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Following the switch statement rules The switch statement is very useful, but unfortunately, you can’t always use it when you might like to. Any switch statement you write must adhere to the following rules: ■ ■ ■ ■ You can use switch only on certain data types, such as int, char, or string. With any other types (including float and double), you must use an if statement. The case labels must be constant expressions, such as 42 if the switch data type is an int, '4' if the switch data type is a char, or "42" if the switch data type is a string. If you need to calculate your case label values at run time, you must use an if statement. The case labels must be unique expressions. In other words, two case labels cannot have the same value. You can specify that you want to run the same statements for more than one value by providing a list of case labels and no intervening statements, in which case the code for the final label in the list is executed for all cases in that list. However, if a label has one or more associated statements, execution cannot fall through to subsequent labels; in this case, the compiler generates an error. The following code fragment illustrates these points: switch (trumps) { case Hearts : case Diamonds : color = "Red"; break; case Clubs : color = "Black"; case Spades : color = "Black"; break; } // Fall-through allowed - no code between labels // Code executed for Hearts and Diamonds // Error - code between labels Note The break statement is the most common way to stop fall-through, but you can also use a return statement to exit from the method containing the switch statement or a throw statement to generate an exception and abort the switch statement. The throw statement is described in Chapter 6, “Managing errors and exceptions.” switch fall-through rules Because you cannot accidentally fall through from one case label to the next if there is any intervening code, you can freely rearrange the sections of a switch statement without affecting its meaning (including the default label, which by convention is usually—but does not have to be—placed as the last label). C and C++ programmers should note that the break statement is mandatory for every case CHAPTER 4 Using decision statements 101
in a switch statement (even the default case). This requirement is a good thing—it is common in C or C++ programs to forget the break statement, allowing execution to fall through to the next label and leading to bugs that are difficult to spot. If you really want to, you can mimic C/C++ fall-through in C# by using a goto statement to go to the following case or default label. Using goto in general is not recommended, though, and this book does not show you how to do it. In the following exercise, you will complete a program that reads the characters of a string and maps each character to its XML representation. For example, the left angle bracket character (), ampersand (&), single quotation mark (‘), and double quotation mark (“) characters. You will write a switch statement that tests the value of the character and traps the special XML characters as case labels. Write switch statements 1. Start Visual Studio 2015 if it is not already running. 2. Open the SwitchStatement project, which is located in the \Microsoft Press\VCSBS\Chapter 4\ SwitchStatement folder in your Documents folder. 3. On the Debug menu, click Start Debugging. Visual Studio 2015 builds and runs the application. The application displays a form containing two text boxes separated by a Copy button. 4. Type the following sample text into the upper text box: inRange = (lo = number); 5. Click Copy. The statement is copied verbatim into the lower text box, and no translation of the characters occurs, as shown in the following screen shot. 102 PART I Introducing Microsoft Visual C# and Microsoft Visual Studio 2015
Page 1 and 2: Microsoft Visual C# Step by Step Ei
Page 3 and 4: PUBLISHED BY Microsoft Press A Divi
Page 5 and 6: PART IV BUILDING UNIVERSAL WINDOWS
Page 7 and 8: Associativity and the assignment op
Page 9 and 10: Summary. . . . . . . . . . . . . .
Page 11 and 12: Using multidimensional arrays . . .
Page 13 and 14: Using properties . . . . . . . . .
Page 15 and 16: Summary. . . . . . . . . . . . . .
Page 17 and 18: Canceling tasks and handling except
Page 20 and 21: Introduction Microsoft Visual C# is
Page 22 and 23: ■ ■ Part III, “Defining exten
Page 24 and 25: System requirements You will need t
Page 26 and 27: For those of you who like to know a
Page 28 and 29: Project/Solution Description Chapte
Page 30 and 31: Project/Solution Description Chapte
Page 32: We want to hear from you At Microso
Page 35 and 36: Note The answers to these questions
Page 37 and 38: The outcome of the || operator is t
Page 39 and 40: notice that the Boolean expression
Page 41 and 42: Cascading if statements You can nes
Page 43 and 44: private void compareClick(object se
Page 45 and 46: else if (leftHandSide.Day > rightHa
Page 47: Understanding switch statement synt
Page 51 and 52: If the current character being copi
Page 54 and 55: Index Symbols & (ampersand) & (AND)
Page 56 and 57: inary operators creating, 232-241 d
Page 58 and 59: code CanExecuteChanged events, 670,
Page 60 and 61: CountdownEvent class concurrent ope
Page 62 and 63: Ellipse class for structures, 209 w
Page 64 and 65: forms exception objects, 144 except
Page 66 and 67: indexers H handheld devices. See Un
Page 68 and 69: lock statements I/O operations asyn
Page 70 and 71: Name property invoking through dele
Page 72 and 73: Parallel.Invoke method okClick meth
Page 74 and 75: eference types number of, 521 Progr
Page 76 and 77: static properties security, unsafe
Page 78 and 79: throughput, improving System.String
Page 80 and 81: views voice activation, 680-695 Win
Page 82: yield keyword blocking current thre

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