An variable is a reserved space in memory with a symbolic name (identifier), in which we can store a value of a specific type.
Variables allow us to store temporary data that we can use later throughout our program.
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Variable Declaration
Declaring a variable in C# involves specifying its type and giving it a name. The basic syntax for declaring a variable is:
type variableName;
For example, to declare an integer variable named age:
int age;
Default Values
In C#, uninitialized variables are assigned default values (depending on their type).
Value Types
For value types, the default values are:
| Type | Default Value |
|---|---|
| int | 0 |
| float | 0.0f |
| double | 0.0d |
| char | '\0' (Unicode null character) |
| bool | false |
Reference Types
For reference types, the default value is always null, which indicates that the variable does not reference any location in memory.
string str; // str is null by default
int[] array; // array is null by default
object myObject; // object is null by default.
This leads to one of the most common errors in programming. If you access a Reference Type variable before creating it, you will get a null exception error because your variable contains nothing.
Before using a Reference Type variable, you must assign an instance to it, or you will have a runtime error (i.e., when the program is running)
Value Assignment
Assigning a value to a variable means giving it an initial value or modifying the value it contains. The syntax for assigning a value to a variable is:
variableName = value;
For example, to assign the value 25 to the variable age:
age = 25;
It is also possible to declare and assign a variable in a single line:
int age = 25;
Practical Examples
Let’s look at some very simple examples of using variables in C#. These examples show how to declare and use variables to perform calculations and manage data.
Student Data Management
In this example, we show how to manage basic data of a student, such as their name and age. This example illustrates how to declare and assign values to variables of different data types.
// We declare variables
public string name; // for the student's name
public int age; // for the student's age
// We assign values to the variables
name = "Ana";
age = 21;
// We print the student's data to the console
Console.WriteLine($"{name} is {age} years old");
Calculating the Area of a Circle
In this example, we calculate the area of a circle using a basic mathematical formula. A constant is declared for the value of PI and a variable for the radius of the circle.
// We declare a constant for the value of PI
const double PI = 3.14159;
// We declare a variable for the radius of the circle
double radius = 5.0;
// We calculate the area of the circle using the formula: area = PI * radius^2
double area = PI * radius * radius;
// We print the result to the console
Console.WriteLine($"The area of the circle is: {area}");
