C# Variables

• Variables stand for a small part of memory you will use in the code.
• The length of the memory you are using depends on data type and its length.
• C# variables are type restricted so you will have to define each of the data type of the variable before you use it.
• Variables can be assign an initial value at the declaration statement.
• After a variable is declared, you can assign a value to it or get it out in the code.

The syntax to define variables is shown below.

<Data Type> <Variable List|Variable = Initial Value[,]>

Example 01-03-01

int i, j=0, k;
double xx;
char ch1 = 'X', ch2='Y';
string s1 = "123abc";
object ob = new Object();

• Line 1: Declare 3 integer type variables i, j and k with j has initial 0 value.
• Line 2: Declare a double type variable.
• Line 3: Declare 2 char variables with initial values.
• Line 4: Declare a string variable s1 with its initial value.
• Line 5: Declare an Object variable ob and instantiate an instance to it.

Note
A variable cannot be declared twice.

• Variable names cannot be defined as keyword. For example, "class", "namespace" or "int". Each keyword has its specific meaning and translated into code by C# compiler.
• Variable names must start with a letter, an underscore or @ symbol.
• The subsequent characters can be letters, underscores or numbers.

The following are the good examples of the variable names.

p45ab
@de123
_123ab

And here is the bad examples.

45xyz
using
It-is-not-right

The reasons are as follows.

• Line 1: The variable name starts with a digit.
• Line 2: The variable name is using a keyword.
• Line 3: The variable name contains minus character(-) which is not allowed.

"=" is the assignment operator. It is also used in declaration statement when a variable is initialized. See the syntax below.

<Variable Name> = <Value of Data Type>;

Example 01-03-02

using System;

namespace TestVariables
{
    class Program
    {
        static void Main(string[] args)
        {
            int i, j = 1, k;
            i = 2;
            Console.WriteLine("i={0}, j={1}", i, j);
            k = i;
            i = j;
            j = k;
            Console.WriteLine("i={0}, j={1}", i, j);
            Console.Read();
        }
    }
}

Output

i=2, j=1
i=1, j=2

• Line 9: Declare 3 integer type variables i, j and k and j is initialized as 1 value. It equals to the following code.

int i, j, k;
j=1;

• Line 10: Assign 2 to i.
• Line 11: Output variable i and j. {0} is place holder and will be replaced with the second parameter i value. {1} is replaced with j value. So the first line of the output is what we expect.
• Line 12: We use this statement to assign i's value to k. So k's is the same as i's after Line 12.
• Line 13: Assign j's value to i.
• Line 14: Assign k's value to j. so Line 12-14 makes i's value and j's value exchanged by the bridge variable k.
• Line 15: Output i and j's value. It is the same as line 11. Check the second output above they were exchanged.

What happened if Line 10 is removed ?

You will get compile error. Because no value is assigned to i before it is outputted.

Note
Variables must be assigned before they are used.