C# 5.0 Programmer's Reference

152 ❘ CHAPTER 7 Program Control Statements
if-else Statements
The if-else statement has the following syntax:
if (condition1) statement1;
else if (condition2) statement2;
else if (condition3) statement3;
...
else statementElse;
The conditions are logical expressions that evaluate to either true or false. The statements are the
statements that should be executed if the corresponding condition is true.
The program evaluates each of the conditions until it finds one that is true. It then evaluates the
corresponding statement and skips all the rest of the series of else and if statements.
Short Circuiting Ifs
Because the program skips any remaining else and if statements, a series of if-else
statements provides short-circuit evaluation similar to the behavior given by the ||
operator. In particular, the program doesn’t evaluate the conditions used by those
statements. Normally, that’s okay and saves the program some time, but it can be a
problem if those conditions invoke methods with side effects.
For example, suppose a condition calls the EmployeeExists method. If that
method opens an employee database and leaves it open, that is a side effect. If you
don’t know whether that condition will be checked, after the series of if-else
statements finishes, you won’t know whether the database has been opened.
The best way to avoid this issue is to not write methods that have unexpected side
effects. For more on side effects, see the discussion of the && and || operators in the
section “Logical Operators” in Chapter 5.
If none of the conditions are true, the program executes statementElse.
For example, consider the following code snippet.
string greeting = "";
if (DateTime.Now.DayOfWeek == DayOfWeek.Monday)
greeting = "Sorry, it's Monday.";
else if (DateTime.Now.DayOfWeek == DayOfWeek.Friday)
greeting = "Finally, it's Friday!";
else
greeting = "Welcome to " + DateTime.Now.DayOfWeek.ToString();
MessageBox.Show(greeting);
The code starts by creating the string variable greeting and initializing it to a blank string. It then
begins a series of if-else statements.
If it’s Monday, the program sets greeting to "Sorry, it's Monday."
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