Essential CSharp 3rd Edition_5

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Essential CSharp 3rd Edition_5

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  1. 541 A nonymous Types and Implicitly Typed Local Variables Listing 14.2: Type Safety and Immutability of Anonymous Types class Program { static void Main() { var patent1 = new { Title = "Bifocals", YearOfPublication = "1784" }; var patent2 = new { YearOfPublication = "1877", Title = "Phonograph" }; var patent3 = new { patent1.Title, Year = patent1.YearOfPublication }; // ERROR: Cannot implicitly convert type 'AnonymousType#1' to 'AnonymousType#2' // patent1 = patent2; // ERROR: Cannot implicitly convert type 'AnonymousType#3' to 'AnonymousType#2' // patent1 = patent3; // ERROR: Property or indexer 'AnonymousType#1.Title' cannot be assigned to -- it is read only' // patent1.Title = "Swiss Cheese"; } } The resultant two compile errors assert the fact that the types are not com- patible, so they will not successfully convert from one to the other. The third compile error is caused by the reassignment of the Title property. Anonymous types are immutable, so it is a compile error to change a property on an anonymous type once it has been instantiated. Although not shown in Listing 14.2, it is not possible to declare a method with an implicit data type parameter (var). Therefore, instances From the Library of Wow! eBook
  2. 542 C hapter 14: Collection Interfaces with Standard Query Operators of anonymous types can only be passed outside the method in which they are created in only two ways. First, if the method parameter is of type object, the anonymous type instance may pass outside the method because the anonymous type will convert implicitly. A second way is to use method type inference, whereby the anonymous type instance is passed as a method type parameter that the compiler can successfully infer. Calling void Method(T parameter) using Function(patent1), therefore, would succeed, although the available operations on parameter within Function() are limited to those supported by object. In spite of the fact that C# allows anonymous types such as the ones shown in Listing 14.1, it is generally not recommended that you define them in this way. Anonymous types provide critical functionality with C# 3.0 support for projections, such as joining/associating collections, as we discuss later in the chapter. However, generally you should reserve anony- mous type definitions for circumstances where they are required, such as aggregation of data from multiple types. ADVANCED TOPIC Anonymous Type Generation Even though Console.WriteLine()’s implementation is to call ToString(), notice in Listing 14.1 that the output from Console.WriteLine() is not the default ToString(), which writes out the fully qualified data type name. Rather, the output is a list of PropertyName = value pairs, one for each property on the anonymous type. This occurs because the compiler over- rides ToString() in the anonymous type code generation, and instead for- mats the ToString() output as shown. Similarly, the generated type includes overriding implementations for Equals() and GetHashCode(). The implementation of ToString() on its own is an important reason that variance in the order of properties causes a new data type to be gener- ated. If two separate anonymous types, possibly in entirely separate types and even namespaces, were unified and then the order of properties changed, changes in the order of properties on one implementation would have noticeable and possibly unacceptable effects on the others’ ToString() From the Library of Wow! eBook
  3. 543 C ollection Initializers results. Furthermore, at execution time it is possible to reflect back on a type and examine the members on a type—even to call one of these members dynamically (determining at runtime which member to call). A variance in the order of members on two seemingly identical types could trigger unex- pected results, and to avoid this, the C# designers decided to generate two different types. Collection Initializers Another feature added to C# in version 3.0 was collection initializers. A collection initializer allows programmers to construct a collection with an initial set of members at instantiation time in a manner similar to array declaration. Without collection initialization, elements had to be explicitly added to a collection after the collection was instantiated—using some- thing like System.Collections.Generic.ICollection’s Add() method. With collection initialization, the Add() calls are generated by the C# com- plier rather than explicitly coded by the developer. Listing 14.3 shows how to initialize the collection using a collection initializer instead. Listing 14.3: Filtering with System.Linq.Enumerable.Where() using System; using System.Collections.Generic; class Program { static void Main() { List sevenWorldBlunders; sevenWorldBlunders = new List() { // Quotes from Ghandi "Wealth without work", "Pleasure without conscience", "Knowledge without character", "Commerce without morality", "Science without humanity", "Worship without sacrifice", "Politics without principle" }; From the Library of Wow! eBook
  4. 544 C hapter 14: Collection Interfaces with Standard Query Operators Print(sevenWorldBlunders); } private static void Print(IEnumerable items) { foreach (T item in items) { Console.WriteLine(item); } } } The syntax is similar not only to the array initialization, but also to an object initializer with the curly braces following the constructor. If no parameters are passed in the constructor, the parentheses following the data type are optional (as they are with object initializers). A few basic requirements are needed in order for a collection initializer to compile successfully. Ideally, the collection type to which a collection ini- tializer is applied would be of a type that implements System.Collec- tions.Generic.ICollection. This ensures that the collection includes an Add() that the compiler-generated code can invoke. However, a relaxed version of the requirement also exists and simply demands that one or more Add() methods exist on a type that implements IEnumerable—even if the collection doesn’t implement ICollection. The Add() methods need to take parameters that are compatible with the values specified in the col- lection initializer. Allowing initializers on collections that don’t support ICollection was important for two reasons. First, it turns out that the majority of collec- tions (types that implement IEnumerable) do not also implement ICollection, thus significantly reducing the usefulness of collection initializers. Second, matching on the method name and signature compatibility with the collection initialize items enables greater diversity in the items ini- tialized into the collection. For example, the initializer now can support new DataStore(){ a, {b, c}} as long as there is one Add() method whose signature is compatible with a and a second Add() method compatible with b, c. From the Library of Wow! eBook
  5. 545 C ollection Initializers Note that you cannot have a collection initializer for an anonymous type since the collection initializer requires a constructor call, and it is impossible to name the constructor. The workaround is to define a method such as static List CreateList(T t) { return new List(); }. Method type inference allows the type parameter to be implied rather than specified explicitly, and so this workaround successfully allows for the creation of a collection of anonymous types. Another approach to initializing a collection of anonymous types is to use an array initializer. Since it is not possible to specify the data type in the constructor, array initialization syntax allows for anonymous array ini- tializers using new[] (see Listing 14.4). Listing 14.4: Initializing Anonymous Type Arrays using System; using System.Collections.Generic; using System.Linq; class Program { static void Main() { var worldCup2006Finalists = new[] { new { TeamName = "France", Players = new string[] { "Fabien Barthez", "Gregory Coupet", "Mickael Landreau", "Eric Abidal", // ... } }, new { TeamName = "Italy", Players = new string[] { "Gianluigi Buffon", "Angelo Peruzzi", "Marco Amelia", "Cristian Zaccardo", // ... } } }; From the Library of Wow! eBook
  6. 546 C hapter 14: Collection Interfaces with Standard Query Operators Print(worldCup2006Finalists); } private static void Print(IEnumerable items) { foreach (T item in items) { Console.WriteLine(item); } } } The resultant variable is an array of the anonymous type items, which must be homogenous since it is an array. What Makes a Class a Collection: IEnumerable By definition, a collection within .NET is a class that, at a minimum, imple- ments IEnumerable (technically, it would be the nongeneric type IEnu- merable). This interface is a key because implementing the methods of IEnumerable is the minimum implementation requirement needed to support iterating over the collection. Chapter 3 showed how to use a foreach statement to iterate over an array of elements. The syntax is simple and avoids the complication of having to know how many elements there are. The runtime does not directly support the foreach statement, however. Instead, the C# compiler transforms the code as described in this section. foreach with Arrays Listing 14.5 demonstrates a simple foreach loop iterating over an array of integers and then printing out each integer to the console. Listing 14.5: foreach with Arrays int[] array = new int[]{1, 2, 3, 4, 5, 6}; foreach (int item in array) { Console.WriteLine(item); } From the Library of Wow! eBook
  7. 547 W hat Makes a Class a Collection: IEnumerable From this code, the C# compiler creates a CIL equivalent of the for loop, as shown in Listing 14.6. Listing 14.6: Compiled Implementation of foreach with Arrays int number; int[] tempArray; int[] array = new int[]{1, 2, 3, 4, 5, 6}; tempArray = array; for (int counter = 0; (counter < tempArray.Length); counter++) { int item = tempArray[counter]; Console.WriteLine(item); } In this example, note that foreach relies on support for the Length property and the index operator ([]). With the Length property, the C# compiler can use the for statement to iterate through each element in the array. foreach with IEnumerable Although the code shown in Listing 14.6 works well on arrays where the length is fixed and the index operator is always supported, not all types of collections have a known number of elements. Furthermore, many of the col- lection classes, including the Stack, Queue, and Dictionary classes, do not support retrieving elements by index. Therefore, a more general approach of iterating over collections of elements is needed. The iterator pattern provides this capability. Assuming you can determine the first, next, and last elements, knowing the count and supporting retrieval of elements by index is unnecessary. The System.Collections.Generic.IEnumerator and nongeneric System.Collections.IEnumerator interfaces (see Listing 14.8) are designed to enable the iterator pattern for iterating over collections of elements, rather than the length-index pattern shown in Listing 14.6. A class diagram of their relationships appears in Figure 14.1. From the Library of Wow! eBook
  8. 548 C hapter 14: Collection Interfaces with Standard Query Operators IEnumerator Interface IEnumerable Interface Properties IDisposable Current Interface Methods Methods GetEnumerator Methods MoveNext Dispose Reset IEnumerable T IEnumerator T Generic Interface Generic Interface IDisposable IEnumerable IEnumerator Methods Properties GetEnumerator Current Figure 14.1: IEnumerator and IEnumerator Interfaces IEnumerator, which IEnumerator derives from, includes three members. The first is bool MoveNext(). Using this method, you can move from one element within the collection to the next while at the same time detecting when you have enumerated through every item. The second member, a read-only property called Current, returns the element cur- rently in process. Current is overloaded in IEnumerator, providing a type-specific implementation of it. With these two members on the collec- tion class, it is possible to iterate over the collection simply using a while loop, as demonstrated in Listing 14.7. (The Reset() method usually throws a NotImplementedException and, therefore, should never be called. If you need to restart an enumeration, just create a fresh enumerator.) Listing 14.7: Iterating over a Collection Using while System.Collections.Generic.Stack stack = new System.Collections.Generic.Stack(); int number; // ... // This code is conceptual, not the actual code. while (stack.MoveNext()) { number = stack.Current; Console.WriteLine(number); } From the Library of Wow! eBook
  9. 549 W hat Makes a Class a Collection: IEnumerable In Listing 14.7, the MoveNext() method returns false when it moves past the end of the collection. This replaces the need to count elements while looping. Listing 14.7 uses a System.Collections.Generic.Stack as the col- lection type. Numerous other collection types exist; this is just one exam- ple. The key trait of Stack is its design as a last in, first out (LIFO) collection. It is important to note that the type parameter T identifies the type of all items within the collection. Collecting one particular type of object within a collection is a key characteristic of a generic collection. It is important that the programmer understands the data type within the collection when adding, removing, or accessing items within the collection. This preceding example shows the gist of the C# compiler output, but it doesn’t actually compile that way because it omits two important details concerning the implementation: interleaving and error handling. State Is Shared The problem with an implementation such as Listing 14.7 is that if two such loops interleaved each other—one foreach inside another, both using the same collection—the collection must maintain a state indicator of the current element so that when MoveNext() is called, the next ele- ment can be determined. The problem is that one interleaving loop can affect the other. (The same is true of loops executed by multiple threads.) To overcome this problem, the collection classes do not support IEnu- merator and IEnumerator interfaces directly. As shown in Figure 14.1, there is a second interface, called IEnumerable, whose only method is GetEnumerator(). The purpose of this method is to return an object that supports IEnumerator. Instead of the collection class maintaining the state, a different class, usually a nested class so that it has access to the internals of the collection, will support the IEnumerator interface and will keep the state of the iteration loop. The enumerator is like a “cursor” or a “bookmark” in the sequence. You can have multiple bookmarks, and moving each of them enumerates over the collection independently of the other. Using this pattern, the C# equivalent of a foreach loop will look like the code shown in Listing 14.8. From the Library of Wow! eBook
  10. 550 C hapter 14: Collection Interfaces with Standard Query Operators Listing 14.8: A Separate Enumerator Maintaining State during an Iteration System.Collections.Generic.Stack stack = new System.Collections.Generic.Stack(); int number; System.Collections.Generic.Stack.Enumerator enumerator; // ... // If IEnumerable is implemented explicitly, // then a cast is required. // ((IEnumerable)stack).GetEnumerator(); enumerator = stack.GetEnumerator(); while (enumerator.MoveNext()) { number = enumerator.Current; Console.WriteLine(number); } Cleaning Up Following Iteration Since the classes that implement the IEnumerator interface maintain the state, sometimes you need to clean up the state after it exits the loop (because either all iterations have completed or an exception is thrown). To achieve this, the IEnumerator interface derives from IDisposable. Enu- merators that implement IEnumerator do not necessarily implement IDis- posable, but if they do, Dispose() will be called as well. This enables the calling of Dispose() after the foreach loop exits. The C# equivalent of the final CIL code, therefore, looks like Listing 14.9. Listing 14.9: Compiled Result of foreach on Collections System.Collections.Generic.Stack stack = new System.Collections.Generic.Stack(); System.Collections.Generic.Stack.Enumerator enumerator; IDisposable disposable; enumerator = stack.GetEnumerator(); try { int number; while (enumerator.MoveNext()) { number = enumerator.Current; Console.WriteLine(number); } } From the Library of Wow! eBook
  11. 551 W hat Makes a Class a Collection: IEnumerable finally { // Explicit cast used for IEnumerator. disposable = (IDisposable) enumerator; disposable.Dispose(); // IEnumerator will use the as operator unless IDisposable // support is known at compile time. // disposable = (enumerator as IDisposable); // if (disposable != null) // { // disposable.Dispose(); // } } Notice that because the IDisposable interface is supported by IEnu- merator, the using statement can simplify the code in Listing 14.9 to that shown in Listing 14.10. Listing 14.10: Error Handling and Resource Cleanup with using System.Collections.Generic.Stack stack = new System.Collections.Generic.Stack(); int number; using( System.Collections.Generic.Stack.Enumerator enumerator = stack.GetEnumerator()) { while (enumerator.MoveNext()) { number = enumerator.Current; Console.WriteLine(number); } } However, recall that the CIL also does not directly support the using key- word, so in reality, the code in Listing 14.9 is a more accurate C# represen- tation of the foreach CIL code. ADVANCED TOPIC foreach without IEnumerable Technically, the compiler doesn’t require that IEnumerable/IEnumera- ble be supported in order to iterate over a data type using foreach. From the Library of Wow! eBook
  12. 552 C hapter 14: Collection Interfaces with Standard Query Operators Rather, the compiler uses a concept known as “duck typing” such that if no IEnumerable/IEnumerable method is found, it looks for the GetEnu- merator() method to return a type with Current() and MoveNext() meth- ods. Duck typing involves searching for a method by name rather than relying on an interface or explicit method call to the method. Do Not Modify Collections during foreach Iteration Chapter 3 showed that the compiler prevents assignment of the foreach variable (number). As is demonstrated in Listing 14.10, an assignment to number would not be a change to the collection element itself, so the C# compiler prevents such an assignment altogether. In addition, neither the element count within a collection nor the items themselves can generally be modified during the execution of a foreach loop. If, for example, you called stack.Push(42) inside the foreach loop, it would be ambiguous whether the iterator should ignore or incorporate the change to stack—in other words, whether iterator should iterate over the newly added item or ignore it and assume the same state as when it was instantiated. Because of this ambiguity, an exception of type System.InvalidOpera- tionException is generally thrown upon accessing the enumerator if the collection is modified within a foreach loop, reporting that the collection was modified after the enumerator was instantiated. Standard Query Operators Besides the methods on System.Object, any type that implements IEnu- merable has only one method, GetEnumerator(). And yet, it makes more than 50 methods available to all types implementing IEnumera- ble, not including any overloading—and this happens without need- ing to explicitly implement any method except the GetEnumerator() method. The additional functionality is provided using C# 3.0’s extension methods and it all resides in the class System.Linq.Enumerable. Therefore, including the using declarative for System.Linq is all it takes to make these methods available. Each method on IEnumerable is a standard query operator; it pro- vides querying capability over the collection on which it operates. In the From the Library of Wow! eBook
  13. 553 S tandard Query Operators following sections, we will examine some of the most prominent of these standard query operators. Many of the examples will depend on an Inventor and/or Patent class, both of which are defined in Listing 14.11. Listing 14.11: Sample Classes for Use with Standard Query Operators using System; using System.Collections.Generic; using System.Linq; public class Patent { // Title of the published application public string Title { get; set; } // The date the application was officially published public string YearOfPublication { get; set; } // A unique number assigned to published applications public string ApplicationNumber { get; set; } public long[] InventorIds { get; set; } public override string ToString() { return string.Format("{0}({1})", Title, YearOfPublication); } } public class Inventor { public long Id { get; set; } public string Name { get; set; } public string City { get; set; } public string State { get; set; } public string Country { get; set; } public override string ToString() { return string.Format("{0}({1}, {2})", Name, City, State); } } class Program { From the Library of Wow! eBook
  14. 554 C hapter 14: Collection Interfaces with Standard Query Operators static void Main() { IEnumerable patents = PatentData.Patents; Print(patents); Console.WriteLine(); IEnumerable inventors = PatentData.Inventors; Print(inventors); } private static void Print(IEnumerable items) { foreach (T item in items) { Console.WriteLine(item); } } } public static class PatentData { public static readonly Inventor[] Inventors = new Inventor[] { new Inventor(){ Name="Benjamin Franklin", City="Philadelphia", State="PA", Country="USA", Id=1 }, new Inventor(){ Name="Orville Wright", City="Kitty Hawk", State="NC", Country="USA", Id=2}, new Inventor(){ Name="Wilbur Wright", City="Kitty Hawk", State="NC", Country="USA", Id=3}, new Inventor(){ Name="Samuel Morse", City="New York", State="NY", Country="USA", Id=4}, new Inventor(){ Name="George Stephenson", City="Wylam", State="Northumberland", Country="UK", Id=5}, new Inventor(){ Name="John Michaelis", City="Chicago", State="IL", Country="USA", Id=6}, new Inventor(){ Name="Mary Phelps Jacob", City="New York", State="NY", Country="USA", Id=7}, }; public static readonly Patent[] Patents = new Patent[] { From the Library of Wow! eBook
  15. 555 S tandard Query Operators new Patent(){ Title="Bifocals", YearOfPublication="1784", InventorIds=new long[] {1}}, new Patent(){ Title="Phonograph", YearOfPublication="1877", InventorIds=new long[] {1}}, new Patent(){ Title="Kinetoscope", YearOfPublication="1888", InventorIds=new long[] {1}}, new Patent(){ Title="Electrical Telegraph", YearOfPublication="1837", InventorIds=new long[] {4}}, new Patent(){ Title="Flying machine", YearOfPublication="1903", InventorIds=new long[] {2,3}}, new Patent(){ Title="Steam Locomotive", YearOfPublication="1815", InventorIds=new long[] {5}}, new Patent(){ Title="Droplet deposition apparatus", YearOfPublication="1989", InventorIds=new long[] {6}}, new Patent(){ Title="Backless Brassiere", YearOfPublication="1914", InventorIds=new long[] {7}}, }; } Listing 14.11 also provides a selection of sample data. Output 14.2 displays the results. OUTPUT 14.2: Bifocals(1784) Phonograph(1877) Kinetoscope(1888) Electrical Telegraph(1837) Flying machine(1903) Steam Locomotive(1815) Droplet deposition apparatus(1989) Backless Brassiere(1914) Benjamin Franklin(Philadelphia, PA) Orville Wright(Kitty Hawk, NC) Wilbur Wright(Kitty Hawk, NC) Samuel Morse(New York, NY) George Stephenson(Wylam, Northumberland) John Michaelis(Chicago, IL) Mary Phelps Jacob(New York, NY) From the Library of Wow! eBook
  16. 556 C hapter 14: Collection Interfaces with Standard Query Operators Filtering with Where() In order to filter out data from a collection, we need to provide a filter method that returns true or false, indicating whether a particular element should be included or not. A delegate expression that takes an argument and returns a Boolean is called a predicate, and a collection’s Where() method depends on predicates for identifying filter criteria, as shown in Listing 14.12. (Technically, the result of the Where() method is a monad which encapsulates the operation of filtering a given sequence with a given predicate.) The output appears in Output 14.3. Listing 14.12: Filtering with System.Linq.Enumerable.Where() using System; using System.Collections.Generic; using System.Linq; class Program { static void Main() { IEnumerable patents = PatentData.Patents; patents = patents.Where( patent => patent.YearOfPublication.StartsWith("18")); Print(patents); } // ... } OUTPUT 14.3: Phonograph(1877) Kinetoscope(1888) Electrical Telegraph(1837) Steam Locomotive(1815) Notice that the code assigns the output of the Where() call back to IEnumerable. In other words, the output of IEnumerable.Where() is a new IEnumerable collection. In Listing 14.12, it is IEnumera- ble. From the Library of Wow! eBook
  17. 557 S tandard Query Operators Less obvious is that the Where() expression argument has not necessar- ily executed at assignment time. This is true for many of the standard query operators. In the case of Where(), for example, the expression is passed in to the collection and “saved” but not executed. Instead, execu- tion of the expression occurs only when it is necessary to begin iterating over the items within the collection. A foreach loop, for example, such as the one in Print() (in Listing 14.11), will trigger the expression to be evaluated for each item within the collection. At least conceptually, the Where() method should be understood as a means of specifying the query regarding what appears in the collection, not the actual work involved with iterating over to produce a new collection with potentially fewer items. Projecting with Select() Since the output from the IEnumerable.Where() method is a new IEnumerable collection, it is possible to again call a standard query operator on the same collection. For example, rather than just filtering the data from the original collection, we could transform the data (see Listing 14.13). Listing 14.13: Projection with System.Linq.Enumerable.Select() using System; using System.Collections.Generic; using System.Linq; class Program { static void Main() { IEnumerable patents = PatentData.Patents; IEnumerable patentsOf1800 = patents.Where( patent => patent.YearOfPublication.StartsWith("18")); IEnumerable items = patentsOf1800.Select( patent => patent.ToString()); Print(items); } // ... } From the Library of Wow! eBook
  18. 558 C hapter 14: Collection Interfaces with Standard Query Operators In Listing 14.13, we create a new IEnumerable collection. In this case, it just so happens that adding the Select() call doesn’t change the output; but this is only because Print()’s Console.WriteLine() call used ToString() anyway. Obviously, a transform still occurred on each item from the Patent type of the original collection to the string type of the items collection. Consider the example using System.IO.FileInfo in Listing 14.14. Listing 14.14: Projection with System.Linq.Enumerable.Select() and new // ... IEnumerable fileList = Directory.GetFiles( rootDirectory, searchPattern); IEnumerable files = fileList.Select( file => new FileInfo(file)); // ... fileList is of type IEnumerable. However, using the projection offered by Select, we can transform each item in the collection to a System.IO.FileInfo object. Lastly, capitalizing on anonymous types, we could create an IEnumera- ble collection where T is an anonymous type (see Listing 14.15 and Output 14.4). Listing 14.15: Projection to an Anonymous Type // ... IEnumerable fileList = Directory.GetFiles( rootDirectory, searchPattern); var items = fileList.Select( file => { FileInfo fileInfo = new FileInfo(file); return new { FileName = fileInfo.Name, Size = fileInfo.Length }; }); // ... From the Library of Wow! eBook
  19. 559 S tandard Query Operators OUTPUT 14.4: { FileName = AssemblyInfo.cs, Size = 1704 } { FileName = CodeAnalysisRules.xml, Size = 735 } { FileName = CustomDictionary.xml, Size = 199 } { FileName = EssentialCSharp.sln, Size = 40415 } { FileName = EssentialCSharp.suo, Size = 454656 } { FileName = EssentialCSharp.vsmdi, Size = 499 } { FileName = EssentialCSharp.vssscc, Size = 256 } { FileName = intelliTechture.ConsoleTester.dll, Size = 24576 } { FileName = intelliTechture.ConsoleTester.pdb, Size = 30208 } { FileName = LocalTestRun.testrunconfig, Size = 1388 } The output of an anonymous type automatically shows the property names and their values as part of the generated ToString() method associ- ated with the anonymous type. Projection using the Select() method is very powerful. We already saw how to filter a collection vertically (reducing the number of items in the collection) using the Where() standard query operator. Now, via the Select() standard query operator, we can also reduce the collection horizontally (making fewer columns) or transform the data entirely. In combination, Where() and Select() provide a means for extracting only the pieces of the original collection that are desirable for the current algorithm. These two methods alone provide a powerful collection manipulation API that would otherwise result in significantly more code that is less readable. ADVANCED TOPIC Running LINQ Queries in Parallel With the abundance of computers having multiple processors and multi- ple cores within those processors, the ability to easily take advantage of the additional processing power becomes far more important. To do this, pro- grams need to be changed to support multiple threads so that work can happen simultaneously on different CPUs within the computer. Listing 14.16 demonstrates one way to do this using Parallel LINQ (PLINQ). From the Library of Wow! eBook
  20. 560 C hapter 14: Collection Interfaces with Standard Query Operators Listing 14.16: Executing LINQ Queries in Parallel // ... IEnumerable fileList = Directory.GetFiles( rootDirectory, searchPattern); var items = fileList. AsParallel() .Select( file => { FileInfo fileInfo = new FileInfo(file); return new { FileName = fileInfo.Name, Size = fileInfo.Length }; }); // ... As Listing 14.16 shows, the change in code to enable parallel support is minimal. All that it uses is a .NET Framework 4 introduced standard query operator, AsParallel(), on the static class System.Linq.Paral- lelEnumerable. Using this simple extension method, however, the run- time begins executing over the items within the fileList collection and returning the resultant objects in parallel. Each parallel operation in this case isn’t particularly expensive (although it is relative to what other exe- cution is taking place), but consider CPU-intensive operations such as encryption or compression. Paralyzing the execution across multiple CPUs can decrease execution time by a magnitude corresponding to the number of CPUs. An important caveat to be aware of (and the reason why AsParallel() appears in an Advanced Block rather than the standard text) is that parallel execution can introduce race conditions such that an operation on one thread can be intermingled with an operation on a different thread, caus - ing data corruption. To avoid this, synchronization mechanisms are required on data with shared access from multiple threads in order to force the operations to be atomic where necessary. Synchronization itself, how- ever, can introduce deadlocks that freeze the execution, further complicat- ing the effective parallel programming. More details on this and additional multithreading topics are covered in Chapter 18 and Chapter 19. From the Library of Wow! eBook
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