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Design Patterns in C#

-
+

Overview

  • The Singleton Pattern
    • -
  • Dependency Injection
    • +
  • 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 @@ -44,7 +45,7 @@
-
+

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
    • @@ -56,7 +57,7 @@
-
+

A singleton implementation could look something like this:

@@ -97,17 +98,126 @@
-
+

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
    • +
+
+
+
    +
  • Simple reflection example to obtain type information:
    int i = 42;
+Type type = i.GetType();
+Console.WriteLine(type);
+
    +
    • +
+
+
+
    +
  • Use strings to invoke methods:
    // 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 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.
    • +
  • Attribute is used by placing its name in square brackets [] above the declaration of the entity you want it to apply to
    [Serializable]
+public class SampleClass
+{
+    // Objects of this type can be serialized.
+}
+
    +
    • +
+
+
+
    +
  • +

    Attributes can also have parameters:

    +
    [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:

    +
    • +
+
+
+
[Conditional("DEBUG")]
+[Conditional("TEST1")]
+void TraceMethod()
+{
+    // ...
+}
+
+
+
+
[Conditional("DEBUG"), Conditional("TEST1")]
+void TraceMethod()
+{
+    // ...
+}
+
+
+
+
+
+

Custom attributes

+
    +
  • You can write your own custom attributes by inheriting from the Attribute class
    // This defaults to Inherited = true.
+public class MyAttribute : Attribute
+{
+    //...
+}
+
    +
    • +
  • Then, you can use it like any other attribute
    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
    • @@ -119,7 +229,7 @@
-
+

The solution

  • What if, instead of directly creating the objects, they were provided by some interface that takes care of the creation?
    • @@ -128,7 +238,7 @@
  • In ASP.NET, this container system is built in
-
+

Dependency injection in ASP.NET

public class HomeController : Controller
 {
@@ -142,7 +252,7 @@
 }
-
+

Design Patterns

  • If the concepts of a singleton and dependency injection flew over your head, don't worry about it
    • diff --git a/15-design-patterns-in-csharp.md b/15-design-patterns-in-csharp.md index a241372..53d573a 100644 --- a/15-design-patterns-in-csharp.md +++ b/15-design-patterns-in-csharp.md @@ -14,6 +14,7 @@ title: 15. Design Patterns in C# ## Overview * The Singleton Pattern +* Reflection and attributes * Dependency Injection ## The Singleton Pattern @@ -82,12 +83,122 @@ class Program - ### 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 + +
    +
    + +* Simple reflection example to obtain type information: + ```csharp + int i = 42; + Type type = i.GetType(); + Console.WriteLine(type); + ``` + +
    +
    + +* 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: + + +
    +
    + +```csharp +[Conditional("DEBUG")] +[Conditional("TEST1")] +void TraceMethod() +{ + // ... +} +``` + +
    +
    + +```csharp +[Conditional("DEBUG"), Conditional("TEST1")] +void TraceMethod() +{ + // ... +} +``` + +
    +
    + +### 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