Add item for page 200

docs/errata/README.md

@@ -4,6 +4,6 @@ If you find any mistakes in the seventh edition, *C# 11 and .NET 7 - Modern Cros
 
 [**Errata** (34 items)](errata.md): Typos, tool user interface changes, or mistakes in code that would cause a compilation error that prevents a successful build.
 
-[**Improvements** (19 items)](improvements.md): Changes to text or code that would improve the content. These are optional.
+[**Improvements** (20 items)](improvements.md): Changes to text or code that would improve the content. These are optional.
 
 All errata and improvements will be included in the 8th edition planned for publishing in November 2023. To be included they must be submitted by mid-September 2023.

docs/errata/improvements.md

@@ -1,4 +1,4 @@
-**Improvements** (19 items)
+**Improvements** (20 items)
 
 If you have suggestions for improvements, then please [raise an issue in this repository](https://github.com/markjprice/cs11dotnet7/issues) or email me at markjprice (at) gmail.com.
 
@@ -9,6 +9,7 @@ If you have suggestions for improvements, then please [raise an issue in this re
 - [Page 153 - Writing a function that returns a value](#page-153---writing-a-function-that-returns-a-value)
 - [Page 161 - Using lambdas in function implementations](#page-161---using-lambdas-in-function-implementations)
 - [Page 179 - Reviewing project packages](#page-179---reviewing-project-packages)
+- [Page 200 - Talking about OOP](#page-200---talking-about-oop)
 - [Page 237 - Implementing functionality using methods](#page-237---implementing-functionality-using-methods)
 - [Page 241 - Defining flight passengers](#page-241---defining-flight-passengers)
 - [Page 251 - Setting up a class library and console application](#page-251---setting-up-a-class-library-and-console-application)
@@ -161,6 +162,43 @@ In Step 4, I tell Visual Studio 2022 readers to select the `appsettings.json` fi
 </ItemGroup>
 ```
 
+# Page 200 - Talking about OOP
+
+In the next edition, I will add a summary table of the different categories of type and their capabilities, as shown in the following table:
+
+|Type|Can be instantiated with `new`|Can be inherited from with `:`|Multiple inheritance|
+|---|---|---|---|
+|`class`|Yes|Yes|No|
+|`sealed class`|Yes|No|No|
+|`abstract class`|No|Yes|No|
+|`interface`|No|Yes|Yes|
+|`struct`|Yes|No|No|
+
+```cs
+// To simplify the examples, I have left out access modifiers.
+
+interface Alpha { void M1(); }
+interface Beta { void M2(); }
+
+// A type (in this example a class) implementing an interface.
+// ": Alpha" means I promise to implement all members of Alpha.
+// "void M1() { }" is that minimum implementation.
+class Gamma : Alpha { void M1() { } } 
+
+// A type (in this example a struct) implementing two interfaces.
+struct Delta : Alpha, Beta { void M1() { } void M2() { } } 
+
+// A sub class inheriting from a base aka super class.
+// : Delta means inherit all members of that base class.
+class Episilon : Delta { } 
+```
+
+For me, the terms *inherit* and *implement* are different, and in the early days of C# and .NET you could strictly apply them to classes and interfaces respectfully. *Inherit* implies some functionality that a sub class gets "for free" from its base aka super class. *Implement* implies some functionality that is NOT inherited but instead MUST be provided by the sub class. 
+
+Before C# 8, interfaces were always purely contracts i.e. there was no functionality that you could *inherit*. In those days, you could strictly use the term *implement* for an interface that represents a list of members that your type must implement, and *inherit* for classes with functionality that your type can inherit and potentially override. 
+
+With C# 8, interfaces can now include default implementations, making them more like abstract classes, and the term *inherit* for an interface that has default implementations does make sense. But I feel uncomfortable with this capability as do many other .NET developers because it messes up what used to be a clean language design.
+
 # Page 237 - Implementing functionality using methods
 
 > Thanks to [Masoud Nazari](https://github.com/MAS-OUD) for raising this [issue on 5 March 2023](https://github.com/markjprice/cs11dotnet7/issues/35).