The switch Statement

Besides the if statement, the other conditional statement in C# is the switch statement. This statement acts much like an extended if statement; it can handle multiple tests in a single statement, checking a test expression against various values and executing code when the test expression matches one of the values you've tested it against. Here's what the switch statement looks like formally in C#:

 switch (  expression  ) {   case  constant-expression  :  statement   jump-statement  [default:  statement   jump-statement  ] } 

Here are the parts of this statement:

• expression An integral or string type expression used to test against.

• statement The embedded statement(s) to be executed if the test expression matches the current case statement's constant-expression .

• jump-statement A jump statement that transfers control out of the case body.

• constant-expression Control is transferred to the body of a case statement according to the value of this expression. A switch statement can include any number of case statements, but note that no two case statements inside the same switch statement can have the same constant-expression .

Here's how it works: Control is transferred to the case statement whose constant-expression matches expression . If no case statement matches the test expression, the code in the default statement (if present) is executed.

FOR C++ PROGRAMMERS

C# case statements work differently than in C++; if you want control to fall through from statement case1 to statement case2 in C#, case1 cannot have any executable statements in its body. C++ switch statements don't support the goto statements we're about to see.

You can see an example, ch01_11.cs, in Listing 1.11. This program asks the user to guess a number (5), and it checks the user 's guess against the mystery number with a switch statement.

Listing 1.11 Using the switch Statement (ch01_11.cs)

 using System; class ch01_11 {   static void Main()   {     Console.Write("Guess my number (1-5): ");     int input = Convert.ToInt32(Console.ReadLine());     switch (input)     {       case 1:         Console.WriteLine("Wrong, sorry.\n");         break;       case 2:       case 3:         Console.WriteLine("Neither 2 nor 3 is correct.\n");         break;       case 4:         goto case 1;       case 5:         Console.WriteLine("Right!\n");         break;       default:         Console.WriteLine("Not a valid guess!\n");         break;     }   } } 

If the user guesses 1, the first case in the switch statement will be executednote the break statement at the end, which terminates the switch statement:

 case 1:   Console.WriteLine("Wrong, sorry.\n");   break; 

If the user guesses 2, control will fall through to case 3 , because there is no code for case 2 (note that if you place any code in case 2 , but omit a break or goto statement, you'll get a compilation error):

 case 2: case 3:   Console.WriteLine("Neither 2 nor 3 is correct.\n");   break; 

If the user guesses 4, we send them to the code for case 1 with a goto statement, which transfers control to the code in the case we specify:

 case 4:   goto case 1; 

And if the user guesses 5, they're right, and we admit the fact:

 case 5:   Console.WriteLine("Right!\n");   break; 

We've taken care of the numbers 15, and if we still haven't left the switch statement, the user must have entered an invalid guess. We can handle that error with the (optional) default statement, which (if present) is executed if no case statement matches:

 default:   Console.WriteLine("Not a valid guess!\n");   break;