417: Converting a string to an int in Visual C#.NET

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Listing 417: Converting a string to an int
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using System; class ExceptionHandling { static void Main() { string firstName; string ageText; int age; ConsoleWriteLine("Hey you!"); ConsoleWrite("Enter your first name: "); firstName = SystemConsoleReadLine(); ConsoleWrite("Enter your age: "); ageText = ConsoleReadLine(); age = intParse(ageText); ConsoleWriteLine( "Hi {0}! You are {1} months old", firstName, age*12); } }
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Output 411 shows the results of Listing 417
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OUTPUT 411:
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Hey you! Enter your first name: Inigo Enter your age: 42 Hi Inigo! You are 504 months old
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Basic Error Handling with Exceptions
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The return value from SystemConsoleReadLine() is stored in a variable called ageText and is then passed to a method on the int data type, called Parse() This method is responsible for taking a string value that represents a number and converting it to an int type
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BEGINNER TOPIC 42 as a String versus 42 as an Integer C# is a strongly typed language Therefore, not only is the data value important, but the type associated with the data is important as well A string value of 42, therefore, is distinctly different from an integer value of 42 The string is composed of the two characters 4 and 2, whereas the int is the number 42
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Given the converted string, the final SystemConsoleWriteLine() statement will print the age in months by multiplying the age value by 12 However, what happens if the user does not enter a valid integer string For example, what happens if the user enters forty-two The Parse() method cannot handle such a conversion It expects the user to enter a string that contains only digits If the Parse() method is sent an invalid value, it needs some way to report this fact back to the caller
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Trapping Errors To indicate to the calling method that the parameter is invalid, intParse() will throw an exception Throwing an exception will halt further execution in the current program flow and instead will jump into the first code block within the call stack that handles the exception Since you have not yet provided any such handling, the program reports the exception to the user as an unhandled exception Assuming there is no registered debugger on the system, the error will appear on the console with a message such as that shown in Output 412 Obviously, such an error is not particularly helpful To fix this, it is necessary to provide a mechanism that handles the error, perhaps reporting a more meaningful error message back to the user
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4: Methods and Parameters
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OUTPUT 412:
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Hey you! Enter your first name: Inigo Enter your age: forty-two Unhandled Exception: SystemFormatException: Input string was not in a correct format at SystemNumberParseInt32(String s, NumberStyles style, NumberFormatInfo info) at ExceptionHandlingMain()
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This is known as catching an exception The syntax is demonstrated in Listing 418, and the output appears in Output 413
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Listing 418: Catching an Exception
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using System; class ExceptionHandling { static int Main() { string firstName; string ageText; int age; int result = 0; ConsoleWrite("Enter your first name: "); firstName = ConsoleReadLine(); ConsoleWrite("Enter your age: "); ageText = ConsoleReadLine(); try { age = intParse(ageText); ConsoleWriteLine( "Hi {0}! You are {1} months old", firstName, age*12); } catch (FormatException ) { ConsoleWriteLine( "The age entered, {0}, is not valid", ageText); result = 1; } catch(Exception exception) {
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ConsoleWriteLine( "Unexpected error: result = 1;
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{0}", exceptionMessage);
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} finally { ConsoleWriteLine("Goodbye {0}", firstName); } return result; } }
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OUTPUT 413:
Enter your first name: Inigo Enter your age: forty-two The age entered, forty-two, is not valid Goodbye Inigo
To begin, surround the code that could potentially throw an exception (age = intParse()) with a try block This block begins with the try keyword It is an indication to the compiler that the developer is aware of the possibility that the code within the block could potentially throw an exception, and if it does, then one of the catch blocks will attempt to handle the exception One or more catch blocks (or the finally block) must appear immediately following a try block The catch block header (see the Advanced Topic titled Generic catch, later in this chapter) optionally allows you to specify the data type of the exception, and as long as the data type matches the exception type, the catch block will execute If, however, there is no appropriate catch block, the exception will fall through and go unhandled as though there were no exception handling The resultant program flow appears in Figure 41 For example, assume the user enters forty-two for the age In this case, intParse() will throw an exception of type SystemFormatException, and control will jump to the set of catch blocks (SystemFormatException indicates that the string was not of the correct format to be parsed appropriately) Since the first catch block matches the type of exception