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Using Variables

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Nội dung Text: Using Variables

Using Variables A variable is a storage location that holds a value. You can think of a variable as a box holding temporary information. You must give each variable in a program a unique name. You use a variable's name to refer to the value it holds. For example, if you want to store the value of the cost of an item in a store, you might create a variable simply called cost, and store the item's cost in this variable. Later on, if you refer to the cost variable, the value retrieved will be the item's cost that you put there earlier. Naming Variables You should adopt a naming convention for variables that help you avoid confusion concerning the variables you have defined. The following list contains some general recommendations: • Don't use underscores. • Don't create identifiers that differ only by case. For example, do not create one variable named myVariable and another named MyVariable for use at the same time, because it is too easy to get them confused. NOTE Using identifiers that differ only by case can limit the ability to reuse classes in applications developed using other languages that are not case sensitive, such as Visual Basic. • Start the name with a lowercase letter. • In a multiword identifier, start the second and each subsequent word with an uppercase letter. (This is called camelCase notation.) • Don't use Hungarian notation. (Microsoft Visual C++ developers reading this book are probably familiar with Hungarian notation. If you don't know what Hungarian notation is, don't worry about it!) IMPORTANT You should treat the first two recommendations as compulsory because they relate to Common Language Specification (CLS) compliance. If you want to write programs that can interoperate with other languages, such as Microsoft Visual Basic .NET, you need to comply with these recommendations. For example, score, footballTeam, _score, and FootballTeam are all valid variable names, but only the first two are recommended. Declaring Variables
Remember that variables are like boxes in memory that can hold a value. C# has many different types of values that it can store and process—integers, floating-point numbers, and strings of characters, to name three. When you declare a variable, you must specify what type of data it will hold. NOTE Microsoft Visual Basic programmers should note that C# does not allow implicit declarations. You must explicitly declare all variables before you can use them if you want your code to compile. You declare the type and name of a variable in a declaration statement. For example, the following statement declares that the variable named age holds int (integer) values. As always, the statement must be terminated with a semi-colon. int age; The variable type int is the name of one of the primitive C# types—integer which is a whole number. (You'll learn about several primitive data types later in this chapter.) After you've declared your variable, you can assign it a value. The following statement assigns age the value 42. Again, you'll see that the semicolon is required. age = 42; The equal sign (=) is the assignment operator, which assigns the value on its right to the variable on its left. After this assignment, the age variable can be used in your code to refer to the value it holds. The next statement writes the value of the age variable, 42, to the console: Console.WriteLine(age); TIP If you leave the mouse pointer over a variable in the Visual Studio 2005 Code and Text Editor window, a ToolTip appears telling you the type of the variable.

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