Finished Defining readonly Members; Finished Defining const Members

csharp_fundamentals/7_building_types.md

@@ -48,4 +48,19 @@ Reference:
 
 Instead of implement property, it can be implemented automatically if it's standard getter & (or private) setter (as read-only object). Besides, we can declare setter to be private
 
-Overall, property provide more concise and convenient way to define than field.
+Overall, property provide more concise and convenient way to define than field.
+
+## Defining readonly & const Members
+
+References:
+* [readonly (C# Reference)](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/readonly)
+* [Constants (C# Programming Guide)](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/constants )
+
+Some class members are decalred to be readonly. There are multiple methods can achieve this objective:
+* `readonly` keyword can be used to create readonly field, that can only be initialized during declaration and overwritten in constructor.
+* `const` is more strict, it can only be initialized in during declaration, and cannot be overwritten in any place. (It must be defined before compilation). 
+  * Constant fileds are NOT variable. 
+  * Local variable within block can be constant.
+  * `const` value can be assigned with `public` access modifier, as it cannot be modified
+  * Normally `const` values are CAPITAL.
+  * `const` value is treaded as `static` (i.e. belong to class, instead of object as it cannot be changed)