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4: Methods and Parameters
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Listing 410: Passing Command-Line Arguments to Main
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using System; using SystemIO; using SystemNet; class Program { static int Main(string[] args) { int result; string targetFileName = ParseCommandLineArgs(args); 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; } private static string ParseCommandLineArgs(string[] args) { string targetFileName = null; switch (argsLength) { case 0: // No URL specified, so display error ConsoleWriteLine(
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Returns and Parameters on Main()
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"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; } return targetFileName; } }
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The results of Listing 410 appear in Output 44
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OUTPUT 44:
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>Downloaderexe ERROR: You must specify the URL to be downloaded Downloaderexe <URL> <TargetFileName>
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If you were successful in calculating the target filename, you would use it to save the downloaded file Otherwise, you would display the help text The Main() method also returns an int rather than a void This is optional for a Main() declaration, but if it is used, the program can return a status code to a caller, such as a script or a batch file By convention, a return other than zero indicates an error Although all command-line arguments can be passed to Main() via an array of strings, sometimes it is convenient to access the arguments from inside a method other than Main() The SystemEnvironmentGetCommandLineArgs() method returns the command-line arguments array in the same form that Main(string[] args) passes the arguments into Main()
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ADVANCED TOPIC Disambiguate Multiple Main() Methods If a program includes two classes with Main() methods, it is possible to specify on the command line which class to use for the Main() declaration cscexe includes an /m option to specify the fully qualified class name of Main()
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4: Methods and Parameters
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BEGINNER TOPIC Call Stack and Call Site As code executes, methods call more methods that in turn call additional methods, and so on In the simple case of Listing 44, Main() calls GetUserInput(), which in turn calls SystemConsoleReadLine(), which in turn calls even more methods internally The set of calls within calls within calls, and so on, is termed the call stack As program complexity increases, the call stack generally gets larger and larger as each method calls another method As calls complete, however, the call stack shrinks until another series of methods are invoked The term for describing the process of removing calls from the call stack is stack unwinding Stack unwinding always occurs in the reverse order of the method calls The result of method completion is that execution will return to the call site, which is the location from which the method was invoked
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So far, this chapter s examples have returned data via the method return This section demonstrates the options of returning data via method parameters and via a variable number of parameters
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BEGINNER TOPIC Matching Caller Variables with Parameter Names In some of the previous listings, you matched the variable names in the caller with the parameter names in the callee (target method) This matching is simply for readability; whether names match is entirely irrelevant to the behavior of the method call Value Parameters By default, parameters are passed by value, which means that the variable s stack data is copied into the target parameter For example, in Listing 411, each variable that Main() uses when calling Combine() will be copied into the parameters of the Combine() method Output 45 shows the results of this listing
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