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copied!<p>I don't see much difference here in my C# version.</p> <p>I think the author of mentioned C# book might try to make his example looks like the original Observer pattern, where there are <code>Subject</code>, <code>ConcreteSubject</code>, <code>Observer</code>, and <code>ConcreteObserver</code> classes. These are actually unnecessary in many conditions. Many times, just subscribe to the event with method is enough.</p> <p>By using event and delegate C# has provided, you can eliminate the need to maintain "observer list" and its related attach/detach methods yourself. They also provide an easy way to notify the subscribed clients of new event.</p> <p><strong>Update:</strong> Just saw @Tomas's implementation. He has good use of C# 3 there. However, if you want to see direct mapping from Ruby code my example below could help.</p> <pre><code>using System; namespace Observer { class Program { static void Main() { Employee fred = new Employee() { Name = "Fred", Title = "Crane Operator", Salary = 40000.0 }; TaxMan tax_man = new TaxMan(); fred.Update += tax_man.OnUpdate; fred.Salary = 50000.0; } } public class Subject { public delegate void UpdateHandler(Subject s); public virtual event UpdateHandler Update; } public class Employee : Subject { public string Name { get; set; } public string Title { get; set; } private double _salary; public double Salary { get { return _salary; } set { _salary = value; if (Update != null) Update(this); } } public override event UpdateHandler Update; } public class TaxMan { public void OnUpdate(Subject s) { if (s is Employee) Console.WriteLine("Send {0} a new tax bill!", (s as Employee).Name); } } } </code></pre>

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