From e532d0b5992260431073b8fa6d39ddfa22f0417e Mon Sep 17 00:00:00 2001
From: Jason Zhu <jasonzhuyq@outlook.com>
Date: Tue, 23 Feb 2021 10:19:58 +1100
Subject: [PATCH] Finished Defining an Abstract Class, repo at f6215b7

---
 csharp_fundamentals/8_oop.md | 88 +++++++++++++++++++++++++++++++++++-
 1 file changed, 87 insertions(+), 1 deletion(-)

diff --git a/csharp_fundamentals/8_oop.md b/csharp_fundamentals/8_oop.md
index 3e0a2bc..35d9c2a 100644
--- a/csharp_fundamentals/8_oop.md
+++ b/csharp_fundamentals/8_oop.md
@@ -79,4 +79,90 @@ Reference:
 
 Abstract class is prerequist of polymorphism
 
-TODO: Understand polymorphism more deeply.
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+TODO: Understand polymorphism more deeply.
+
+## Defining an Interface
+
+Reference:
+* [Very Useful: Interface in C# (dotnettutorial)](https://dotnettutorials.net/lesson/interface-c-sharp/)
+* [Very Useful: Multiple Inheritance in C#](https://dotnettutorials.net/lesson/multiple-inheritance-csharp/)
+* [Interfaces (C# Programming Guide)](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/interfaces/)
+
+.NET provides an alternative approach known as the interface to support the concept of **multiple inheritances**, (as .NET does not support by nature)
+
+Interface force the class signatures, while ignoring implementation in details.
+
+### Declare interface
+
+Using `interface` to declare an interface.
+
+```csharp
+// SYNTAX:
+public interface InterfaceName
+{
+    //only abstract members:
+    // Abstract methods
+    // Properties
+    // Indexes
+    // Events
+}
+```
+
+e.g.
+```csharp
+public interface Example
+{
+    void show();
+}
+```
+
+### Working with interface
+
+Interface provides a forceful rule for classes to implementation. Hence, multi-inheritance is enabled
+
+```csharp
+namespace InterfaceDemo
+{
+    public interface Area
+    {
+        void area(double a, double b);
+    }
+    class Rectangle : Area
+    {
+        public void area(double a, double b)
+        {
+            double areaRectangle;
+            areaRectangle = a * b;
+            Console.WriteLine("the area of rectangle is :" + areaRectangle);
+        }
+    }
+    class Circle : Area
+    {
+        static double PI = 3.14;
+        public void area(double a, double b)
+        {
+            double areaCircle;
+            areaCircle = PI * a * a;
+            Console.WriteLine("the area of Circle is :" + areaCircle);
+        }
+    }
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            Area a = new Rectangle();
+            a.area(5, 6);
+            a = new Circle();
+            a.area(7, 0);
+            Console.WriteLine("Press any key to exist.");
+            Console.ReadKey();
+        }
+    }
+}
+```
+
+### Tips
+
+* Naming Convention: Interface is begin by `I`. e.g. `IBook`
+* Interfaces are far more commen than abstract class
+* When class inherit from interface, methods from interface must be implemented, even if it's abstract class, and implemented methods are virtual
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