3.2.3 Virtual Function Members

chap03.md

@@ -445,6 +445,34 @@ TODO: Omit remaining content
 
 ### 3.2.3 Virtual Function Members
 
+* A *function* marked as `virtual` can be **overriden** by subclasses for more specific implementation
+* Who be declared `virtual`? ANS: Methods, properties, indexers, events
+
+```csharp
+// Define a virtual method to be overridden
+public class Asset
+{
+    public string Name;
+    public virtual decimal Liability => 0; // Expression-bodied properties
+}
+
+// subclass override
+public class House : Asset
+{
+    public decimal Mortgage;
+    public override decimal Liability => Mortgage;
+}
+
+// Initialization of overridden field
+House mansion = new House { Name = "McMansion", Mortgage = 250000 };
+Asset a = mansion;
+Console.WriteLine(a.Liability); // 250000
+```
+
+#### Covariant return types (C# 9)
+
+TODO: Omit
+
 ### 3.2.4 Abstract Classes and Abstract Members
 
 ### 3.2.5 Hiding Inherited Members