c# qr codes The Where method that is used in our sample query in C#.NET

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Listing 3.2 The Where method that is used in our sample query
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public static IEnumerable<TSource> Where<TSource>( this IEnumerable<TSource> source, Func<TSource, Boolean> predicate) { foreach (TSource element in source) { if (predicate(element)) yield return element; } }
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But where does this Where method come from Is it a method of the IEnumerable<T> interface Well, no. As you may have guessed if you remember chapter 2, it s an extension method. This can be detected by the presence of the this keyword on the first parameter of the method B. The extension methods we see here (Where, OrderByDescending, and Select) are provided by the System.Linq.Enumerable class. The name of this class comes from the fact that the extension methods it contains work on IEnumerable<T> objects.
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Introducing sequences
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In LINQ, the term sequence designates everything that implements IEnumerable<T>.
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Let s take another look at the Where method. Note that it uses the yield return C statement added in C# 2.0. This and the IEnumerable<TSource> return type in the signature make it an iterator. We ll now take some time to review background information on iterators before getting back to our example.
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Refresher on iterators
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An iterator is an object that allows you to traverse through a collection s elements. What is named an iterator in .NET is also known as a generator in other languages such as Python, or sometimes a cursor, especially within the context of a database. You may not know what an iterator is, but you surely have used several of them before! Each time you use a foreach loop (For Each in VB.NET), an iterator is involved. (This isn t true for arrays because the C# and VB.NET compilers optimize foreach and For Each loops over arrays to replace the use of iterators by a simple loop, as if a for loop were used.) Every .NET collection (List<T>, Dictionary<T>, and ArrayList for example) has a method named GetEnumerator that returns an object used to iterate over its contents. That s what foreach uses behind the scenes to iterate on the items contained in a collection. If you re interested in design patterns, you can study the classical Iterator pattern. This is the design iterators rely on in .NET. An iterator is similar, in its result, to a traditional method that returns a collection, because it generates a sequence of values. For example, we could create the following method to return an enumeration of integers:
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int[] OneTwoThree() { return new [] {1, 2, 3}; }
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However, the behavior of an iterator in C# 2.0 or 3.0 is very specific. Instead of building a collection containing all the values and returning them all at once, an iterator returns the values one at a time. This requires less memory and allows the caller to start processing the first few values immediately, without having the complete collection ready. Let s look at a sample iterator to understand how it works. An iterator is easy to create: it s simply a method that returns an enumeration and uses yield return to provide the values.
LINQ building blocks
Listing 3,3 shows an iterator named OneTwoThree that returns an enumeration containing the integer values 1, 2, and 3:
Listing 3.3 Sample iterator (Iterator.csproj)
using System; using System.Collections.Generic; static class Iterator { static IEnumerable<int> OneTwoThree() { Console.WriteLine("Returning 1"); yield return 1; Console.WriteLine("Returning 2"); yield return 2; Console.WriteLine("Returning 3"); yield return 3; } static void Main() { foreach (var number in OneTwoThree()) { Console.WriteLine(number); } }
Here are the results of this code sample s execution:
Returning 1 1 Returning 2 2 Returning 3 3
As you can see, the OneTwoThree method does not exit until we reach its last statement. Each time we reach a yield return statement, the control is yielded back to the caller method. In our case, the foreach loop does its work, and then control is returned to the iterator method where it left so it can provide the next item. It looks like two methods, or routines, are running at the same time. This is why .NET iterators could be presented as a kind of lightweight coroutine. A traditional method starts its execution at the beginning of its body each time it is called. This kind of method is named a subroutine. In comparison, a coroutine is a
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