C# 4 and .NET 4

Predefined Data Types ❘ 31
...
public static void Main()
{
int j = 30;
Console.WriteLine(j);
Console.WriteLine(ScopeTest2.j);
}
...
If you were accessing an instance field (a field that belongs to a specific instance of the class), you would
need to use the this keyword instead.
Constants
As the name implies, a constant is a variable whose value cannot be changed throughout its lifetime.
Prefixing a variable with the const keyword when it is declared and initialized designates that variable as a
constant:
const int a = 100; // This value cannot be changed.
Constants have the following characteristics:
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They must be initialized when they are declared, and after a value has been assigned, it can never be
overwritten.
The value of a constant must be computable at compile time. Therefore, you can’t initialize a constant
with a value taken from a variable. If you need to do this, you will need to use a read-only field (this is
explained in Chapter 3).
Constants are always implicitly static. However, notice that you don’t have to (and, in fact, are not
permitted to) include the static modifier in the constant declaration.
At least three advantages exist for using constants in your programs:
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Constants make your programs easier to read by replacing magic numbers and strings with readable
names whose values are easy to understand.
Constants make your programs easier to modify. For example, assume that you have a SalesTax
constant in one of your C# programs, and that constant is assigned a value of 6 percent. If the sales
tax rate changes at a later point in time, you can modify the behavior of all tax calculations simply by
assigning a new value to the constant; you don’t have to hunt through your code for the value .06 and
change each one, hoping that you will find all of them.
Constants help prevent mistakes in your programs. If you attempt to assign another value to a constant
somewhere in your program other than at the point where the constant is declared, the compiler
will flag the error.
Predefined daTa TyPes
Now that you have seen how to declare variables and constants, let’s take a closer look at the data types
available in C#. As you will see, C# is much stricter about the types available and their definitions than
some other languages are.
Value Types and reference Types
Before examining the data types in C#, it is important to understand that C# distinguishes between two
categories of data type:
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Value types
Reference types
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