in bookList where testBook.Author == "Jesse Liberty" select new { testBook.Title, testBook.Author };
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Notice that this class doesn t have a name; it doesn t really need one, because you re using it only in this one spot. Therefore, this feature is called an anonymous type. Based on the select statement, the compiler determines the number and types of the properties for the class (two strings, in this case), and creates the class accordingly. This code won t work yet, though. You re assigning the results of the query (now a collection of anonymous objects) to a collection of type <Book>. Obviously, that s a type mismatch, and you ll need to change the type. But what do you change it to, if you don t know the name of the anonymous type That s where implicitly typed variables come in. As we mentioned way back in 3, C# has the ability to infer the type of a variable based on the value you re assigning to it. Even though you don t know the name of the anonymous type, the compiler has assigned it as an identifier, and can recognize that type when it s used. Therefore, your new query looks like this:
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var resultsAuthor = from testBook in bookList where testBook.Author == "Jesse Liberty" select new { testBook.Title, testBook.Author };
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Now resultsAuthor is a collection of anonymous objects, and the compiler is perfectly fine with that. All you need to know is that resultsAuthor is a collection that implements IEnumerable, and you can go ahead and use it to output the results:
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Console.WriteLine("Books by Jesse Liberty:"); foreach (var testBook in resultsAuthor)
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Anonymous Types and Implicitly Typed Variables |
{ Console.WriteLine("{0}, by {1}", testBook.Title, testBook.Author); }
We ve replaced the Book type in the foreach loop with var, but the compiler still knows what type testBook is, because it s a member of the collection resultsAuthor, and the compiler knows what type that is, even if you don t. These changes are shown in Example 21-2, although we ve omitted the Book class definition and the creation of the List for space, because those haven t changed.
using using using using System; System.Collections.Generic; System.Linq; System.Text;
namespace Example_21_2_ _ _ _Anonymous_Types { // simple book class public class Book { ... } class Program { static void Main(string[] args) { List<Book> bookList = new List<Book> { ... }; // find books by Jesse Liberty var resultsAuthor = from testBook in bookList where testBook.Author == "Jesse Liberty" select new { testBook.Title, testBook.Author }; Console.WriteLine("Books by Jesse Liberty:"); foreach (var testBook in resultsAuthor) { Console.WriteLine("{0}, by {1}", testBook.Title, testBook.Author); } } } }
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21: LINQ
Lambda Expressions
Back in 17 we mentioned that lambda expressions were created for use with LINQ, to create expressions that return a method instead of a single return value. The same query we ve been using all along could be written like this with lambda expressions:
var resultsAuthor = bookList.Where(bookEval => bookEval.Author == "Jesse Liberty");
As we mentioned in the previous section, the keyword var lets the compiler infer that resultsAuthor is an IEnumerable collection. You can interpret this whole statement as fill the IEnumerable collection resultsAuthor from the collection bookList with each member such that the Author property is equal to the string Jesse Liberty . The variable bookEval isn t declared anywhere; it can be any valid name. The Boolean expression on the righthand side is projected onto the variable, which is passed to the Where method to use to evaluate the collection. This method syntax takes some getting used to, and it can be easier to use LINQ s query syntax, but you should know how to use the alternative. This example is shown in Example 21-3.
using using using using System; System.Collections.Generic; System.Linq; System.Text;
namespace Example_21_3_ _ _ _Lambda_Expressions { // simple book class public class Book { ... } class Program { static void Main(string[] args) { List<Book> bookList = new List<Book> { ... }; // find books by Jesse Liberty var resultsAuthor = bookList.Where(bookEval => bookEval.Author == "Jesse Liberty");
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