What are and how to use functions in C#

A function is a reusable block of code that performs a specific task when called or invoked from another part of the program.

Functions are a fundamental part of programming, as they allow for breaking the code into smaller, manageable units (this facilitates understanding, maintenance, and debugging of the software).

Functions in C# can take zero or more parameters as input, perform calculations or manipulations based on those parameters, and return an optional result as output.

The basic syntax for defining a function in C# is as follows:

[modifier] return_type function_name([parameter_list])
{
    // Function code
    return return_value; // Optional
}

Basic Example

Below is a basic example of a function that adds two numbers:

public int Add(int num1, int num2)
{
    return num1 + num2;
}

Function Calls

To call (invoke) a function and execute its code, simply use the function name followed by parentheses, passing the parameter values if necessary.

Here’s an example of code that calls the Add function and stores the result in a variable:

int result = Add(5, 3);

Parameters

Functions in C# can have one or more input parameters, which are values passed to the function when called. These parameters are used within the function to perform calculations and determine the result to return.

For example, in the following example, the function CalculateRectangleArea has two parameters, width and height, both of type double.

double CalculateRectangleArea(double width, double height)
{
    // Code to calculate the area of a rectangle
    double area = width * height;
    return area;
}

Return Value

Functions can return a value as a result using the return keyword. The type of the returned value must match the return type specified in the function signature.

For example, in the CalculateCircleArea function, it returns a double type.

static double CalculateCircleArea(double radius)
{
    // Code to calculate the area of a circle
    double area = Math.PI * radius * radius;
    return area;  // return of area
}

Only one value can be returned from a function. Although it is possible to return a grouping of several values (like a tuple, struct, or an object).

Functions Without Return

If a function does not return any value, the void keyword is used as the return type.

public void Greet(string name)
{
    Console.WriteLine($"Hello, {name}!");
}

Practical Examples

public static string EvenOdd(int number)
{
    if (number % 2 == 0) // Checks if the number is divisible by 2
    {
        return "even"; // Returns "even" if the number is divisible by 2
    }
    else
    {
        return "odd"; // Returns "odd" if the number is not divisible by 2
    }
}

public static double CalculateCircleArea(double radius)
{
    return Math.PI * radius * radius; // Calculates the area using the formula πr²
}

public static int Maximum(int a, int b)
{
    if (a > b) // Compares the two numbers
    {
        return a; // Returns 'a' if it is greater than 'b'
    }
    else
    {
        return b; // Returns 'b' if it is greater than or equal to 'a'
    }
}

public static double CelsiusToFahrenheit(double celsius)
{
    return (celsius * 9/5) + 32; // Applies the conversion formula
}

public static bool IsPrime(int number)
{
    if (number <= 1) return false; // Numbers less than or equal to 1 are not prime
    for (int i = 2; i < number; i++) // Iterates from 2 to number - 1
    {
        if (number % i == 0) return false; // If the number is divisible by 'i', it is not prime
    }
    return true; // If no divisor is found, the number is prime
}

public static int SumArray(int[] numbers)
{
    int sum = 0; // Initializes the sum to 0
    foreach (int number in numbers)
    {
        sum += number; // Adds each element of the array to 'sum'
    }
    return sum; // Returns the total sum
}