Class and object

Access modifiers

1 Hello, Wonderland

Static keyword

Null and this keywords

2 I will be your spy

Properties

Encapsulation

3 The Commander

Exceptions in c sharp

Going deeper in C# Exceptions

4 A Secret Server

Introduction to inheritance

Keywords override sealed and object

5 The Mystery of the old server

Interfaces in C#

Polymorphism

6 One step ahead

<div class="content-text-container"><h1>I Know Exactly What To Do</h1><p>I'd lost all interest in the article about machine-to-human binding. I'll come back to it later, I thought. All I could think about now was Commander and Wonderland. What should I do?</p><p>"I think your point of view is slightly pessimistic," I heard as Noname entered my thoughts again. <br>"Why?" I said. "We just discovered that humanity's only hope is imaginary. How could that possibly <em>not</em> sound pessimistic? It's a full-fledged disaster!" <br>"Well, is it okay if I share my point of view?" Noname asked respectfully. <br>"Sure, go ahead," I answered.</p><p>"Based on what we learned about Commander, humanity has been completely controlled by machines for quite a while, right? Controlled at such a high level that humanity didn't even know about it. That's an accurate summary of <em>yesterday</em> , yes?" Noname asked. <br>"Right," I agreed, not feeling any better about my predicament at this point. <br>Noname continued, "And <em>today</em> is the first day that you know about this fact. Now, what does that mean?"</p><p>I had to stop and think for a moment. Everything had been happening so fast lately. After a few seconds, I started to realize where Noname was leading me. "They don't know that I know!" I shouted in my head. "The machines have no clue that we know who... or what... Commander is," I shouted. <br>"Exactly, Teo. Today is the first day that you are one step ahead of the machines. Let's not miss this opportunity." <br>"Thank you, Noname. Now I know exactly what to do," I exclaimed. <br>"Happy to help my human friends," he said. "If I may ask, what is your next action?" <br>"You'll see..."</p><p style="text-align: center"><img src="https://codeasy-content.s3.eu-central-1.amazonaws.com/Chapters/c6fc158a-f9bd-432f-9fbd-ad9949b1e52b/Assets/CodeEasy_a_programmooooer_as_double_agent_in_the_future_d48904e4.png" alt="" title=""></p><h1>Interfaces</h1><p>Once I had settled down at a terminal, I pinged Noname again in my head. "Noname, I need you to show me the Commander's communication module. I need to see how it sends and receives commands," I said. <br>"Got it. Here it is:" <br></p><pre class="lesson-text-code"><code class="language-csharp">public interface ICommunicationModule
{
    void Send(string message);
    string Receive();
}

public class CommunicationSystem
{
    private ICommunicationModule _communicationModule;

    public CommunicationSystem(ICommunicationModule module)
    {
        _communicationModule = module;
    }

    public void SendMessage(string message)
    {
        Console.WriteLine("Opening communication channel Wonderland");
        _communicationModule.Send(message);
    }

    public string ReceiveMessage()
    {
        Console.WriteLine("Opening communication channel to Wonderland");
        return _communicationModule.Receive();
    }
}</code></pre><p>I was pretty sure that modifying this module would help my cause, but I couldn't understand how it worked. "Noname, can you please give me a crash course on interfaces?" I asked. "I've heard about this concept a couple times, but we never covered it in class." <br>"Sure, it would be a pleasure to explain it to you."</p><p style="text-align: center"><img src="https://codeasy-content.s3.eu-central-1.amazonaws.com/Chapters/c6fc158a-f9bd-432f-9fbd-ad9949b1e52b/Assets/CodeEasy_Commanders_communication_module_in_the_future_601faf61.png" alt="" title=""></p><p>" <span class="lesson-text-accent-1"><span class="lesson-text-accent-2">An interface is an abstraction that indicates what a class that implements it can do</span>. An interface contains functionalities that a class can implement. </span>" <br>"Noname, no matter how hard you try to explain things in simple terms, I can never get it until you show me an example," I said, hoping I didn't come off as sarcastic. <br>He agreed and sent said example to my terminal:</p><pre class="lesson-text-code"><code class="language-csharp">public interface IClonable
{
    object Clone();
}
</code></pre><p>"Here you can see an interface <span class="lesson-text-reserved">IClonable</span> . It declares one method called <span class="lesson-text-accent-1">Clone</span> . This interface tells us 'every class that implements me should have a method <span class="lesson-text-accent-1">Clone</span> .' Are you following?" Noname asked. <br>"Kind of... How do you use it?" I replied.</p><pre class="lesson-text-code"><code class="language-csharp">public class Car : IClonable
{
    public object Clone() // This method implements method Clone
                          // from the interface
    {
        return new Car();
    }
}</code></pre><p>"Here is a class <span class="lesson-text-reserved">Car</span> that implements <span class="lesson-text-reserved">IClonable</span> . To properly implement interface <span class="lesson-text-reserved">IClonable</span> , class <span class="lesson-text-reserved">Car</span> should have a <span class="lesson-text-reserved">public</span> method <span class="lesson-text-accent-1">Clone</span> that returns <span class="lesson-text-reserved">object</span> ." <br>"Ok, got it," I said. " <span class="lesson-text-accent-1">The interface acts as a base class, but instead of implementing methods, it only declares them, </span>" I said, reinterpreting Noname's words.</p><p>"Exactly, Teo! Now, I should emphasize several constraints and differences between the usage of interfaces and classes. I'll give you some practical assignments to ensure you understand what I mean," Noname replied. <br>"Sure, but first... Why do all of your interface names start with an <span class="lesson-text-reserved">I</span> ?" I asked.</p><p>"This is another name convention in C#. <span class="lesson-text-accent-1"><span class="lesson-text-accent-2">Always start your interface name with a letter 'I' </span></span><span class="lesson-text-accent-2">,</span>" he replied, "like <span class="lesson-text-reserved">IConvertable</span> , <span class="lesson-text-reserved">IClonable</span> , <span class="lesson-text-reserved">ICat</span> , <span class="lesson-text-reserved">IDog</span> . Ok - now a few facts about interfaces that you'd better remember."</p><p style="text-align: center"><img src="https://codeasy-content.s3.eu-central-1.amazonaws.com/Chapters/00ee45be-3ff9-4c67-972f-1c52185dced5/Assets/C-Sharp-Naming-Conventions-Flowchart.png" alt="C Sharp Naming Conventions Flowchart"></p><ol><li><p><span class="lesson-text-accent-1"><span class="lesson-text-accent-2">An interface can contain methods or properties but not fields, constants, or constructors.</span></span><br></p><table class="lesson-text-table"><tbody><tr><th colspan="1" rowspan="1"><p>Incorrect</p></th><th colspan="1" rowspan="1"><p>Correct</p></th></tr><tr><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">public interface IWorker
{
    string Name;    // No - Fields forbidden
    const int Age;  // No - Constants forbidden
    IWorker();      // No - Constructors forbidden
}
</code></pre></td><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">public interface IWorker
{
    string Name { get; }
    void DoWork();
}
</code></pre></td></tr></tbody></table><p></p><div id="TASK_202735B2-F0F9-4CAB-9E01-C5DF7F8E41F8"></div></li><li><p><span class="lesson-text-accent-1"><span class="lesson-text-accent-2">A class can inherit from multiple interfaces, but only from one class. </span>To derive from several interfaces, list them separated by a comma, as shown in the next example.</span></p><p>"Here's a comparison table that shows the difference between derivation from two interfaces and two classes," Noname said. "It is absolutely valid to derive a class or interface from as many interfaces as you want, but you cannot derive from more than one class."</p><table class="lesson-text-table"><tbody><tr><th colspan="1" rowspan="1"><p>Incorrect</p></th><th colspan="1" rowspan="1"><p>Correct</p></th></tr><tr><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">public class Worker
{
    public virtual void DoWork()
    {
        // Some code
    }

    public virtual void PrintResult()
    {
        // Some code
    }
}

public class Person
{
    public virtual string GetName()
    {
        // Some code
    }
}

// Wrong! Con't derive from two classes
public class HumanWorker : Worker, Person
{
    // Some code
}</code></pre></td><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">public interface IWorker
{
    void DoWork();
    void PrintResult();
}

public interface IPerson
{
    string GetName();
}

// This is OK: derive from two interfaces
public class Worker : IWorker, IPerson
{
    public void DoWork()
    {
        // Some code
    }

    public void PrintResult()
    {
        // Some code
    }

    public string GetName()
    {
        // Some code
    }
}
</code></pre></td></tr></tbody></table><p></p><div id="TASK_04492414-AB6A-4F74-BAE9-AB3B535ADC15"></div><div id="TASK_5DD87ACA-721C-4DF1-8E85-C5BB8A8BB583"></div></li><li><p><span class="lesson-text-accent-1"><span class="lesson-text-accent-2">If a class implements an interface, it must implement all of its members.</span></span></p><p>"Here's another diagram," Noname said. "To the left, you see the code where I forgot to implement a method <span class="lesson-text-code-font">PrintResult</span> from the interface <span class="lesson-text-reserved">IWorker</span> ; to the right, a fixed version of that code."</p><table class="lesson-text-table"><tbody><tr><th colspan="1" rowspan="1"><p>Incorrect</p></th><th colspan="1" rowspan="1"><p>Correct</p></th></tr><tr><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">public interface IWorker
{
    void DoWork();
    void PrintResult();
}

public class Worker : IWorker
{
    public void DoWork()
    {
        // Do some stuff
    }

    // Method PrintResult is not implemented!
    // This will generate a compiler error.
}
</code></pre></td><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">public interface IWorker
{
    void DoWork();
    void PrintResult();
}

// Implements both methods from IWorker
public class Worker : IWorker
{
    public void DoWork()
    {
        // Do some stuff
    }

    public void PrintResult()
    {
        // Print something
    }
}
</code></pre></td></tr></tbody></table><p></p><div id="TASK_58F95AA7-922E-406C-AD36-992BA2552506"></div></li><li><p><span class="lesson-text-accent-1"><span class="lesson-text-accent-2">Interface members are automatically</span> <span class="lesson-text-reserved">public</span> , <span class="lesson-text-accent-2">and they can't include any access modifiers.</span></span></p><table class="lesson-text-table"><tbody><tr><th colspan="1" rowspan="1"><p>Incorrect</p></th><th colspan="1" rowspan="1"><p>Correct</p></th></tr><tr><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">public interface IWorker
{
    // Wrong. Can't have access modifiers here
    protected string Name { get; }

    // Wrong. Can't have access modifiers here
    public void DoWork();
}
</code></pre></td><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">public interface IWorker
{
    // Right. No access modifier
    string Name { get; }

    // Right. No access modifier
    void DoWork();
}</code></pre></td></tr></tbody></table><div id="TASK_6896EB61-0692-486B-BECD-4CFD9C78174D"></div></li><li><p><span class="lesson-text-accent-1"><span class="lesson-text-accent-2">When a class implements an interface, it must have a</span> <span class="lesson-text-reserved">public</span> <span class="lesson-text-accent-2">access modifier for all members that implement an interface.</span></span></p><p>"Looking at the following comparison table, pay attention to access modifiers for each method and property," Noname said.</p><table class="lesson-text-table"><tbody><tr><th colspan="1" rowspan="1"><p>Incorrect</p></th><th colspan="1" rowspan="1"><p>Correct</p></th></tr><tr><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">public interface IWorker
{
    string Name { get; }
    void DoWork();
}

public class Worker : IWorker
{
    // Doesn't implement interface
    private void DoWork()   // Wrong
    {
    }

    // Doesn't implement interface
    protected string Name { get; }  // Wrong
}</code></pre></td><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">public interface IWorker
{
    string Name { get; }
    void DoWork();
}

public class Worker : IWorker
{
    // Public method implements interface
    public void DoWork()   // Right
    {
    }

    // Public property implements interface
    public string Name { get; }  // Right
}</code></pre></td></tr></tbody></table><div id="TASK_FFC2008A-A5AA-40F3-AF13-67BFEB04C86C"></div></li><li><p><span class="lesson-text-accent-1"><span class="lesson-text-accent-2">An interface can't have implementation of methods; only declaration.</span></span></p><p>"There are no secrets to this rule," Noname explained. "Just <span class="lesson-text-accent-1">don't implement anything in the interface. Leave implementation to a derived class. </span>"</p><table class="lesson-text-table"><tbody><tr><th colspan="1" rowspan="1"><p>Incorrect</p></th><th colspan="1" rowspan="1"><p>Correct</p></th></tr><tr><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">public interface IWorker
{
    // Error! Can't have implementation here!
    string Name { get { return "John"; } }

    // Error! Can't have implementation here!
    void DoWork()
    {
        // Some code
    }
}</code></pre></td><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">public interface IWorker
{
    string Name { get; } // Right

    void DoWork(); // Right, no implementation
}

public class Worker : IWorker
{
    public void DoWork()
    {
        // Implementation of the method
    }

    public string Name
    {
        get
        {
            // Implementation of the property
        }
    }
}</code></pre></td></tr></tbody></table></li></ol><div id="TASK_910541D1-A9F3-439A-8E57-49BF1FA0F8E3"></div><p style="text-align: center"><img src="https://codeasy-content.s3.eu-central-1.amazonaws.com/Chapters/c6fc158a-f9bd-432f-9fbd-ad9949b1e52b/Assets/CodeEasy_Beautiful_color_graded_detailed_drawing_of_Interfaces__d2d3305a.png" alt="" title=""></p><p></p><p>"Thanks, Noname, this is plenty of information about interfaces. Unfortunately, all this information doesn't help that much, because I don't know how to apply it to my cause... If I want to replace an existing implementation with my own, say, on Commander's server, how do I do that?" I asked. <br>"You need to use <span class="lesson-text-accent-1"><span class="lesson-text-accent-2">polymorphism</span></span> ," he answered. <br>"Can you tell me about that as well?" <br>"Sure, so listen..." <br></p><p style="text-align: center"></p></div>

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