* Using namespaces helps you to organize the scope of your project, so that...
* Variable names do not clash
* Related variables are well-contained in their own namespaces
* $\Rightarrow$ easy to import what you need
* By default, when creating a new project and adding classes in Visual Studio, everything is contained within a namespace named after your project name
Using namespaces helps you to organize the scope of globally available data in your project, meaning that related objects in a well-named namespace makes it easy to import what you need
---
By default, when creating a new project and adding classes in Visual Studio, everything is contained within a namespace named after your project name
* If you need a class, interface, etc to be accessible from anywhere, create it within that namespace and set it as `public`
If you need a class or an enum to be accessible from anywhere, create it within that namespace and set it as public
<divclass='columns'markdown='1'>
<divmarkdown='1'>
```csharp
// Note that namespaces can have
// subdirectories, as Models here
namespace MyAwesomeApp.Models
{
public class Student
{
}
}
```
// This is how you would use the
</div>
<divmarkdown='1'>
```csharp
// This is how you would use the
// Student class in other code
using MyAwesomeApp.Models
```
# Classes
</div>
</div>
__Classes __ in C# are blueprints for a __type __ of __object__
* Wait, what's a class, though?
We have already used a class named _Program _ when creating a new console application:
---
## Classes
* [Classes](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/classes) in C# are blueprints for specific kinds of collections of data & functionality
* ***Instance*** of a class is an implementation of that class, also referred to as an ***object***
* As C# is an object-oriented language, almost everything has to happen in a class
* Thus, we have already used a class named `Program` when creating a new console application:
```csharp
class Program
{
static void Main(string[] args)
{
You could even create new instances of Program
}
}
```
* If we wanted, we could just write the entire program with thousands of lines of code on one page inside the main function
* This kind of program would of course be impossible to work with within a group or organization:
* No one should have to scroll through tens of thousands of lines of code to find that one line that causes problems…
* At least make them browse through hundreds of classes instead :)
* Classes provide __encapsulation __ which result in __shorter files__ , __less repetitive code__ and overall __better readability__
### Why classes?
# Creating a Class
* Classes are a core feature of the ***object-oriented programming*** paradigm popularized by Java in the 90s
* (See [Lecture 8](8-inheritance-and-abstract-classes#extra-object-oriented-programming))
* Classes provide *__encapsulation__*: Connected concepts and functionalities are neatly contained within a class
* This will ***ideally*** result in shorter files, less repetitive code and ***hopefully*** overall better readability
* No paradigm is idiot-proof, though...
* Shorter files can also mean that it's hard to find the actual functionality from hundreds of short files full of intermediate classes and class factories
You can create a new class by writing
### Creating a class
* You can create a new class by writing
```csharp
class Student
{
}
Create variables inside a class just like before inside the Program class:
```
* Create variables inside a class just like before inside the `Program` class:
```csharp
class Student
{
int id;
string name;
}
```
* These variables declared directly inside classes are called *__fields__*.
❕
Variables declared directly inside classes are called __fields.__
# Referencing a Class
### Instancing a class
New instances of a class can be created with the __new __ keyword
<divclass='columns'markdown='1'>
<divmarkdown='1'>
All instances of a class have exactly the same fields and methods, but they can have different values
* New instances of a class can be created with the `new` keyword
* All instances of a class have exactly the same fields and methods, but they can have different values
* In the following example a few ***objects*** of class `Student` are created
* However, these objects have no functionality yet
In the following example a few __objects __ of __type __ Student are created
</div>
<divmarkdown='1'>
However, these objects have no functionality yet
```csharp
namespace MyAwesomeApp
{
class Student
{
}
class Program
{
static void Main(string[] args)
{
Student student1 = new Student();
Student student2 = new Student();
Student student3 = new Student();
}
}
}
```
# Object Variables
</div>
</div>
The variables inside of an object can be accessed with the '.' operator
### Object variables
However, the following syntax results in an error:
* Variables inside of an object can be accessed with the `.` operator
* However, the following syntax results in an error:
| Student class: | Program class: |
| :-: | :-: |
| class Student{ int id;} | class Program{static void Main(string[] args){ Student student = new Student(); student.id = 12345678;}} |
<divclass='columns'markdown='1'>
<divmarkdown='1'>
Student class:
```csharp
class Student
{
int id;
}
```
Error:
Let's fix the previous example by changing the _ ___access specifier__ of the 'id' variable to __public__ :
| class Student{ public int id;} | class Program{static void Main(string[] args){ Student student = new Student(); student.id = 12345678;}} |
| | |
</div>
<divmarkdown='1'>
Program class:
The value of the variable 'id' of the object 'student' is now 12345678
```csharp
class Program
{
static void Main(string[] args)
{
Student student = new Student();
student.id = 12345678;
}
}
```
# Access Specifiers
</div>
</div>
__Access specifiers__ can be used to get additional level of protection inside classes
---
Variables specified with __private __ are accessible only inside the containing class
Let's fix the previous example by changing the *__access specifier__* of the variable `id` to `public`:
Variables specified with __public __ are accessible outside of the class
The __default __ access for any variable is private!
<divclass='columns'markdown='1'>
<divmarkdown='1'>
Student class:
```csharp
class Student
{
public int id;
}
```
</div>
<divmarkdown='1'>
```csharp
class Program
{
static void Main(string[] args)
{
Student student = new Student();
student.id = 12345678;
}
}
```
int id; // Accessible only inside the class
</div>
</div>
private string name; // Accessible only inside the class
* The value of the variable `id` of the object `student` is now `12345678`
public string address; // Accessible everywhere within the namespace
## Access Specifiers
* *__Access specifiers__* can be used to get additional level of protection inside classes
* Variables specified with `private` are accessible only inside the containing class
* This is the ***default*** access!
* Variables specified with `public` are accessible outside of the class
```csharp
class Student
{
int id; // Accessible only inside the class
private string name; // Accessible only inside the class
public string address; // Accessible everywhere within the namespace
}
```
---
Continuing on the class in the previous slide, if we follow the 'student' variable with a dot, VS intellisense will only suggest the 'address' variable, because it was the only public variable of the 'Student' class
* Continuing on the class in the previous example: if we follow the `student` variable with a dot, Visual Studio IntelliSense will only suggest the `address` variable, because it was the only `public` variable of the `Student` class!
# Classes - Example
## Classes: An example
class Student
<divclass='columns12'markdown='1'>
<divmarkdown='1'>
```csharp
class Student
{
public int id;
public string name;
}
```
class Program
</div>
<divmarkdown='1'>
```csharp
class Program
{
static void Main(string[] args)
{
Student student1 = new Student();
Student student2 = new Student();
student1.id = 22225555;
student1.name = "Johannes Kantola";
student2.id = 44441111;
student2.name = "Rene Orosz";
Console.WriteLine(student1.name); // Outputs Johannes Kantola
Console.WriteLine(student1.name);
// Outputs Johannes Kantola
}
}
```
# Exercise 1
</div>
</div>
Create a console application that has a class User which contains two fields: string userName and string password
## Exercise 1
<!--_class: "exercise invert" -->
Create a main loop where the user is asked repeatedly for a new username and password. The values are stored to a new User instance and that instance is saved to a list of users
1) Create a console application that has a class `User` that contains two fields: `string userName` and `string password`
2) Create a main loop where the user is asked repeatedly for a new username and password. The values are stored in a new `User` instance and that instance is saved to a list of users
3) Print all stored usernames every time a new user is created
Print all stored usernames every time a new user is created
## Class methods
# Class Methods
* As mentioned in [Lecture 6](6-methods), we can create methods inside of our classes:
As mentioned in the previous lecture, we can create methods inside of our classes:
<divclass='columns'markdown='1'>
<divmarkdown='1'>
```csharp
class Person
{
public string firstName;
public string lastName;
public string FullName()
{
return firstName + " " + lastName;
}
}
```
class Program
</div>
<divmarkdown='1'>
```csharp
class Program
{
static void Main(string[] args)
{
Person someDude = new Person();
someDude.firstName = "Johannes";
someDude.lastName = "Kantola";
Console.WriteLine(someDude.FullName());
// Outputs Johannes Kantola
}
}
```
If the method is public, it can be called from outside of the class
</div>
</div>
# Constructors
* If the method is public, it can be called from outside of the class
Constructors are class methods which are called once the object is initialized
## Constructors
Constructors are created by creating a class method with the same name as the class:
* Constructors are class methods which are called once the object is initialized
* Constructors are created by creating a class method with the same name as the class:
class User
<divclass='columns'markdown='1'>
<divmarkdown='1'>
```csharp
class User
{
public User()
{
Console.WriteLine
("New user created!");
}
}
```
| | |
| :-: | :-: |
</div>
<divmarkdown='1'>
```csharp
class Program
{
static void Main(string[] args)
{
User user = new User();
// Outputs "New user created!"
}
}
```
❕
</div>
</div>
In Visual Studio, just write "ctor" and press tab twice to quickly create a constructor
* ***Note:*** In Visual Studio, just write `ctor` and press tab twice to quickly create a constructor!
# Constructors with Parameters
## Constructors with parameters
You can pass in parameters to the constructor at initialization:
* You can pass in parameters to the constructor at initialization:
class Car
<divclass='columns'markdown='1'>
<divmarkdown='1'>
```csharp
class Car
{
private string model;
private int year;
private int doors;
public Car(string modelName, int modelYear)
{
model = modelName;
year = modelYear;
doors = 5;
}
}
```
class Program
</div>
<divmarkdown='1'>
```csharp
class Program
{
static void Main(string[] args)
{
Car lada = new Car("Niva", 1984);
Car ford = new Car("Focus", 2010);
// Both cars have 5 doors by default
}
}
```
</div>
</div>
# Exercise 2
Create a console application with a class Animal, that contains
two strings: name and sound, and
a method Greet() that prints " __*name*__ says __*sound*__ !"
Create a few animals with different names and sounds
## Exercise 2
<!--_class: "exercise invert" -->
Call their greet methods from the main program
1) Create a console application with a class Animal, that contains
* two strings, `name` and `sound`
* a method `Greet()` that prints `<name> says <sound>!` (without the angled brackets)
2) Create a few animals with different names and sounds
3) Call their greet methods from the main program
# Properties
## Properties
When working with C#, you will eventually see __properties __ being used at some point
* When working with C#, you will eventually see [properties](https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/properties) being used at some point
* They are fields with defined `get` and `set`***accessors***
* These accessors are often referred to as a ***getter*** and a ***setter***
* Properties allow the manipulation of private fields from outside of the class:
Properties allow the manipulation of private fields from outside of the class:
<divclass='columns'markdown='1'>
<divmarkdown='1'>
```csharp
class User
{
private int id;
public int Id
{
get { return id; }
set { id = value; }
}
}
```
class Program
</div>
<divmarkdown='1'>
```csharp
class Program
{
static void Main(string[] args)
{
User admin = new User();
admin.Id = 12345678;
Console.WriteLine(admin.Id);
// This outputs 12345678
}
}
```
# Auto Properties
### Auto Properties
Auto properties are a shorthand version of the same:
* Auto properties are a shorthand version of the same:
class User
<divclass='columns'markdown='1'>
<divmarkdown='1'>
```csharp
class User
{
public int Id { get; set; }
}
```
class Program
</div>
<divmarkdown='1'>
```csharp
class Program
{
static void Main(string[] args)
{
User admin = new User();
admin.Id = 12345678;
Console.WriteLine(admin.Id);
// This outputs 12345678
}
}
```
❕
In Visual Studio, just write "prop" and press tab twice to quickly create an auto property
</div>
</div>
# Properties
* ***Note:*** In Visual Studio, just write `prop` and press tab twice to quickly create an auto property
* __Why use properties if we could just use public fields?__
* We don't always want to expose the exact same variable that is used inside of the class, to outside
* Properties allow class fields to be __read-only__ or __write-only__ :
* public int Id { get; } // This field is read-onlypublic int Password { private get; set; } // This field is write-only
* Properties are supported in __interfaces__ , while fields are not
* We will cover interfaces later
* There are a bunch of other reasons, but they are outside the scope of this course
---
### Why properties?
Lets say we have a variable in our class which is queued for calculating something important
Now some other class changes that variable -> Problem
* Why use properties if we could just use public fields?
* We don't always want to expose the exact same variable that is used inside the class to outside
* Properties allow class fields to be *__read-only__* or *__write-only__*:
```csharp
public int Id { get; } // This field is read-only
public int Password { private get; set; } // This field is write-only
```
* Also, we can add whatever functionality we want to getters and setters, which makes properties a very flexible feature
* Don't know when a property is being changed? Just print its value to the console in the setter.
* Properties are supported in *__interfaces__*, while fields are not
* We will cover interfaces later in [Lecture 9](9-interfaces#interfaces)
# Value and Reference Types
## Value and Reference Types
* If this program is executed, what will be the output?
* int, double, char and other primitive data types are of __value type__
* The variable stores the __actual data__ in memory
* See the example below. If this program is executed, what will be the output?
* `int`, `double`, `char` and other primitive data types are of the*__value type__*
* The variable stores the *__actual data__* in memory
* Every copy of the variable will be stored in a separate location
* class Program{ static void Main(string[] args) { int user = 1; int otherUser = user; user = 2; Console.WriteLine( "Value of otherUser: " + otherUser); }}
<divclass='columns21'markdown='1'>
<divmarkdown='1'>
* When this program is executed, what will be the output?
* Strings, Arrays and Classes are of __reference type __
* The variable stores the address of the data being stored in memory
* Every copy of the variable points to the same address
```csharp
class Program{
static void Main(string[] args)
{
int user = 1;
int otherUser = user;
user = 2;
Console.WriteLine("Value of otherUser: " + otherUser);
* Another example: When this program is executed, what will be the output?
* `string`, `Array` and Classes are of the *__reference type__
* The variable stores ***the address of the data*** being stored in memory
* Every copy of the variable points to the same ***address***
user.id = 1;
<divclass='columns'markdown='1'>
<divmarkdown='1'>
```csharp
class Program
{
static void Main(string[] args)
{
User user = new User();
user.id = 1;
User otherUser = user;
user.id = 2;
Console.WriteLine(
"Value of otherUser.id: " + otherUser.id);
}
}
```
class User
</div>
<divmarkdown='1'>
```csharp
class User
{
public int id;
}
```
Location in memory
</div>
</div>
---
<!-- Location in memory
int userId = 42;
@ -542,17 +596,17 @@ User user = new User(42);
User otherUser = user;
| user { id = 42 } |
| :-: |
# Exercise 3
Classes and enums can be created to a separate file. To quickly create a new class, right mouse click you project (not solution) name in solution explorer -> Add -> Class… Name your class "TestClass"
| :-: | -->
Back in your main method, create an instance of TestClass. The compiler should not give any errors.
## Exercise 3
<!--_class: "exercise invert" -->
Now rename the namespace that your TestClass is in, to "TestNamespace". Go back to your main method. The TestClass instantiation should not work anymore. Try to fix that without going back to TestClass.
1) Classes can be stored in a separate file. To quickly create a new class in Visual Studio: right mouse click your project name (not the solution!) in Solution Explorer *-> Add -> Class...*
* Name your class `TestClass`.
2) Back in your main method, create an instance of `TestClass`. The compiler should not give any errors.
3) Now rename the namespace that your `TestClass` is in to `TestNamespace`. Go back to your main method. The `TestClass` instantiation should not work anymore. Try to fix the issue without going back to `TestClass`!
# Exercise 4
<!-- ## Exercise 4
Modify previous solution by using methods and class.
Modify previous solution by using methods and class. -->
borb
2 weeks ago
