using System;

using System.Collections.Generic;

using System.Linq;

using System.Data.SqlClient;

​

public class OKSolution 

{

    private IDatabase db;

    public bool AddUser(string userName) 

    {

        // Don't ask first. Just try.

        try 

        {

            db.Execute("INSERT INTO user (userrname, email, ...)");

            return true;

        } 

        catch (SqlException) {

            // Username was taken.

            return false;

        }

    }

    public void RegisterUser()      

    {       

        bool userCreated = false;       

        do {

           Console.WriteLine("Enter your username :");

           var userName = Console.ReadLine();

           userCreated = AddUser(userName);

        } while (!userCreated); 

    }

}

​

​

public class BadSolution

{

    private IDatabase db;

    public bool AddUser(string userName) 

    {

        // Check if username is taken

        bool alreadyTaken = db.Query("SELECT * FROM User where username=="+userName).Any();

        if (alreadyTaken)

           return false;

        // Not taken. Insert user.

        db.Execute("INSERT INTO user (userrname, email, ...)");

        return true;

    }

    public void RegisterUser()      

    {       

        bool userCreated = false;       

        do {

           Console.WriteLine("Enter your username :");

           string userName = Console.ReadLine();

           userCreated = AddUser(userName);

        } while (!userCreated); 

    }

}

​

​

// A pretend db.

public interface IDatabase {

   IEnumerable<object> Query(string sql); 

   void Execute(string sql);

}