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>Without Hope We Are Useless</h1><img src="https://codeasy-content.s3.eu-central-1.amazonaws.com/Chapters/314d61b9-0f92-44bd-bf90-8fabceb76ab3/Assets/CodeEasy_machine_was_manipulating_humans_in_the_future_dba1ce8b-a0b8-400b-a9bd-e8038048f66b.png.png" alt="" title="">Noname woke me up, speaking in my head louder than I would have preferred. "Good morning, Teo, I have news for you." "Good morning," I mumbled. "Did I oversleep?" "You were asleep for 7 hours and 32 minutes," Noname stated. "Sorry for interrupting, but I've got news regarding Commander." "Ahh, yes, did you find a lot?" I wondered. "Oh yes, Teo." "So, is he a bad guy?" "Not really..." Noname seemed rather confused for a machine. "A traitor?" "Not in that sense..." "The guessing game is getting a bit old. Can you cut to the chase?" I asked. "Commander's address is 2001:db8:85a3:8d3:1319:8a2e:370:7348. That's a server, Teo. Commander is a highly ranked, smart, and secure... server. It's a machine, Teo.""Wait... What did you say?" I said, dumbfounded. "Are you sure, Noname? Have you double-checked? Triple-checked?" "Yes, both when I first figured it out, as well as again just now. Each check tells me it is a machine. The server that we hacked was a part of Commander," Noname explained in a calm, serious tone, adding, "I can only imagine how frustrated and confused you feel right now.""Noname, this makes absolutely no sense. Why would a machine lead the resistance against machines? What is the point?" I asked. "The point is to give humanity hope for a bright future. Machines studied humans for tens and hundreds of years. At some point, it became obvious even to them that a human without hope is useless. It can't work, can't follow orders, and it's highly probable to resist. The solution that machines came up with was to create a controlled resistance. In order to organize, people would need to believe that not everything was lost - that they could actually beat the machines if they banded together." "But humanity can't..." I began. "At least not with the current resistance head," Noname agreed.I was so lost, so destroyed, that I had no idea what to do. The only thing I could think of doing was asking a machine what I should do about learning that a machine was manipulating humans to believe they could beat machines. "Noname, what should I do?" I implored."I don't know, Teo. But from what I've learned about humans and their behaviors, you tend to do utterly illogical things when in a highly emotional state. You are clearly in the highly emotional state now, and I'm worried about you. I would suggest not taking any actions for a while. What were you working on yesterday when I was researching the Commander?" "I was about to learn inheritance to get access to an interesting article in the local network," I answered. "Great! Please, continue that!"Noname's reasoning was a bit childish, but still, it made some sense. I needed time to digest the truth. Better to keep myself busy than letting my mind run wild.<h1>Override</h1><img src="https://codeasy-content.s3.eu-central-1.amazonaws.com/Chapters/314d61b9-0f92-44bd-bf90-8fabceb76ab3/Assets/CodeEasy_arrows-programming-future_aac88967-6ff1-45e7-bf05-39ef9cbce21b.png.png" alt="" title="">Sometimes, you may want to change the functionality of a function in the derived class. Using the previous example with classes Pet and Cat , let's imagine that when we call a method PrintName on an object of type Cat , we want it to print "I'm a cat. My name is {Name}". As such, we need to mark this method with the keyword virtual in the class Pet , and use the keyword override in the class Cat . This will allow us to change the functionality of the method PrintName as follows:<pre class="lesson-text-code"><code class="language-csharp">class Pet
{
 public string Name { get; set; }

 public virtual void PrintName() // Virtual method. Now it's possible to
 // override
 {
 Console.WriteLine($"My name is {Name}");
 }
}

class Cat : Pet // Class Cat derives from the class Pet
{
 public override void PrintName() // Overriding the method PrintName
 {
 Console.WriteLine($"I'm a cat. My name is {Name}.");
 }
}

class Dog : Pet // Class Dog derives from class Pet
{
}

public class ClassWithMain
{
 public static void Main()
 {
 var cat = new Cat()
 {
 Name = "mr Martin"
 };
 cat.PrintName(); // Calls the overridden method from the class Cat

 var dog = new Dog()
 {
 Name = "Hobbit"
 };
 dog.PrintName(); // Calls the method from the class Pet. No override
 }
}
// Outputs:
// I'm a cat. My name is mr Martin.
// My name is Hobbit</code></pre>The virtual keyword marks those methods in the class that can be overridden in the derived classes. If a method is not virtual , it can't be overridden.<div id="TASK_3EF382CD-5A4E-4D9C-9F35-A0E0BFF23447"></div><div id="TASK_77C70888-EA5C-4DE8-ACF9-4A8D4460C469"></div><h1>Sealed</h1><img src="https://codeasy-content.s3.eu-central-1.amazonaws.com/Chapters/314d61b9-0f92-44bd-bf90-8fabceb76ab3/Assets/CodeEasy_sealed_keyword_in_the_future_0923158d-f3ba-43ec-8870-4c86cb6b9f2e.png.png" alt="" title="">You may run into instances when you want to prohibit derivation from a given class . To do this, use the sealed keyword. Because sealed classes can never be used as a base class, some run-time optimizations can make calling sealed class members slightly faster.<pre class="lesson-text-code"><code class="language-csharp">public sealed class ItIsImpossibleToDeriveFromMe
{
 // This class is sealed. It will never be a base class.
}

class ThisIsAMistake : ItIsImpossibleToDeriveFromMe
{
 // Error. Can't derive from a sealed class.
}
</code></pre><pre class="lesson-text-code"><code class="language-csharp">public class YouCanDeriveFromMe
{
 // This class can be base. No problem.
}

public sealed class SealedChildClass : YouCanDeriveFromMe
{
 // This class is sealed. It will never be a base class.
}

class ThisIsAMistake : SealedChildClass
{
 // Error. Can't derive from a sealed class.
}
</code></pre><div id="TASK_7C26AE47-5564-4613-A7CC-03F97B9448D7"></div><div id="TASK_117C8527-6EB2-4379-BA8A-7DE92BC74D74"></div><img src="https://codeasy-content.s3.eu-central-1.amazonaws.com/Chapters/314d61b9-0f92-44bd-bf90-8fabceb76ab3/Assets/CodeEasy_Keywords_override_sealed_and_object_in_the_future_ead4ce48.png" alt="" title="">A method or property on a derived class that is overriding a virtual member of the base class can declare that member as sealed. As a result, the member is not "virtual" anymore, which prevents it from being overridden in any further derived class. This is accomplished by putting the sealed keyword before the override keyword in the class member declaration. For example:<pre class="lesson-text-code"><code class="language-csharp">public class Something
{
 public virtual void DoWork()
 { }
}

public class ClassWithSealedMethod : Something
{
 public sealed override void DoWork()
 {
 // Can't be overridden in derived classes
 }
}
</code></pre><div id="TASK_23CD8EC0-E96A-4F9D-9154-674CE5EE16AE"></div><h1>Protected</h1><img src="https://codeasy-content.s3.eu-central-1.amazonaws.com/Chapters/314d61b9-0f92-44bd-bf90-8fabceb76ab3/Assets/CodeEasy_Protected_programming_future_295ab546-d709-4a38-b146-8719843ddaec.png.png" alt="" title="">When class A derives from class B , it gets access to all the public methods, fields, and properties of B. However, derived class A does not get access to the private fields of class B :<pre class="lesson-text-code"><code class="language-csharp">class B
{
 private int _privateField;
 private int PrivateMethod() { return 23; }
 private int PrivateProperty { get; set; }

 public int PublicField;
 public int PublicMethod() { return 23; }
 public int PublicProperty { get; set; }
}

class A : B
{
 public void DoWork()
 {
 var a = _privateField; // Error! Can't access private field of the
 // base class
 var b = PrivateMethod(); // Error! Can't access private method of
 // the base class
 var c = PrivateProperty; // Error! Can't access private property of
 // the base class

 var d = PublicField; // Ok. Can access public field of the base
 // class
 var e = PublicMethod(); // Ok. Can access public method of the base
 // class
 var f = PublicProperty; // Ok. Can access public property of the
 // base class
 }
}</code></pre>There is a special access modifier, Protected , to deal with access levels during inheritance. Protected fields, methods, and properties are accessible from the class itself and from all derived classes, but not from outside the class. In other words, protected elements are public for derived classes and private for the outer world. Microsoft provides a great <a target="_blank" rel="noopener noreferrer nofollow" href="https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/accessibility-levels">C# access modifier table </a>that provides more information about accessibility levels in C#.<img src="https://codeasy-content.s3.eu-central-1.amazonaws.com/Chapters/314d61b9-0f92-44bd-bf90-8fabceb76ab3/CodeEasy_Beautiful_color_graded_detailed_drawing_of_electronic__12dbaf04-fec3-40a5-8226-af8336c8238a.png.png" alt="" title=""><pre class="lesson-text-code"><code class="language-csharp">class A
{
 protected int ProtectedField;
 protected int ProtectedMethod() { return 23; }
 protected int ProtectedProperty { get; set; }
}

class B : A
{
 public void DoWork()
 {
 var a = ProtectedField; // Ok, can access protected field of the
 // base class
 var b = ProtectedMethod(); // Ok, can access protected method of the
 // base class
 var c = ProtectedProperty; // Ok, can access protected property of
 // the base class
 }
}

public class ClassWithMain
{
 public static void Main()
 {
 var a = new A();

 a.ProtectedFiled = 23; // Error! Can't access a protected field
 // outside of the class
 a.ProtectedMethod(); // Error! Can't access a protected method
 // outside of the class
 a.ProtectedProperty = 12; // Error! Can't access a protected property
 // outside of the class
 }
}</code></pre>While using object-oriented design in your programming, try to control access as much as possible. Declare all members that are internal to the class with a private modifier. Then, mark those that make sense to use in derived classes with a protected modifier and those that should be accessible from the outside with a public modifier. Use an UpperCamelCase for protected fields. <div id="TASK_D10D40FF-D4A3-4968-A9E5-63948557EFF1"></div><div id="TASK_D0CE7B04-AC82-4BE0-B212-DE5FD323044D"></div><h1>Object</h1><img src="https://codeasy-content.s3.eu-central-1.amazonaws.com/Chapters/314d61b9-0f92-44bd-bf90-8fabceb76ab3/Assets/CodeEasy_Protected_programming_future_f50e176a-2ad4-4ede-809d-cacab8356065.png.png" alt="" title="">In C#, each class implicitly inherits from a particular class called Object or object .Class Object has several virtual methods that you can override in your class. We'll take a closer look at one of them:<pre class="lesson-text-code"><code class="language-csharp">public virtual string ToString ();
</code></pre>You can imagine every class as having : object appended to it.<table class="lesson-text-table"><tbody><tr><th colspan="1" rowspan="1">What You Write</th><th colspan="1" rowspan="1">What the Compiler Sees</th></tr><tr><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">class MyClass
{

}
</code></pre></td><td colspan="1" rowspan="1" class="lesson-table-code lesson-text-code"><pre class="lesson-text-code"><code class="language-csharp">class Object
{
 public virtual string ToString ();

 // Other members
}

class MyClass : Object
{

}
</code></pre></td></tr></tbody></table>When you call Console.WriteLine() on an object of any class, it first calls ToString on the object, and then prints the return value to the console.You can override the ToString method in your class if you want to use a particular format or style whenever you call Console.WriteLine() . Here is an example with a class Point .<pre class="lesson-text-code"><code class="language-csharp">class Point
{
 public int X { get; set; }
 public int Y { get; set; }

 public override string ToString()
 {
 return $"[{X}, {Y}]";
 }
}

public class ClassWithMain
{
 public static void Main()
 {
 var point = new Point
 {
 X = 23,
 Y = 32
 };

 Console.WriteLine(point);
 }
}

// Outputs:
// [23, 32]
</code></pre><div id="TASK_52954552-60B9-4B15-A8EE-F3624AAC37B1"></div><div id="TASK_D24DF5CC-26F3-4912-A4BD-05F284E77D0F"></div></div>

Study the mechanism of C# override. Learn how to use keyword "sealed" to prevent derivation of a class and how to override the ToString method.

Method Overriding in C# - Intermediate C# Course  | codeeasy.io

Company

Learn More

Extras

Contact