Advanced Topic: Nested using Declaratives in C#.NET

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Advanced Topic: Nested using Declaratives
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Not only can you have using declaratives at the top of a file, but you also can include them at the top of a namespace declaration For example, if a new namespace, AwlMichaelisEssentialCSharp, were declared, it would be possible to add a using declarative at the top of the namespace declaration (see Listing 47)
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Listing 47 Specifying the using Directive inside a Namespace Declaration
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namespace AwlMichaelisEssentialCSharp { using System; class HelloWorld { static void Main() { // No need to qualify Console with System // because of the using directive above ConsoleWriteLine("Hello, my name is Inigo Montoya"); } } }
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The results of Listing 47 appear in Output 43
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Output 43
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Hello, my name is Inigo Montoya
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The difference between placing the using declarative at the top of a file rather than at the top of a namespace declaration is that the declarative is active only within the namespace declaration If the code includes a new namespace declaration above or below the AwlMichaelisEssentialCSharp declaration, then the using System directive within a different namespace would not be active Code seldom is written this way, especially given the standard practice of a single type declaration per file
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Aliasing
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The using directive also has a provision for aliasing a namespace or type An alias is an alternate name that you can use within the scope to which the using directive applies The two most common reasons for aliasing are to disambiguate two types that have the same name and to abbreviate a long name In Listing 48, for example, the CountDownTimer alias is declared as a means of referring to the
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type SystemTimersTimer Simply adding a using SystemTimers directive will not sufficiently enable the code to avoid fully qualifying the Timer type The reason is that SystemThreading also includes a type called Timer, and therefore, just using Timer within the code will be ambiguous
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Listing 48 Declaring a Type Alias
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using System; using SystemThreading; using CountDownTimer = SystemTimersTimer; class HelloWorld { static void Main() { CountDownTimer timer; // } }
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Listing 48 uses an entirely new name, CountDownTimer, as the alias It is possible, however, to specify the alias as Timer, as shown in Listing 49
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Listing 49 Declaring a Type Alias with the Same Name
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using System; using SystemThreading; // Declare alias Timer to refer to SystemTimersTimer to // avoid code ambiguity with SystemThreadingTimer using Timer = SystemTimersTimer; class HelloWorld { static void Main() { Timer timer; // } }
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Because of the alias directive, "Timer" is not an ambiguous reference Furthermore, to refer to the SystemThreadingTimer type, you will have to either qualify the type or define a different alias
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Returns and Parameters on Main()
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So far, declaration of an executable's Main() method has been the simplest declaration possible You have not included any parameters or return types in your Main() method declarations However, C# supports the ability to retrieve the command-line arguments when executing a program, and it is possible to return a status indicator from the Main() method The runtime passes the command-line arguments to Main() using a single string array parameter All you need to do to retrieve the parameters is to access the array, as demonstrated in Listing 410 The purpose of this program is to download a file whose location is given by a URL The first command-line argument identifies the URL, and the optional second argument is the filename to which to save the file The listing begins with a switch statement that evaluates the number of parameters (argsLength) as follows
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1 If there are zero parameters, display an error indicating that it is necessary to provide the URL 2 If there is only one argument, calculate the second argument from the first argument 3 The presence of two arguments indicates the user has provided both the URL of the resource and the download target filename
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Listing 410 Passing Command-Line Arguments to Main
using System; using SystemIO; using SystemNet; class Program { static int Main(string[] args) { int result; string targetFileName = null; switch (argsLength) { case 0: // No URL specified, so display error ConsoleWriteLine( "ERROR: You must specify the " + "URL to be downloaded"); break; case 1: // No target filename was specified targetFileName = PathGetFileName(args[0]); break; case 2: targetFileName = args[1]; break; } if (targetFileName != null)
{ WebClient webClient = new WebClient(); webClientDownloadFile(args[0], targetFileName); result = 0; } else { ConsoleWriteLine( "Downloaderexe <URL> <TargetFileName>"); result = 1; } return result; } }