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A Simple Query
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At this point, it will be helpful to work through a simple LINQ example The following program uses a query to obtain the positive values contained in an array of integers:
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// Create a simple LINQ query using System; using SystemLinq; class SimpQuery { static void Main() { int[] nums = { 1, -2, 3, 0, -4, 5 };
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19:
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// Create a query that obtains only positive numbers var posNums = from n in nums where n > 0 select n; ConsoleWrite("The positive values in nums: "); // Execute the query and display the results foreach(int i in posNums) ConsoleWrite(i + " "); ConsoleWriteLine(); } }
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PART I
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This program produces the following output:
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The positive values in nums: 1 3 5
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As you can see, only the positive values in the nums array are displayed Although quite simple, this program demonstrates the key features of LINQ Let s examine it closely The first thing to notice in the program is the using directive:
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using SystemLinq;
To use the LINQ features, you must include the SystemLinq namespace Next, an array of int called nums is declared All arrays in C# are implicitly convertible to IEnumerable<T> This makes any C# array usable as a LINQ data source Next, a query is declared that retrieves those elements in nums that are positive It is shown here:
var posNums = from n in nums where n > 0 select n;
The variable posNums is called the query variable It refers to the set of rules defined by the query Notice it uses var to implicitly declare posNums As you know, this makes posNums an implicitly typed variable In queries, it is often convenient to use implicitly typed variables, although you can also explicitly declare the type (which must be some form of IEnumerable<T>) The variable posNums is then assigned the query expression All queries begin with from This clause specifies two items The first is the range variable, which will receive elements obtained from the data source In this case, the range variable is n The second item is the data source, which in this case is the nums array The type of the range variable is inferred from the data source In this case, the type of n is int Generalizing, here is the syntax of the from clause: from range-variable in data-source The next clause in the query is where It specifies a condition that an element in the data source must meet in order to be obtained by the query Its general form is shown here: where boolean-expression
Part I:
The C# Language
The boolean-expression must produce a bool result (This expression is also called a predicate) There can be more than one where clause in a query In the program, this where clause is used:
where n > 0
It will be true only for an element whose value is greater than zero This expression will be evaluated for every n in nums when the query executes Only those values that satisfy this condition will be obtained In other words, a where clause acts as a filter on the data source, allowing only certain items through All queries end with either a select clause or a group clause This example employs the select clause It specifies precisely what is obtained by the query For simple queries, such as the one in this example, the range value is selected Therefore, it returns those integers from nums that satisfy the where clause In more sophisticated situations, it is possible to finely tune what is selected For example, when querying a mailing list, you might return just the last name of each recipient, rather than the entire address Notice that the select clause ends with a semicolon Because select ends a query, it ends the statement and requires a semicolon Notice, however, that the other clauses in the query do not end with a semicolon At this point, a query variable called posNums has been created, but no results have been obtained It is important to understand that a query simply defines a set of rules It is not until the query is executed that results are obtained Furthermore, the same query can be executed two or more times, with the possibility of differing results if the underlying data source changes between executions Therefore, simply declaring the query posNums does not mean that it contains the results of the query To execute the query, the program uses the foreach loop shown here:
foreach(int i in posNums) ConsoleWriteLine(i + " ");
Notice that posNums is specified as the collection being iterated over When the foreach executes, the rules defined by the query specified by posNums are executed With each pass through the loop, the next element returned by the query is obtained The process ends when there are no more elements to retrieve In this case, the type of the iteration variable i is explicitly specified as int because this is the type of the elements retrieved by the query Explicitly specifying the type of the iteration variable is fine in this situation, since it is easy to know the type of the value selected by the query However, in more complicated situations, it will be easier (or in some cases, necessary) to implicitly specify the type of the iteration variable by using var
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