Custom Constants

12.52m = 492.9124in

If you use this 39.37 many times in your program, at one time, you may make a mistake and type it as 3937 or 3.937 or else. Consider the following program:

using System;

class Program
{
    static void Main()
    {
        double Meter, Inch;

        Meter = 12.52;
        Inch = Meter * 39.37;
        Console.Write(Meter);
        Console.Write("m = ");
        Console.Write(Inch);
        Console.WriteLine("in\n");

        Meter = 12.52;
        Inch = Meter * 3.937;
        Console.Write(Meter);
        Console.Write("m = ");
        Console.Write(Inch);
        Console.WriteLine("in\n");

        Meter = 12.52;
        Inch = Meter * 393.7;
        Console.Write(Meter);
        Console.Write("m = ");
        Console.Write(Inch);
        Console.WriteLine("in\n");
    }
}

This would produce:

12.52m = 492.9124in

12.52m = 49.29124in

12.52m = 4929.124in

Because of mistakes in the way to represent the number, the same calculation produces different results. To make sure that this is unlikely, you can instead use a variable that holds the value. Then, when you need that value, you can access the variable instead of the value itself. A number such as 39.37 is called a constant.

A constant is a value that never changes such as 244, "ASEC Mimosa", 39.37, or True. These are constant values you can use in your program any time. You can also declare a variable and make it a constant; that is, use it so that its value is always the same.

To create a constant, type the const keyword to its left. When declaring a constant, you must initialize it with an appropriate value. Here is an example:

const double ConversionFactor = 39.37;

Once a constant has been created and it has been appropriately initialized, you can use its name where the desired constant would be used. Here is an example of a constant variable used various times:

using System;

class Exercise
{
    static void Main()
    {
        const double ConversionFactor = 39.37;
        double Meter, Inch;

        Meter = 12.52;
        Inch = Meter * ConversionFactor;
        Console.Write(Meter);
        Console.Write("m = ");
        Console.Write(Inch);
        Console.WriteLine("in\n");

        Meter = 12.52;
        Inch = Meter * ConversionFactor;
        Console.Write(Meter);
        Console.Write("m = ");
        Console.Write(Inch);
        Console.WriteLine("in\n");

        Meter = 12.52;
        Inch = Meter * ConversionFactor;
        Console.Write(Meter);
        Console.Write("m = ");
        Console.Write(Inch);
        Console.WriteLine("in\n");
    }
}

This would produce:

12.52m = 492.9124in

12.52m = 492.9124in

12.52m = 492.9124in

Notice that, this time, the calculation is more accurate. Also, this time, if you mistype the name of the variable in an operation, you would receive a compiler error, giving you the time to fix it.

To initialize a constant variable, the value on the right side of the assignment operator "=" must be a constant or a value that the compiler can determine as constant. Instead of using a known constant, you can also assign it another variable that has already been declared as constant.

There are two main categories of constants you will use in your programs. You can create your own constant as we saw above. The C# language also provides various constants. Some constants are part of the C# language. Some other constants are part of the .NET Framework. Before using a constant, of course, you must first know that it exists. Second, you must know how to access it. A constant that is part of the C# language can be accessed anywhere in your code. Those constant are normally defined in the System namespace. Other constant arte defined in various appropriate namespaces.

null: The null keyword is a constant used to indicate that a variable doesn't hold a known value

PI: PI is a constant used as the ratio of the circumference of a circle to its diameter. PI is defined in Math. To use it, you would type Math.PI.