csharp-basics/15-design-patterns-in-csharp.md

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title: 15. Design Patterns in C#
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# Design Patterns in C#

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## Overview

* The Singleton Pattern
* Reflection and attributes
* Dependency Injection

## The Singleton Pattern

### The problem

* In most cases, it makes no sense to create an instance of a class every time its members need to be accessed
  * For example, a shared resource manager that is being called from multiple classes
* While a static class could be used for this, there are some problems:
  * As stated in [lecture 10](10-static-members-methods-and-classes#StaticClasses), static classes can only have static members
  * Static classes cannot be instantiated, so a reference to them cannot be passed around as a parameter
  * Static classes cannot inherit from other classes or implement interfaces
  * [And many more... ](https://www.c-sharpcorner.com/UploadFile/akkiraju/singleton-vs-static-classes/)

### The solution

* The  __singleton__ class is a class that benefits from all the perks of a non-static class (non-static members, inheritance, referencing…), but only one (or zero) instances of it ever exists during the lifetime of your application
* For example, reading from / writing to a file that should be accessible to multiple clients, should be made into a singleton
  * Instead of every client directly accessing the same file (and possibly causing massive performance issues), the singleton is instantiated once and a reference to it is provided to clients
  * The singleton could take care of queueing the read/write requests and be the only entity accessing the actual file

---

A singleton implementation could look something like this:

<div class='columns' markdown='1'>
<div markdown='1'>

  ```csharp
  class Singleton
  {
    private static Singleton instance = null;

    private Singleton() { }
    public void MySingletonFunction()
    {
      Console.WriteLine
        ("This function is accessible anywhere!");
    }

    public static Singleton Instance
    {
      get
      {
        if (instance == null)
          instance = new Singleton();
        return instance;
      }
    }
  }
  ```
</div>
<div markdown='1'>

```csharp
class Program
{
  static void Main(string[] args)
  {
    Singleton.Instance.MySingletonFunction();
    // Outputs: "This function is accessible
    // from everywhere!"
  }
}
```
</div>
</div>

### Implementing a singleton pattern

* The exact implementation of the singleton is out of the scope of this course, but it is important to understand that it exists and what its purpose is
* Multitude of examples for different use cases are available and can be found by googling

## Reflection

* Reflective programming or reflection is the ability for the program to examine or modify its own structure and behaviour
* C# has special reflection methods that we can use to get information about types

<div class='columns' markdown='1'>
<div markdown='1'>

* Simple reflection example to obtain type information:
  ```csharp
  int i = 42;
  Type type = i.GetType();
  Console.WriteLine(type);
  ```

</div>
<div markdown='1'>

* Use strings to invoke methods:
  ```csharp
  // Without reflection
  var foo = new Foo();
  foo.PrintHello();

  // With reflection
  Object foo = Activator.CreateInstance
    ("complete.classpath.and.Foo");
  MethodInfo method = foo
    .GetType()
    .GetMethod("PrintHello");
  method.Invoke(foo, null);
  ``` 

### Attributes

* [Attributes](https://learn.microsoft.com/en-us/dotnet/standard/attributes/
) can be used to extend methods, classes, or even entire programs with new  ***metadata***
  * Metadata is information about the types defined
* Attributes use reflection so the program can examine its own metadata
* Here's an example of the built-in [SerializableAttribute](https://learn.microsoft.com/en-us/dotnet/api/system.serializableattribute?view=net-9.0).
* Attribute is used by placing its name in square brackets `[]` above the declaration of the entity you want it to apply to
  ```csharp
  [Serializable]
  public class SampleClass
  {
      // Objects of this type can be serialized.
  }
  ```

---

* Attributes can also have parameters:
  ```csharp
  [Obsolete("Will be removed in next version.")]
  public static int Add(int a, int b)
  {
      return (a + b);
  }
  ```

* You can add multiple attributes either on separate lines or by separating them with a comma:


<div class='columns' markdown='1'>
<div markdown='1'>

```csharp
[Conditional("DEBUG")]
[Conditional("TEST1")]
void TraceMethod()
{
    // ...
}
```

</div>
<div markdown='1'>

```csharp
[Conditional("DEBUG"), Conditional("TEST1")]
void TraceMethod()
{
    // ...
}
```

</div>
</div>

### Custom attributes

* You can write your own [custom attributes](https://learn.microsoft.com/en-us/dotnet/standard/attributes/writing-custom-attributes) by inheriting from the `Attribute` class
  ```csharp
  // This defaults to Inherited = true.
  public class MyAttribute : Attribute
  {
      //...
  }
  ```
* Then, you can use it like any other attribute
  ```csharp
  public class MyClass
  {
      [MyAttribute]
      public virtual void MyMethod()
      {
          //...
      }
  }
  ```

## Dependency Injection

### The problem

* Traditionally, when new objects of classes are instantiated, the consuming class handles the creation of the objects
* Many classes change their functionality throughout the development of any project
  * This means that also  _every single_  consuming class has to change
  * This is called  *__tight coupling__*

### The solution

* What if, instead of directly creating the objects, they were provided by some interface that takes care of the creation?
* This way, even if the base class changes, the consuming classes won't care because they only know about the provider
* This provider is called  *__Container__*, and the functionality being injected is called  *__Service__*
* In ASP.NET, this container system is built in

### Dependency injection in ASP.NET

```csharp
public class HomeController : Controller
{
  private readonly IUserRepository _userRepository;

  public HomeController(IUserRepository userRepository)
  {
    _userRepository = userRepository;
  }
  // User repository including all users is now accessible in HomeController
}
```

## Design Patterns

* If the concepts of a singleton and dependency injection flew over your head, don't worry about it
* The important thing is to know they exist so that when they come up again in ASP.NET, you have already familiarized yourself with the terms
  * Thus, understanding the logic behind ASP.NET becomes less overwhelming
* There are many more design patterns, see the material  [here](https://www.c-sharpcorner.com/UploadFile/bd5be5/design-patterns-in-net/)
  ```csharp
  public void ConfigureServices(IServiceCollection services)
  {
    services.AddSingleton<IDateTime, SystemDateTime>();
    services.AddControllersWithViews();
  }
  ```