C# Variables

Key Variable Concepts

int number;
string text;

int number = 10;
string text = "Hello";

int x = 5;
x = 10; // New value

const double PI = 3.14159;

🔹 Declaring Variables

In C#, variables are declared by specifying a data type followed by a meaningful name. You can initialize them immediately or separately for clarity. Proper variable declaration—such as int age = 30; or string name = "Alice";—lays the foundation for organized, maintainable code and efficient memory usage in any application.

// Declare and initialize
int age = 30;
string name = "Alice";
double price = 19.99;
bool isActive = true;

// Declare first, initialize later
int count;
count = 100;

Console.WriteLine(age);      // Output: 30
Console.WriteLine(name);     // Output: Alice
Console.WriteLine(price);    // Output: 19.99
Console.WriteLine(isActive); // Output: True
Console.WriteLine(count);    // Output: 100

🔹 Variable Naming Rules

Follow C# naming conventions to write clean, readable, and error-free code. Variable names must begin with a letter or underscore, can include letters, digits, and underscores, and are case-sensitive. Adhering to rules—like using camelCase for local variables—improves code clarity, collaboration, and long-term maintainability in development projects.

// Valid variable names
int age;
string firstName;
double _price;
int number123;

// Invalid variable names (will cause errors)
// int 123number;  // Cannot start with a number
// string first-name;  // Cannot use hyphens
// double my price;  // Cannot have spaces

// Case-sensitive examples
int myVariable = 10;
int MyVariable = 20;
Console.WriteLine(myVariable); // Output: 10
Console.WriteLine(MyVariable); // Output: 20

🔹 Multiple Variables

Declare multiple variables of the same type in a single line to write concise and efficient C# code. Using comma-separated lists—like int x = 5, y = 10, z = 15;—reduces redundancy, improves readability, and is especially useful when initializing related values such as coordinates, scores, or configuration settings in your program.

// Declare multiple variables in one line
int x = 5, y = 10, z = 15;

Console.WriteLine(x); // Output: 5
Console.WriteLine(y); // Output: 10
Console.WriteLine(z); // Output: 15

// You can also assign the same value
int a, b, c;
a = b = c = 50;

Console.WriteLine(a); // Output: 50
Console.WriteLine(b); // Output: 50
Console.WriteLine(c); // Output: 50

🔹 Constants

Use the const keyword to define immutable values that remain fixed throughout program execution. Constants like const double Pi = 3.14159; ensure data integrity, prevent accidental changes, and improve code readability. They are ideal for mathematical values, configuration settings, or application-wide identifiers that should never be modified.

// Declare a constant
const double PI = 3.14159;
const int MAX_USERS = 100;
const string APP_NAME = "MyApp";

Console.WriteLine(PI);        // Output: 3.14159
Console.WriteLine(MAX_USERS); // Output: 100
Console.WriteLine(APP_NAME);  // Output: MyApp

// This would cause an error:
// PI = 3.14; // Cannot modify a constant

🔹 Variable Scope

Understand variable scope in C# to manage where variables are accessible and avoid common errors. Local variables declared inside a method are only accessible within that method, while class-level variables have broader access. Proper scope management prevents naming conflicts, reduces bugs, and promotes cleaner, more modular code design in complex applications.

class Program
{
    // Class-level variable (accessible everywhere in the class)
    static int globalVar = 100;
    
    static void Main()
    {
        // Local variable (only accessible in Main method)
        int localVar = 50;
        
        Console.WriteLine(globalVar); // Output: 100
        Console.WriteLine(localVar);  // Output: 50
        
        DisplayMessage();
    }
    
    static void DisplayMessage()
    {
        Console.WriteLine(globalVar); // Output: 100
        // Console.WriteLine(localVar); // Error: localVar not accessible here
    }
}