Microsoft Visual C# 2005 Unleashed
| Every modern programming language supports the notion of branching and conditional logic. This gives your applications the ability to do different things based on current input, events, error conditions, or any condition that can be expressed logically. This section first shows you how C# allows you to form logical expressions and how to create code blocks that are executed conditionally based on those expressions. Then you'll see a couple of shortcuts and extras that C# contains to make using certain types of common logic statements even easier. Introduction to Boolean Expressions
A Boolean expression is a code statement that will eventually evaluate to either true or false At the most basic level, all Boolean expressions, no matter how long or complex, are still going to evaluate to either true or false. The simplest of all Boolean expressions is equality. This expression is used to test whether or not one value is equivalent to another value. This can be something simple, such as 2 == 4
This expression will evaluate to false because 2 is not equal to 4. It can also be something complex, such as MyObject.MyProperty == YourObject.YourProperty
This expression could evaluate to anything, and would be determined at runtime. If you are familiar with C, C++, or even C#, you know that the == (double-equals) is a logical Boolean operator, and the = (single equals) is an assignment operator used to set values. A common source of runtime and compile-time errors revolves around using these operators in the wrong place. Table 2.1 shows the logical operators available in the C# language and provides a brief description of their purpose.
Using Basic Conditional Statements
In the preceding section you were introduced to the tools you need in order to form Boolean expressions. These will allow you to assert whether or not an expression is true. When you have such an assertion, you need to be able to provide some conditional statements in your code to allow you to do something meaningful based on the results of a Boolean expression. This section will show you the basic conditional statements that are at the heart of almost all logic code in C#. Using If/Else Statements
The format of an if statement is as follows: if ( expression ) code_block else if ( expression_1 ) code_block else if ( expression_2 ) code_block else code_block Both the else if and else sections are optional, and are only required if you want your code to perform alternate tasks when the original Boolean expression evaluates to false. Because each of the code blocks in the preceding example can also contain their own if statements, you can nest your conditional code nearly as deep as you like. However, good style and etiquette recommend that you avoid deep nesting because it makes your code difficult to read and analyze. The following sample if statements illustrate using if simply, nested, and with else statements: if ( strInput == "Hello" ) Console.WriteLine("You said Hello"); if ( strInput2 == "Goodbye" ) Console.WriteLine("You said Goodbye"); else if ( strInput2 == "Later" ) Console.WriteLine("You didn't say goodbye, you said Later.") else { if (strInput3 == "Hola") if (strInput4 == "Senor") Console.WriteLine("Hello!"); }
Using the Switch Statement
If you want to test a single variable against a list of possible values using just the standard if/else keywords, you will end up with code that looks like the following: if (val == 1) ... else if (val == 2) ... else if (val == 3) ... else if (val == 4) ... else ... Although this may get the job done, it's not the most elegant or the most easily readable block of code. For this situation, C# has the switch statement, which allows you to combine several logic tests into a single expression, as shown in the following example: switch (val) { case 1: ... break; case 2: ... break; case 3: ... break; default: ... }
Using the Goto Statement
The goto statement has two main uses; one is to transfer control from one case within a switch statement, and the other is to break out of loops. You will see more on loops later in this chapter. The following is a sample of a switch statement that utilizes the goto keyword for transferring control: switch (val) { case 1: ... break; case 2: ... goto case 1; break; case 3: ... goto case 1; break; default: ... }
Using Advanced Conditional Statements
One of the most common uses for an if/else statement is to conditionally print something or render something through Windows Forms or Web Forms. For example, suppose that you want to print the word "Good" if the profit margin is greater than 20, or "Bad" if the profit margin is less than or equal to 20. You might use an if/else that looks like the following: if (profitMargin > 20) Console.WriteLine("The profit margin is Good!"); else Console.WriteLine("The profit margin is Bad!");
This is perfectly fine, but C# includes a ternary operator called the conditional operator that allows you to embed an if/else combination in a single line. You can rewrite the preceding code segment in a single line as follows: Console.WriteLine("The profit margin is " + (profitMargin > 20) ? "Good" : "Bad"); The ternary conditional operator has the following format: expression ? return_when_true : return_when_false
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