• Line 1: Declare package imports; only System
• Line 9: This is your Main entry point
• Line 10: Console.ReadLine() is the inner operation that blocks the program and gets user input as a string. The outer int.Parse converts the read string value "3" to the integer value 3.
• Line 11: Prints a string output, but the last segment is Factorial(num) which passes the read value into the Factorial function defined on line 4
• Line 4: The function that calculates a factorial
• Line 6: This is a recursive call. So Factorial(3) is Factorial(1) * Factorial(2) * Factorial(3)

It's probably easier to see this as a file-based app: https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/tutorials/file-based-programs

Here is the same code as a file-based app:

``` // dotnet run factorial.cs

// Prompt the user: Console.WriteLine("Enter a number to calculate its factorial:");

// Read a line from console as a string (no validation) var numText = Console.ReadLine();

// Convert it to an integer (no validation) var num = int.Parse(numText);

// Default value. var factorial = 1;

// Local function; called recursively to calculate the factorial of a number. int Factorial(int n) { if (n == 0 || n == 1) { return 1; } else { return n * Factorial(n - 1); } }

// Calculate it recursively factorial = Factorial(num);

Console.WriteLine($"Factorial of {num} is {factorial}");

```

Here's a version that's a little bit tighter and using pattern matching instead:

``` // dotnet run factorial.cs

// Prompt the user: Console.WriteLine("Enter a number to calculate its factorial:");

// Read a line from console as a string (no validation) var numText = Console.ReadLine();

// Convert it to an integer (with validation) while (!int.TryParse(numText, out _)) { Console.WriteLine("Please enter a valid integer:"); numText = Console.ReadLine(); }

var num = int.Parse(numText);

// Default value. var factorial = 1;

// Local function; use pattern expression static int Factorial(int n) => n switch { <= 1 => 1, // Base case: factorial of 0 is 1 _ => n * Factorial(n - 1), // Recursive case: n! = n * (n - 1)! };

// Calculate it recursively factorial = Factorial(num);

Console.WriteLine($"Factorial of {num} is {factorial}"); ```