5.4 Constants

Variables are a powerful tool, but there are times when you want to manipulate a defined value, one whose value you want to ensure remains constant. A constant is like a variable in that it can store a value. However, unlike a variable, you cannot change the value of a constant while the program runs.

For example, you might need to work with the Fahrenheit freezing and boiling points of water in a program simulating a chemistry experiment. Your program will be clearer if you name the variables that store these values FreezingPoint and BoilingPoint, but you do not want to permit their values to be changed while the program is executing. The solution is to use a constant. Constants come in three flavors: literals, symbolic constants, and enumerations.

5.4.1 Literal Constants

A literal constant is just a value. For example, 32 is a literal constant. It does not have a name; it is just a literal value. And you can't make the value 32 represent any other value. The value of 32 is always 32. You can't assign a new value to 32, and you can't make 32 represent the value 99 no matter how hard you might try.

5.4.2 Symbolic Constants

Symbolic constants assign a name to a constant value. You declare a symbolic constant using the following syntax:

const type identifier = value;

The const keyword is followed by a type, an identifier, the assignment operator (=), and the value with which you'll initialize the constant.

This is similar to declaring a variable, except that you start with the keyword const and symbolic constants must be initialized. Once initialized, a symbolic constant cannot be altered. For example, in the following declaration, 32 is a literal constant and FreezingPoint is a symbolic constant of type int:

const int FreezingPoint = 32;

Example 5-4 illustrates the use of symbolic constants.

Example 5-4. Using symbolic constants

class Values
{
   static void Main( )
   {
      const int FreezingPoint = 32;   // degrees Farenheit
      const int BoilingPoint = 212;

      System.Console.WriteLine("Freezing point of water: {0}", 
            FreezingPoint );
      System.Console.WriteLine("Boiling point of water: {0}", 
            BoilingPoint );
      //BoilingPoint = 21;   

   }
}

Example 5-4 creates two symbolic integer constants: FreezingPoint and BoilingPoint. See the sidebar Naming Conventions for a discussion of how to name symbolic constants.

Naming Conventions Microsoft has promulgated white papers on how you should name the variables, constants, and other objects in your program. They define two types of naming conventions: Camel notation and Pascal notation. In Camel notation, names begin with a lowercase letter. Multi-word names (such as "my button") are written with no spaces and no underscore and with each word after the first capitalized. Thus, the correct name for "my button" is myButton. Pascal notation is just like Camel notation except that the first letter is also uppercase (FreezingPoint). Microsoft suggests that variables be written with Camel notation and constants with Pascal notation. In later chapters, you'll learn that member variables are named using Camel notation, while methods and classes are named using Pascal notation.

These constants serve the same purpose as using the literal values 32 and 212 for the freezing and boiling points of water, respectively, in expressions that require them. However, because the constants have names, they convey far more meaning. Also, if you decide to switch this program to Celsius, you can reinitialize these constants at compile time to 0 and 100, respectively, and all the rest of the code should continue to work.

To prove to yourself that the constant cannot be reassigned, try un-commenting the last line of the preceding program, by removing the two slash marks.

BoilingPoint = 21;

When you recompile, you receive this error:

error CS0131: The left-hand side of an assignment must be 
a variable, property or indexer

5.4.3 Enumerations

Enumerations provide a powerful alternative to literal or simple symbolic constants. An enumeration is a distinct value type, consisting of a set of named constants (called the enumerator list).

In Example 5-4, you created two related constants:

const int FreezingPoint = 32;  
const int BoilingPoint = 212;

You might want to add a number of other useful constants to this list as well, such as:

const int LightJacketWeather = 60;
const int SwimmingWeather = 72;
const int WickedCold = 0;

Notice, however, that this process is somewhat cumbersome; also this syntax shows no logical connection among these various constants. C# provides an alternate construct, the enumeration, which allows you to group logically related constants, as in the following:

enum Temperatures
{
   WickedCold = 0,
   FreezingPoint = 32,
   LightJacketWeather = 60,
   SwimmingWeather = 72,
   BoilingPoint = 212,
}

The complete syntax for specifying an enumeration uses the enum keyword, follows:

[attributes ] [modifiers ] enum  identifier  
[: base-type ] { enumerator-list }; 

In a specification statement like the preceding example, anything in square brackets is optional. Thus, you can declare an enum with no attributes, modifiers, or base-type.

The optional attributes and modifiers are considered later in this book. For now, let's focus on the rest of this declaration. An enumeration begins with the keyword enum, which is generally followed by an identifier, in this case "Temperatures":

enum Temperatures

The base-type is the underlying type for the enumeration. You might specify that you are declaring constant ints, constant longs, etc. If you leave out this optional value (and often you will), it defaults to int, but you are free to use any of the integral types (e.g., ushort, long) except for char. For example, the following fragment declares an enumeration with unsigned integers (uint) as the base-type:

enum ServingSizes : uint
{
    Small = 1,
    Regular = 2,
    Large = 3
}

Notice that an enum declaration ends with the enumerator list, which contains the constant assignments for the enumeration, each separated by a comma. Example 5-5 rewrites Example 5-4 to use an enumeration.

Example 5-5. Using an enumeration

class Values
{
   // declare the enumeration
   enum Temperatures
   {
      WickedCold = 0,
      FreezingPoint = 32,
      LightJacketWeather = 60,
      SwimmingWeather = 72,
      BoilingPoint = 212,
   }

   static void Main( )
   {

      System.Console.WriteLine("Freezing point of water: {0}", 
         (int) Temperatures.FreezingPoint );
      System.Console.WriteLine("Boiling point of water: {0}", 
         (int) Temperatures.BoilingPoint );
   }
}

In Example 5-5, you declare an enumerated constant called Temperatures. When you want to use any of the values in an enumeration in a program, the values of the enumeration must be qualified by the enumeration name.

You cannot just refer to FreezingPoint; instead, you use the enumeration identifier (Temperature) followed by the dot operator and then the enumerated constant (FreezingPoint). This is called qualifying the identifier FreezingPoint. Thus, to refer to the FreezingPoint, you use the full identifier Temperature.FreezingPoint.

You might want to display the value of an enumerated constant to the console, as in the following:

Console.WriteLine("The freezing point of water is {0}",
   (int) Temperature.FreezingPoint);

To make this work properly, you must cast the constant to its underlying type (int). When you cast a value you tell the compiler "I know that this value is really of the indicated type." In this case you are saying "treat this enumerated constant as an int." Since the underlying type is int, this is safe to do. See the sidebar Casting.

Casting Objects of one type can be converted into objects of another type. This is called casting. Casting can be either implicit or explicit. An implicit conversion happens automatically; the compiler takes care of it for you. If you have a short, and you assign it to a variable of type int, the compiler automatically (and silently) casts it for you. You don't have to take any action. This is safe, because an int variable can hold any value that might have been in a short variable. short myShort = 5; // other code here... int myint = myShort; // implicit conversion Explicit conversions happen when you specifically cast a value to a different type by writing the new type in parentheses. The semantics of an explicit conversion are "Hey! Compiler! I know what I'm doing." This is sometimes called "hitting it with the big hammer" and can be very useful or very painful, depending on whether your thumb is in the way. If you convert from int to short you can lose information. If the value in the int is greater than 32,767, it will be truncated in the conversion to a short. The compiler will not perform an implicit conversion from int to short: short myShort; int myInt = 500; myShort = myInt; // won't compile Instead, you must explicitly cast the value: short myShort; int myInt = 500; myShort = (short) myInt; // explicit cast

In Example 5-5, the values in the two enumerated constants FreezingPoint and BoilingPoint are both cast to type integer; then that integer value is passed to WriteLine( ) and displayed.

Each constant in an enumeration corresponds to a numerical value. In Example 5-5, each enumerated value is an integer. If you don't specifically set it otherwise, the enumeration begins at 0 and each subsequent value counts up from the previous. Thus, if you create the following enumeration:

enum SomeValues
{
   First,
   Second,
   Third = 20,
   Fourth
}

the value of First will be 0, Second will be 1, Third will be 20, and Fourth will be 21.