N-Tier: Begginer's guide in designing their application

A great way to design your own application is to use the N-tier architecture.
 
This is just a simple tip that I will share and it is for you to check out.
 
Note: The following layers are exposed as different projects under one solution.
 
Object Layer: Create a class with properties:

public class Person
{
   public string Firstname {get;set;}
   public string Lastname {get;set;}
   public int Age {get;set;}
}

Data Access Layer: The layer that interacts with database:
 

public sealed class PersonDAL
{
   Person person = new Person();
   public static int GetAge(string firstname, string lastname)
   {
      try
      {
         SQLConnection sqlcon = new SQLConnection(ConfigurationManager
                                                      .ConnectionStrings["myDB"]
                                                      .ConnectionString);
         SQLCommand sqlcmd = new SQLCommand("SELECT age FROM Person_tb WHERE
                               firstname=@firstname AND lastname=@lastname",
                               sqlconn);
         sqlcmd.CommandType = CommandType.CommandText;
         sqlcmd.Parameters.AddWithValue("@firstname", firstname);
         sqlcmd.Parameters.AddWithValue("@lastname", lastname);
         sqlcon.Open();
         int retval = (int)sqlcmd.ExecuteScalar();
      }
      catch(Exception ex)
      {
         //Log ex
      }
      finally
      { 
         sqlcmd.Dispose();
         sqlcon.Close();
         sqlcon = null;
      }
   }
}

Business Logic Layer: The layer that serves as bridge for DAL and Application Layer:
 

public sealed class PersonBLL
{
   public static int GetAge(string firstname, string lastname)
   {
      return PersonDAL.GetAge(firstname, lastname);
   }
}

Application/Presentation Layer: The layer where users input data:
 

Person person = new Person();
person.Firstname = "Eduard";
person.Lastname = "Lu";
 
string firstname = person.Firstname;
string lastname = person.Lastname;
 
txtAge.Text = PersonBLL.GetAge(firstname, lastname).ToString();

 
There are still a lot of ways to optimize this one. You can add more layers which your application might be needing. For example, add a Utilities Layer wherein you can place your configuration/settings and your error/transaction logger. In my example, you can create a function that will return the connection string for your application to communicate with your database server.
 
Also, creating stored procedure is better than using command text. In my example, I used command text just to show you my query.
 
Message me if you have concerns. :)