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The easiest way to figure out which expression type to use is to examine an existing lambda expression in the Visual Studio debugger. We continue this discussion online, at http://www.albahari.com/expressions/. 366 | Chapter 8: LINQ Queries
9 LINQ Operators This chapter describes each of the LINQ query operators. As well as serving as a reference, two of the sections, “Projecting” on page 375 and “Joining” on page 370, cover a number of conceptual areas: • Projecting object hierarchies • Joining with Select, SelectMany, Join, and GroupJoin • Outer range variables in query expressions All of the examples in this chapter assume that a names array is defined as follows: string[] names = { "Tom", "Dick", "Harry", "Mary", "Jay" }; Examples that query a database assume that a typed DataContext variable called dataContext is instantiated as follows: var dataContext = new NutshellContext ("connection string..."); ... public class NutshellContext : DataContext { public NutshellContext (string cxString) : base (cxString) {} public Table Customers { get { return GetTable(); } } public Table Purchases { get { return GetTable(); } } } [Table] public class Customer { [Column(IsPrimaryKey=true)] public int ID; [Column] public string Name; [Association (OtherKey="CustomerID")] public EntitySet Purchases = new EntitySet(); } 367
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C# 4.0 www.free-ebooks-library.com
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C# 4.0 in a Nutshell, Fourth Editio
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Structs 89 Access Modifiers 90 Inte
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Annotations 440 Projecting into an
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Emitting Generic Methods and Types
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Groups 985 Replacing and Splitting
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If you’re already familiar with C
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This icon signifies a tip, suggesti
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I have the highest praise for Jon S
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Properties, methods, and events In
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Figure 1-1. This depicts the topics
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object o1 = "foo.doc"; object o2 =
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The first statement computes the ex
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The name of the C# compiler is csc.
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With contextual keywords, ambiguity
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Custom Type Examples Just as we can
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} } The public keyword The public k
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Figure 2-2. Assignment copies a val
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Technically, the CLR positions fiel
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Numeric literal type inference By d
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+ Addition − Subtraction * Multip
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In this case, x and y are implicitl
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which breaks equality and compariso
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Strings and Characters C#’s char
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Arrays An array represents a fixed
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int[,] matrix = new int[,] { {0,1,2
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Variables and Parameters A variable
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The following code outputs 0, becau
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static void Foo (ref int p) { p = p
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for (int i = 0; i < ints.Length; i+
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var—Implicitly Typed Local Variab
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Operator Precedence and Associativi
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Category Operator symbol Operator n
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Expression statements x = 1 + 2; //
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The switch statement switch stateme
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For example, the following prints t
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The return statement The return sta
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Rules Within a Namespace Name scopi
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namespace N2 { class Class3 : Class
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The :: token is used for namespace
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Fields allow the following modifier
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Overloading constructors A class or
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Object Initializers Versus Optional
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Properties allow the following modi
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Here’s how we could use this inde
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If a static constructor throws an u
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In addition, each participant can i
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Casting and Reference Conversions A
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public class Asset { public string
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Overrider over = new Overrider(); B
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Constructor and field initializatio
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Boxing and Unboxing Boxing is the a
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The ToString Method The ToString me
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Changing struct to class makes this
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Interfaces An interface is similar
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type that implements ISerializable
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public class RichTextBox : TextBox
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Center } A translation between the
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Unfortunately, Enum.IsDefined does
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Generic Types C# generics and C++ t
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static void Swap (ref T a, ref T b)
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Generic Constraints By default, a t
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Self-Referencing Generic Declaratio
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Classes Covariance and contravarian
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The IEnumerator and IEnumerable int
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The reason is that Max needs to be
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Technically, we are specifying a me
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Multicast delegate example Suppose
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The Func and Action Delegates With
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The following, however, is permitte
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Events When using delegates, two em
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public class PriceChangedEventArgs
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if (price == value) return; OnPrice
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Here is an example that illustrates
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Capturing Outer Variables A lambda
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Anonymous Methods Anonymous methods
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OUTPUT: x cannot be zero program co
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• The try block finishes normally
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The other common scenario is to ret
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indeterminate state that is the res
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class Enumerable // Typically imple
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Iterator Semantics An iterator is a
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yield return prevFib; int newFib =
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object o = "string"; int? x = o as
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Console.WriteLine (n | f); // (null
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== != > < >=
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Note n = (Note)554.37; // explicit
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Extension Method Chaining Extension
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static void Main() { string[] strin
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Now let’s change the static type
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Dynamic binding also incurs a perfo
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The static type of a variable decla
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The call to Foo(o,d) is dynamically
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parameters. The following attribute
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Value types declared inline within
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} } Pointers to Unmanaged Code Poin
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Compiler warnings can be extremely
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... Indicates a multiline code sam
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M:NS.MyClass.Z(System.Char[ ],Syste
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Table 5-1. C#, CLR, and .NET Framew
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Text Processing The System.Text nam
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this metadata at runtime, and do su
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infrastructure. Web Forms has been
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Provider layer The provider model d
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Remoting and Web Services Remoting
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ecause in Turkey, char.ToUpper ('i'
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string empty = ""; Console.WriteLin
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The static Join method does the rev
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order—but even then not exactly a
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The following performs a case-insen
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UTF-32 is the least space-efficient
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Two-word characters are called surr
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An excellent article on the rationa
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You can also construct a DateTime w
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The Add method adds a TimeSpan to a
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You can construct a DateTime that d
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source and destination TimeZoneInfo
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Western Australia is actually uniqu
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ToString and Parse The simplest for
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The next example formats a DateTime
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The following custom format provide
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Letter Meaning Sample input Result
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Figure 6-1. Composite NumberStyles
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The format strings "r", "R", and "u
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Other Conversion Mechanisms In the
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XmlConvert If you’re dealing with
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The .NET Framework helps you with t
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The Round method lets you specify t
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If you don’t want reproducibility
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static object GetBoxedIntegralValue
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Tuples Framework 4.0 provides a new
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Value Versus Referential Equality T
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Why the Complexity? You might wonde
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The IEquatable interface A conseque
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How to override equality semantics
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Implementing IEquatable For complet
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The .NET Framework also provides pl
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if (Equals (other)) return 0; // Fa
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Environment The static System.Envir
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7 Collections The .NET Framework pr
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By defining a single method retunin
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You can also subclass an existing c
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{ get { if (currentIndex == −1) t
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The ICollection and IList Interface
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IList and IList IList is the standa
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Because Array is a class, arrays ar
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void WriteFirstValue (Array a) { Co
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public static int FindIndex (T[] ar
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The Comparison delegate follows the
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List and ArrayList The generic List
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Such casts cannot be verified by th
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Here’s a demonstration on the use
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public sealed class BitArray : ICol
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public virtual SortedSet GetViewBet
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IDictionary IDictionary defines the
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Its underlying hashtable works by c
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var sorted = new SortedList (); for
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public class Animal { public string
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A keyed collection has some resembl
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clumsy hook methods like Collection
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Recall the requirements of a hashta
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IComparer and Comparer Comparers ar
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{ public abstract int Compare (stri
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8 LINQ Queries LINQ, or Language In
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We could further shorten our code b
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Page 341 and 342: Figure 8-2. Query syntax The compil
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Page 345 and 346: Reevaluation Subqueries provide ano
Page 347 and 348: How Deferred Execution Works Query
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Page 407 and 408: Joining If you’re used to writing
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Page 413 and 414: You can create and query lookups ma
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Page 419 and 420: from file in files group file.ToUpp
Page 421 and 422: In the next example, we filter the
Page 423 and 424: public static IEnumerable Cast (IE
Page 425 and 426: Method Description SQL equivalents
Page 427 and 428: Count simply enumerates over a sequ
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Page 433 and 434: 10 LINQ to XML The .NET Framework p
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XElement client = config.Element ("
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); new XElement ("lastname", c.Last
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Child Node Navigation Return type M
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The first call to Elements binds to
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Return type Members Defined in IEnu
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The ReplaceXXX methods are equivale
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Setting Values There are two ways t
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var e1 = new XElement ("test", "Hel
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Console.WriteLine (doc.Root.Parent
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XML file into another. Second, name
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Specifying Namespaces in the X-DOM
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foreach (XElement e in data.Descend
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A public type such as string doesn
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know about IQueryable, so it forces
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ObjectGraph.cs Program.cs Propertie
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11 Other XML Technologies The Syste
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Reading Nodes The units of an XML s
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Element----------> quote CDATA-----
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eader.ReadStartElement ("customerLi
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The XmlReader must be positioned on
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XmlWriter XmlWriter is a forward-on
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Patterns for Using XmlReader/XmlWri
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if (isEmpty) return; // element! w
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using (XmlWriter w = XmlWriter.Crea
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Setting the InnerText property repl
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XPath queries are expressed in term
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The [] operator filters a selection
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OUTPUT: Jim Thomas XSD and Schema V
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settings.ValidationEventHandler +=
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12 Disposal and Garbage Collection
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These rules are also helpful when w
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The idea is that a consumer of this
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public void Test() { byte[] myArray
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Figure 12-1. Roots deleted right aw
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objects with finalizers (because su
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public class TempFileRef { public r
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Figure 12-2. Heap generations The l
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dynamically tweaks the thresholds f
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Timers Forgotten timers can also ca
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var sb = new StringBuilder ("this i
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public TDelegate Target { get { var
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13 Diagnostics and Code Contracts W
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MyCompany.Widgets.Gadget; #endif Yo
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Both the Debug and Trace classes pr
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EventLog.CreateEventSource ("DemoAp
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if (list.Contains(item)) return fal
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The binary rewriter After compiling
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Preconditions should access only me
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• It’s time-consuming. • Call
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The binary rewriter moves postcondi
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For the reason just stated, postcon
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A special mechanism lets specify a
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Handling this event is particularly
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3. In the debug configuration of th
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Attaching and Breaking The static D
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If you instantiate a StackTrace obj
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2. Decide on a source name and crea
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The following sections illustrate h
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{ if (!PerformanceCounterCategory.E
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If you later want to define more co
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Figure 14-1. Stream architecture St
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In the following example, we use a
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Seeking A stream is seekable if Can
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OpenWrite and Create differ in beha
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[Flags] public enum FileAccess { Re
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Pipes are stream-based, so one proc
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Console.WriteLine (Encoding.UTF8.Ge
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Console.WriteLine (fs.Position); //
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is passed in (optionally through an
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using (TextWriter w = File.CreateTe
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public void SaveData (Stream s) { v
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The File Class File is a static cla
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using System.Runtime.InteropService
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FileInfo offers most of the File’
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Special Folders You must delete the
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watcher.Created += FileCreatedChang
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Memory-Mapped Files and Shared Memo
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using (TextWriter w = new StreamWri
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Isolation Types Isolated storage ca
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IsolatedStorageFile is poorly named
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s.WriteByte (100); f.DeleteFile (@"
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Figure 15-1. Network architecture I
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IPAddress a3 = IPAddress.Parse ("[3
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Uri u = new Uri ("page.html", UriKi
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2. Assign the Proxy property. 3. As
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If you don’t have a proxy, you mu
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Windows protocol that boils down to
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2. Examine the response object’s
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System.IO.File.WriteAllBytes ("Sear
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you greater privileges in browsing
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In this example, we sleep for 500 m
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without needing more threads—by f
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Using DNS In all these examples, yo
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Using TCP TCP and UDP constitute th
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Hello Hello right back! In this exa
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In this example, we write each mail
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Serialization Engines There are fou
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an arbitrary XML structure. For ins
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It requires, however, that you expl
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Specifying a name and namespace dec
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The solution is to specify all perm
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we would see the same address detai
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Null and Empty Values There are two
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Subclassed Collection Elements The
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[DataMember (Name="Addresses")] Ser
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The following example illustrates a
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The two formatters are otherwise ex
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[OnSerializing] and [OnSerialized]
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Binary Serialization with ISerializ
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Subclassing Serializable Classes In
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XmlSerializer xs = new XmlSerialize
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and you write a reusable method to
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If, on the other hand, you want the
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instead add [XmlElement] to the col
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public void WriteXml (XmlWriter wri
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Assemblies are structured similarly
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To embed an application manifest fi
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Table 17-1. Assembly members Functi
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Delay Signing In an organization wi
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If the assembly has no AssemblyVers
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Figure 17-3. Unsigned file warning
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Signing with signtool.exe You can A
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• Updating assemblies in the GAC
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.mresource public banner.jpg { // O
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GetManifestResourceNames returns th
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(The first argument must be namespa
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We then retrieved the greeting at r
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where to look. You (or Visual Studi
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Loading from a filename You can see
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if (!File.Exists (path)) return nul
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for the presence of a library in it
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18 Reflection and Metadata As we sa
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Obtaining array types As we just sa
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Base Types and Interfaces Type expo
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Console.WriteLine (staticD.DynamicI
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When calling GetMembers, you can pa
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C# construct Method to use Name to
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Console.WriteLine (unbound.Property
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from m in typeof (Enumerable).GetMe
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The following example retrieves all
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There’s no nongeneric type we can
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Modules Calling GetTypes on a multi
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The AttributeUsage Attribute Attrib
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Retrieving Attributes at Runtime Th
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AppDomain.CurrentDomain.ReflectionO
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gen.Emit (OpCodes.Ldc_I4, 123); //
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gen.Emit (OpCodes.Mul); // Multiply
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Type[] strT = { typeof (string) };
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The DefineType method also accepts
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We could just as easily call a dyna
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mb.DefineParameter (1, ParameterAtt
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ILGenerator setGen = setter.GetILGe
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myFieldBuilder.SetCustomAttribute (
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Awkward Emission Targets All the ex
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and object references. In creating
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Here is a sample of the output our
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The next step is to call FormatOper
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The point where we call ResolveMemb
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trees (the same expression trees in
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yte b = 3; Console.WriteLine ((b +
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DynamicVisit on each person in the
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Anonymously Calling Members of a Ge
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Notice that we used dynamic member
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The following does a similar thing
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In the following example, we use Ir
Page 771 and 772:
In this chapter, we discuss the two
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Framework 4.0 deprecates support fo
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When you call AddPermission, the pe
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if you access the chosen file by ca
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Elevation of Privilege Let’s supp
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The Transparency Model in CLR 4.0 T
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Figure 20-1. Transparency model; on
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GC.AddMemoryPressure (1000000); //
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stack, the WindowFromPoint method w
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string dir = Path.Combine (System.R
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The first step in doing this is to
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A feature called User Access Contro
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If you replace requireAdministrator
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Table 20-8. Encryption and hashing
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The GetBytes method on an Encoding
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As well as a key, we made up an IV,
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w.WriteLine ("Small and secure!");
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pairs. A key pair contains a public
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using (var rsaPublicPrivate = new R
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C# allows you to execute code in pa
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Getting Started A client program (C
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Thread.Sleep(0) relinquishes the th
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string text = "t1"; Thread t1 = new
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If this program is called with no a
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The try/catch statement in this exa
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There are a few things to be wary o
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static void Main() { ThreadPool.Que
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we’re using it to pass the method
Page 827 and 828:
Fortunately, the pool manager has a
Page 829 and 830:
class ThreadUnsafe { static int _va
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As with the Enter method, it’s ov
Page 833 and 834:
Instruction atomicity is a differen
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BeginInvoke instead of Invoke. Alte
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static void Main() { for (int i = 1
Page 839 and 840:
class ThreadSafe { static List _li
Page 841 and 842:
In the absence of documentation, it
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The code is similar for Windows For
Page 845 and 846:
It’s also possible to safely assi
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The following implicitly generate f
Page 849 and 850:
As it happens, Intel’s X86 and X6
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Exchange and CompareExchange method
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former two are based on the common
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Two-way signaling Let’s say we wa
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Signaling Constructs and Performanc
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Signaling worker... Started - Some
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and the lock statement, you can ach
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This means that despite appearances
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using System; using System.Threadin
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We don’t pulse after dequeuing, a
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Use of PulseAll would forgo fairnes
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The Event-Based Asynchronous Patter
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class Program { static BackgroundWo
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Reached 20% Reached 40% Reached 60%
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For a discussion as to why aborting
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Cancellation Tokens Framework 4.0 p
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if (_expensive == null) _expensive
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you want to keep data isolated, ens
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from under their feet, as long as t
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Here’s the result: Thread B added
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var rw = new ReaderWriterLockSlim()
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class SystemTimer { static void Mai
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22 Parallel Programming In this cha
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Data parallelism is also conducive
Page 897 and 898:
the code that must execute sequenti
Page 899 and 900:
Why Isn’t AsParallel the Default?
Page 901 and 902:
Most query operators change the ind
Page 903 and 904:
.Select (i => wordList [localRandom
Page 905 and 906:
class Camera { public readonly int
Page 907 and 908:
foreach (int n in parallelQuery) Do
Page 909 and 910:
Figure 22-4. Chunk versus range par
Page 911 and 912:
string text = "Let's suppose this i
Page 913 and 914:
public static void Invoke (Parallel
Page 915 and 916:
var random = new Random(); string[]
Page 917 and 918:
public static ParallelLoopResult Fo
Page 919 and 920:
• Wait on a set of tasks—withou
Page 921 and 922:
The task queuing problem arises bec
Page 923 and 924:
TaskCreationOptions atp = TaskCreat
Page 925 and 926:
As soon as task1 (the antecedent) f
Page 927 and 928:
Conditional continuations By defaul
Page 929 and 930:
This writes “Done” after writin
Page 931 and 932:
var factory = new TaskFactory ( Tas
Page 933 and 934:
Flatten and Handle Both PLINQ and t
Page 935 and 936:
• The concurrent stack, queue, an
Page 937 and 938:
Concurrent bags are ideal when the
Page 939 and 940:
public class PCQueue : IDisposable
Page 941 and 942:
If you leave a spinlock contended f
Page 943 and 944:
How to use SpinWait There are two w
Page 945:
spinWait.SpinOnce(); } } Here’s h
Page 948 and 949:
If you don’t need such concurrenc
Page 950 and 951:
• The underlying implementation
Page 952 and 953:
public class Server { public void S
Page 954 and 955:
void Read() { _stream.BeginRead (_d
Page 956 and 957:
_stream = c.GetStream(); var task =
Page 958 and 959:
exception is encountered, it’s no
Page 960 and 961:
When writing asynchronous methods,
Page 963 and 964:
24 Application Domains An applicati
Page 965 and 966:
Creating and Destroying Application
Page 967 and 968:
When additional application domains
Page 969 and 970:
static void Main() { AppDomain newD
Page 971 and 972:
class Client { // Here's a static f
Page 973 and 974:
it appears as though it was a direc
Page 975 and 976:
namespace Plugin.Extensions { publi
Page 977 and 978:
25 Native and COM Interoperability
Page 979 and 980:
class Test { [DllImport("kernel32.d
Page 981 and 982:
StringBuilder class require copying
Page 983 and 984:
Shared Memory Memory-mapped files,
Page 985 and 986:
The second application subscribes t
Page 987 and 988:
If you’re targeting Framework 4.0
Page 989 and 990:
With the first use of the fixed key
Page 991 and 992:
The root interface in COM is IUnkno
Page 993 and 994:
workBook.SaveAs (@"c:\test.xlsx", P
Page 995 and 996:
Embedding Interop Types We said pre
Page 997 and 998:
26 Regular Expressions The regular
Page 999 and 1000:
Regex r = new Regex (@"sausages?");
Page 1001 and 1002:
Character Sets Character sets act a
Page 1003 and 1004:
The problem is that our * quantifie
Page 1005 and 1006:
The positive lookahead ensures that
Page 1007 and 1008:
The following example matches a sim
Page 1009 and 1010:
Extracting "name = value" pairs (on
Page 1011 and 1012:
Console.WriteLine (m.Groups ["tag"]
Page 1013 and 1014:
Expression Meaning Inverse (“not
Page 1015:
Table 26-11. Miscellaneous construc
Page 1018 and 1019:
catch A keyword for the clause in a
Page 1020 and 1021:
goto A jump statement that jumps to
Page 1022 and 1023:
ef A parameter and argument modifie
Page 1024 and 1025:
volatile A field modifier instructi
Page 1026 and 1027:
A with enums, 98 with nullable type
Page 1028 and 1029:
inary numbers (converting), 233 bin
Page 1030 and 1031:
COM type system, 970 IUnknown and I
Page 1032 and 1033:
time zones, 213-218 TimeSpan, 206 D
Page 1034 and 1035:
Array class, 277 enumerators, 143 I
Page 1036 and 1037:
formatting, 219-225 composite forma
Page 1038 and 1039:
IndexOf method, 196, 279 IndexOfAny
Page 1040 and 1041:
Count and LongCount, 407 Min and Ma
Page 1042 and 1043:
memory barriers and volatility, 825
Page 1044 and 1045:
PipeStream class, 547 anonymous pip
Page 1046 and 1047:
Remoting, 952 remove keyword, 124 R
Page 1048 and 1049:
two-word characters and, 206 String
Page 1050 and 1051:
DateTime and, 213 DateTimeOffset an
Page 1052 and 1053:
loading and parsing, 416 names and
Page 1055:
About the Authors Joe Albahari is t
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