User Input and Output in C#

# CHAPTER 6

User Input and Output #

1. Introduction #

2. Learning Objectives #

• Use Console.ReadLine() to read text input from the user.

• Convert (Parse) string input into numerical data types.

• Handle formatting output using modern String Interpolation.

• Build a functional User Registration form.

3. Reading Input with Console.ReadLine() #

using System;

namespace InputDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Enter your username: "); // Write keeps cursor on same line
            string username = Console.ReadLine();
            
            Console.WriteLine("Welcome, " + username + "!");
        }
    }
}

4. Parsing Strings into Numbers #

// int age = Console.ReadLine(); // ERROR! Cannot convert string to int

You must Parse the string into an integer. You can do this using the Convert class or the type's built-in .Parse() method.

Console.Write("Enter your age: ");

// Method 1: Convert class
int age1 = Convert.ToInt32(Console.ReadLine()); 

// Method 2: Type parsing (Preferred by many developers)
int age2 = int.Parse(Console.ReadLine()); 

Console.WriteLine("You will be " + (age1 + 1) + " next year.");

5. Output Formatting (String Interpolation) #

C# 6.0 introduced String Interpolation, which is the modern standard for formatting. You simply put a $ before the string, and you can place variables directly inside { } brackets.

string name = "Alice";
int age = 28;

// Modern String Interpolation
Console.WriteLine($"Hello {name}, you are {age} years old.");

6. Mini Project: User Registration Form #

using System;

namespace RegistrationForm
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("--- NEW USER REGISTRATION ---");
            
            Console.Write("First Name: ");
            string fName = Console.ReadLine();
            
            Console.Write("Last Name: ");
            string lName = Console.ReadLine();
            
            Console.Write("Age: ");
            int age = int.Parse(Console.ReadLine());
            
            Console.Write("Account Balance ($): ");
            decimal balance = decimal.Parse(Console.ReadLine());
            
            Console.WriteLine("\n--- REGISTRATION SUCCESSFUL ---");
            // Using String Interpolation
            Console.WriteLine($"User: {fName} {lName}");
            Console.WriteLine($"Age: {age}");
            Console.WriteLine($"Balance: ${balance}");
        }
    }
}

7. Common Mistakes #

• Crashing on Parse: If you use int.Parse() and the user types "Hello" instead of a number, the program will crash with a FormatException. (We will learn how to handle this safely using int.TryParse() later).

• Forgetting the $ symbol: If you write "Hello {name}" without the $, it will literally print "Hello {name}" to the screen instead of replacing it with the variable's value.

8. Best Practices #

• Always use Console.Write() when asking for input so the user types their answer on the same line as the prompt.

• Always use String Interpolation ($"") instead of the + operator when concatenating multiple variables into a string. It is faster and far easier to read.

9. Exercises #

1. 1. Write a program that asks for a temperature in Celsius, converts it to Fahrenheit (C * 9/5) + 32, and prints the result using string interpolation.

1. 2. Write a program that asks for an item price and a quantity, calculates the total, and prints it out.

10. MCQs with Answers #

11. Interview Questions #

• Q: Explain the difference between int.Parse() and Convert.ToInt32(). What happens if you pass null to both? (Answer: Parse throws an exception, Convert returns 0).

• Q: What is String Interpolation, and why is it preferred over string concatenation?

12. Summary #

13. Next Chapter Recommendation #