LTE Mars Rover - Designing Logger

ConstructorInjection.cs

public class ItemRepository
{
  private readonly string _connectionString;
  
  // By adding the dependency here, there's no way you that
  // an ItemRepository could be created without specifying
  // a connection string.
  public ItemRepository(string connectionString)
  {
    _connectionString = connectionString;
  }
}

// Example usage
// If the connectionString isn't provided, then
// the code doesn't compile!
var itemRepository = new ItemRepository("Data Source=localhost;Initial Catalog=AdventureWorks;Integrated Security=True")

MethodInjection.cs

public class PurchaseWorkflow
{
  // By adding the NotificationStrategy here,
  // there's no way that a developer can call 
  // the Complete method without specifying 
  // a way to notify the customer.
  public void Complete(Order order, INotificationStrategy strategy)
  {
    string receiptDetails = CreateReceipt(order);
    strategy.SendNotification(order.Customer, receiptDetails);
  }
}

// Example Usage
var order = new Order("Cameron", 22.50);
var workflow = new PurchaseWorkflow();
// If a NotificationStrategy isn't passed in, this code won't compile.
workflow.Complete(order, new TextNotificationStrategy()); 

PropertyInjection.cs

public class PurchaseWorkflow
{
  // By having the notification be set as a property, 
  // developer can set this dependency once and then
  // only change it when needed.
  // However, there's nothing forcing a developer to 
  // set this property.
  public INotificationStrategy NotificationStrategy {get; set;}
  
  public void Complete(Order order)
  {
    string receiptDetails = CreateReceipt(order);
    NotificationStrategy.SendNotification(order.Customer, receiptDetails);
  }
}

// The Mistake
var order = new Order("Cameron", 22.50);
var workflow = new PurchaseWorkflow();
workflow.Complete(order); // This will cause a NullReferenceException to be thrown

// Proper Usage
var order = new Order("Cameron", 22.50);
var workflow = new PurchaseWorkflow();
// As long as you remember to set this property, things should work!
workflow.NotificationStrategy = new TextNotificationStrategy();
workflow.Complete(order);