diff --git a/csharp_fundamentals/8_oop.md b/csharp_fundamentals/8_oop.md
index 35d9c2a..8ea1397 100644
--- a/csharp_fundamentals/8_oop.md
+++ b/csharp_fundamentals/8_oop.md
@@ -165,4 +165,13 @@ namespace InterfaceDemo
 
 * 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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+* When class inherit from interface, methods from interface must be implemented, even if it's abstract class, and implemented methods are virtual
+
+## Writing Grades to a File & Using IDisposable
+
+Reference:
+* [Implement a Dispose method (.NET fundamentals)](https://docs.microsoft.com/en-us/dotnet/standard/garbage-collection/implementing-dispose)
+* [Usage and Importance of Using in C# -> Using Statment (for garbage collection)](https://www.c-sharpcorner.com/UploadFile/manas1/usage-and-importance-of-using-in-C-Sharp472/)
+
+* .NET has garbage collector to recycle memory. We also has some resource that want to release once used as soon as possible. .NET provide `IDisposable` interface to notify and automatically recycle resource.
+* `using` statement 
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