SRP Example - Create User Name

using System;

namespace ConsoleUI
{
    class Program
    {
        static void Main(string[] args)
        {
            StandardMessages.WelcomeMessage();

            Person user = PersonDataCapture.Capture();

            bool isUserValid = PersonValidator.Validate(user);

            if (isUserValid == false)
            {
                StandardMessages.EndApplication();
                return;
            }

            AccountGenerator.CreateAccount(user);

            StandardMessages.EndApplication();
        }
    }
}

using System;

namespace ConsoleUI
{
    class Person
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
    }
}

using System;

namespace ConsoleUI
{
    class StandardMessages
    {
        public static void WelcomeMessage()
        {
            Console.WriteLine("Welcome to my application!");
        }

        public static void EndApplication()
        {
            Console.ReadLine();
        }

        public static void DisplayValidationError(string fieldName)
        {
            Console.WriteLine($"You did not give us a valid {fieldName}!");
        }
    }
}

using System;

namespace ConsoleUI
{
    class PersonValidator
    {
        public static bool Validate(Person person)
        {
            // Checks to be sure the first and last names are valid
            if (string.IsNullOrWhiteSpace(person.FirstName))
            {
                StandardMessages.DisplayValidationError("first name");
                return false;
            }

            if (string.IsNullOrWhiteSpace(person.LastName))
            {
                StandardMessages.DisplayValidationError("last name");
                return false;
            }

            return true;
        }

    }
}

using System;

namespace ConsoleUI
{
    class PersonDataCapture
    {
        public static Person Capture()
        {
            // Ask for user data
            Person output = new Person();

            Console.Write("What is your first name: ");
            output.FirstName = Console.ReadLine();

            Console.Write("What is your last name: ");
            output.LastName = Console.ReadLine();

            return output;
        }

    }
}

using System;

namespace ConsoleUI
{
    class AccountGenerator
    {
        public static void CreateAccount(Person user)
        {
            // Create a username for the person
            Console.WriteLine($"Your username is { user.FirstName.Substring(0, 1) }{ user.LastName }");
        }
    }
}