customer-fetcher.cs

public class CustomersFetcher
{
  public IEnumerable<CustomerDetails> GetSleepingCustomers()
  {
    List<CustomerDetails> result = new List<CustomerDetails>();
    var connectionString = "...";
    using (var connection = new OleDbConnection(connectionString))
    {
      connection.Open();
      var command = new OleDbCommand("select * from Customers where LastPurchasedAt < ?", connection);
      var todayMinus30 = DateTime.Today.AddDays(-30);
      var oleDbParameter = new OleDbParameter { OleDbType = OleDbType.Date, Value = todayMinus30 };
      command.Parameters.Add(oleDbParameter);
      using (command)
      using (var reader = command.ExecuteReader(CommandBehavior.CloseConnection))
      {
        while (reader.Read())
        {
          var customerDetails = new CustomerDetails
                        {
                          FirstName = (string) reader["FirstName"],
                          LastName = (string) reader["LastName"],
                          EmailAddress = (string) reader["EmailAddress"]
                        };
          result.Add(customerDetails);
        }
      }
    }
    return result;
  }
}
 
public class CustomerDetails
{
  public string FirstName;
  public string LastName;
  public string EmailAddress;
}

customer-reminder.cs

public class CustomersReminder
{
  public void Remind()
  {
    var connectionString = "...";
    using (var connection = new OleDbConnection(connectionString))
    {
      connection.Open();
      var command = new OleDbCommand("select * from Customers where LastPurchasedAt < ?", connection);
      var todayMinus30 = DateTime.Today.AddDays(-30);
      var oleDbParameter = new OleDbParameter {OleDbType = OleDbType.Date, Value = todayMinus30};
      command.Parameters.Add(oleDbParameter);
      using (command)
      using (var reader = command.ExecuteReader(CommandBehavior.CloseConnection))
      {
        while (reader.Read())
        {
          var firstName = (string) reader["FirstName"];
          var lastName = (string) reader["LastName"];
          var email = (string)reader["EmailAddress"];
          SendMailTo(firstName, lastName, email);
        }
      }
    }
  }
 
  private void SendMailTo(string firstName, string lastName, string email)
  {
    var mailMessage = new MailMessage("[email protected]", email);
    mailMessage.Subject = "It's been a while...";
    mailMessage.Body = string.Format("Hi there, {0} {1}, please try our new sales!", firstName, lastName);
    var smtpClient = new SmtpClient();
    try
    {
      smtpClient.Send(mailMessage);
    }
    catch (Exception ex)
    {
      // do something here, like logging or alerting..
    }
  }
}

customer-reminder2.cs

public class CustomersReminder
{
  public void Remind()
  {
    var customersFetcher = new CustomersFetcher();
    var emailSender = new EmailSender();
    foreach (var customer in customersFetcher.GetSleepingCustomers())
    {
      emailSender.SendRemindMessage(customer.FirstName, customer.LastName, customer.EmailAddress);
    }
  }
}

customer-reminder3.cs

public class CustomersReminder
{
  public void Remind(CustomersFetcher customersFetcher, EmailSender emailSender)
  {
    foreach (var customer in customersFetcher.GetSleepingCustomers())
    {
      emailSender.SendRemindMessage(customer.FirstName, customer.LastName, customer.EmailAddress);
    }
  }
}

CustomersFetcher.cs

public class CustomersFetcher
{
  public IEnumerable<CustomerDetails> GetSleepingCustomers()
  {
    List<CustomerDetails> result = new List<CustomerDetails>();
    var connectionString = "...";
    using (var connection = new OleDbConnection(connectionString))
    {
      connection.Open();
      var command = new OleDbCommand("select * from Customers where LastPurchasedAt < ?", connection);
      var todayMinus30 = DateTime.Today.AddDays(-30);
      var oleDbParameter = new OleDbParameter { OleDbType = OleDbType.Date, Value = todayMinus30 };
      command.Parameters.Add(oleDbParameter);
      using (command)
      using (var reader = command.ExecuteReader(CommandBehavior.CloseConnection))
      {
        while (reader.Read())
        {
          var customerDetails = new CustomerDetails
                        {
                          FirstName = (string) reader["FirstName"],
                          LastName = (string) reader["LastName"],
                          EmailAddress = (string) reader["EmailAddress"]
                        };
          result.Add(customerDetails);
        }
      }
    }
    return result;
  }
}
 
public class CustomerDetails
{
  public string FirstName;
  public string LastName;
  public string EmailAddress;
}

CustomersReminder1a.cs

public class CustomersReminder
{
  public void Remind()
  {
    var connectionString = "...";
    using (var connection = new OleDbConnection(connectionString))
    {
      connection.Open();
      var command = new OleDbCommand("select * from Customers where LastPurchasedAt < ?", connection);
      var todayMinus30 = DateTime.Today.AddDays(-30);
      var oleDbParameter = new OleDbParameter {OleDbType = OleDbType.Date, Value = todayMinus30};
      command.Parameters.Add(oleDbParameter);
      using (command)
      using (var reader = command.ExecuteReader(CommandBehavior.CloseConnection))
      {
        while (reader.Read())
        {
          var firstName = (string) reader["FirstName"];
          var lastName = (string) reader["LastName"];
          var email = (string)reader["EmailAddress"];
          SendMailTo(firstName, lastName, email);
        }
      }
    }
  }
 
  private void SendMailTo(string firstName, string lastName, string email)
  {
    var mailMessage = new MailMessage("[email protected]", email);
    mailMessage.Subject = "It's been a while...";
    mailMessage.Body = string.Format("Hi there, {0} {1}, please try our new sales!", firstName, lastName);
    var smtpClient = new SmtpClient();
    try
    {
      smtpClient.Send(mailMessage);
    }
    catch (Exception ex)
    {
      // do something here, like logging or alerting..
    }
  }
}

CustomersReminder2.cs

public class CustomersReminder
{
  public void Remind()
  {
    var customersFetcher = new CustomersFetcher();
    var emailSender = new EmailSender();
    foreach (var customer in customersFetcher.GetSleepingCustomers())
    {
      emailSender.SendRemindMessage(customer.FirstName, customer.LastName, customer.EmailAddress);
    }
  }
}

CustomersReminder3.cs

public class CustomersReminder
{
  public void Remind(CustomersFetcher customersFetcher, EmailSender emailSender)
  {
    foreach (var customer in customersFetcher.GetSleepingCustomers())
    {
      emailSender.SendRemindMessage(customer.FirstName, customer.LastName, customer.EmailAddress);
    }
  }
}

example1.cs

var customersReminder = new CustomersReminder();
customersReminder.Remind(new CustomersFetcher(), new EmailSender());

example2.cs

public interface ICustomersFetcher
{
  IEnumerable<CustomerDetails> GetSleepingCustomers();
}
 
public interface IEmailSender
{
  void SendRemindMessage(string firstName, string lastName, string email);
}

example3.cs

public class CustomersFetcher : ICustomersFetcher
{ ... }

example4.cs

public void Remind(ICustomersFetcher customersFetcher, IEmailSender emailSender)
{ ... }

example5.cs

public class ImageProcessing
{
  public void ProcessImage(Uri imageUrl, IDownloader downloader, IImageResizer imageResizer, IFileSystem fileSystem)
  {
    byte[] imageBytes = downloader.Get(imageUrl);
    Image bitmap = Image.FromStream(new MemoryStream(imageBytes));
    Image afterResize = imageResizer.ResizeToMax(bitmap, 100);
    fileSystem.StoreImage(afterResize);
  }
}
 
public interface IFileSystem
{
  void StoreImage(Image image);
}
 
public interface IImageResizer
{
  Image ResizeToMax(Image bitmap, int maxWidthOrHeight);
}
 
public interface IDownloader
{
  byte[] Get(Uri imageUrl);
}

implementations1.cs

public class Rectangle : IShape
{
  private readonly double a;
  private readonly double b;
 
  public Rectangle(double a, double b)
  {
    this.a = a;
    this.b = b;
  }
 
  public double GetArea()
  {
    return a*b;
  }
}
 
public class Circle : IShape
{
  private readonly double radius;
 
  public Circle(double radius)
  {
    this.radius = radius;
  }
 
  public double GetArea()
  {
    const double pi = Math.PI; // ?
    return pi * radius * radius;
  }
}

implementations2.cs

public static void DoSomething(IShape shape)
{
  Console.WriteLine("the area is {0:00}", shape.GetArea());
}

interface-shape.cs

public interface IShape
{
  double GetArea();
}