Querying In-Memory Data by Using Query Expressions in .NET

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Querying In-Memory Data by Using Query Expressions
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Filtering Data
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The Select method enables you to specify, or project, the elds that you want to include in the enumerable collection. However, you might also want to restrict the rows that the enumerable collection contains. For example, suppose you want to list the names of all companies in the addresses array that are located in the United States only. To do this, you can use the Where method, as follows:
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IEnumerable<string> usCompanies = addresses.Where(addr => String.Equals(addr.Country, United States )) .Select(usComp => usComp.CompanyName); foreach (string name in usCompanies) { Console.WriteLine(name); }
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Syntactically, the Where method is similar to Select. It expects a parameter that de nes a method that lters the data according to whatever criteria you specify. This example makes use of another lambda expression. The type addr is an alias for a row in the addresses array, and the lambda expression returns all rows where the Country eld matches the string United States . The Where method returns an enumerable collection of rows containing every eld from the original collection. The Select method is then applied to these rows to project only the CompanyName eld from this enumerable collection to return another enumerable collection of string objects. (The type usComp is an alias for the type of each row in the enumerable collection returned by the Where method.) The type of the result of this complete expression is therefore IEnumerable<string>. It is important to understand this sequence of operations the Where method is applied rst to lter the rows, followed by the Select method to specify the elds. The foreach statement that iterates through this collection displays the following companies:
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If you are familiar with SQL, you are aware that SQL enables you to perform a wide variety of relational operations besides simple projection and ltering. For example, you can specify that you want data to be returned in a speci c order, you can group the rows returned according to one or more key elds, and you can calculate summary values based on the rows in each group. LINQ provides the same functionality. To retrieve data in a particular order, you can use the OrderBy method. Like the Select and Where methods, OrderBy expects a method as its argument. This method identi es the
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Part III
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expressions that you want to use to sort the data. For example, you can display the names of each company in the addresses array in ascending order, like this:
IEnumerable<string> companyNames = addresses.OrderBy(addr => addr.CompanyName).Select(comp => comp.CompanyName); foreach (string name in companyNames) { Console.WriteLine(name); }
This block of code displays the companies in the addresses table in alphabetical order:
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If you want to enumerate the data in descending order, you can use the OrderByDescending method instead. If you want to order by more than one key value, you can use the ThenBy or ThenByDescending method after OrderBy or OrderByDescending. To group data according to common values in one or more elds, you can use the GroupBy method. The next example shows how to group the companies in the addresses array by country:
var companiesGroupedByCountry = addresses.GroupBy(addrs => addrs.Country); foreach (var companiesPerCountry in companiesGroupedByCountry) { Console.WriteLine( Country: {0}\t{1} companies , companiesPerCountry.Key, companiesPerCountry.Count()); foreach (var companies in companiesPerCountry) { Console.WriteLine( \t{0} , companies.CompanyName); } }
By now you should recognize the pattern! The GroupBy method expects a method that speci es the elds to group the data by. There are some subtle differences between the GroupBy method and the other methods that you have seen so far, though. The main point of interest is that you don t need to use the Select method to project the elds to the result. The enumerable set returned by GroupBy contains all the elds in the original source collection, but the rows are ordered into a set of enumerable collections based on the eld identi ed by the method speci ed by GroupBy. In other words, the result of the GroupBy method is an enumerable set of groups, each of which is an enumerable set of rows. In the example just shown, the enumerable set companiesGroupedByCountry is a set of countries. The items in this set are themselves enumerable collections containing the companies for each country
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