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.

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

    consttype 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 3-4 illustrates the use of symbolic constants.

class Values
{
 static void Main(  )
 {
 const int FreezingPoint = 32; // degrees Fahrenheit
 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 3-4 creates two symbolic integer constants: FreezingPoint and BoilingPoint. See the sidebar, "Naming Conventions,” for a discussion of how to name symbolic constants.

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. It might seem easier to just use the literal values 32 and 212 instead of going to the trouble of declaring the constants, but 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:

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

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 3-4, 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, as follows:

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

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, or something else. 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 (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 3-5 rewrites Example 3-4 to use 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 3-5, 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.” Because the underlying type is int, this is safe to do. (See the sidebar, "Casting.”)

    short myShort = 5;
    // other code here...
    int myint = myShort; // implicit conversion

    int myInt = 5;
    // other code here...
    short myShort = (short) myInt; // explicit conversion

In Example 3-5, 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 3-5, 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.