Sealing

Classes should have two possibilities: extendable and non-instatiatable OR non-extendable and instantiatable.
Classes MUST ALWAYS have either abstract or sealed as attributes for the class.

This helps drive good object oriented practices and avoids a lot of complications that appear when classes unintended for extension start to get extended.

AddTwoInts/AddTwoIntsTests.cs

@@ -28,7 +28,7 @@ public abstract class Integer
         protected abstract int Value();
     }
 
-    public class IntegerOf : Integer
+    public sealed class IntegerOf : Integer
     {
         private readonly int _origin;
 
@@ -37,7 +37,7 @@ public class IntegerOf : Integer
         protected override int Value() => _origin;
     }
 
-    public class Sum : ISum
+    public sealed class Sum : ISum
     {
         private readonly int _augend;
         private readonly int _addend;