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Lesson 22 of 60 15 minIntermediate

Inheritance

Build new classes from existing ones: base and derived classes, the is-a rule, base constructors, protected members and sealed.

Beams, columns and braces are all structural members. They each have a mark, a material and a length, and each also has something of its own. Writing those common parts three times would be repetitive and error-prone. Inheritance lets a class reuse and extend another class.

Members.cs
public class StructuralMember                 // base class (parent){    public string Mark { get; set; } = "";    public string Material { get; set; } = "S355";    public double Length { get; set; }     public string Describe() => $"{Mark}: {Material}, {Length} m";} public class Beam : StructuralMember          // derived class (child){    public string Profile { get; set; } = "IPE200";} public class Column : StructuralMember{    public bool IsBraced { get; set; }} var b = new Beam { Mark = "B1", Length = 6, Profile = "IPE300" };Console.WriteLine(b.Describe());   // inherited methodConsole.WriteLine(b.Profile);      // its own property
Output
B1: S355, 6 m
IPE300

class Beam : StructuralMember reads “Beam inherits from StructuralMember”. A Beam automatically has everything public and protected that StructuralMember has, plus what it adds itself.

Interactive

Explore a class family

Click a class to see everything it has: what it inherits and what it adds.

Members of Beam
  • Profiledeclared here
  • SpanLoadKndeclared here
  • Markinherited from StructuralMember
  • Materialinherited from StructuralMember
  • Lengthinherited from StructuralMember
  • Describe()inherited from StructuralMember
  • ToString()inherited from object
  • Equals(object)inherited from object
  • GetHashCode()inherited from object
  • GetType()inherited from object
new Beam() runs constructors in this order
  1. object()
  2. StructuralMember()
  3. Beam()
“Is-a” check
x = new ();
Compiles: a Beam IS-A StructuralMember. Through x you can only use StructuralMember members.

The is-a rule

Use inheritance only when the child truly is a kind of the parent: a Beam is a StructuralMember. Because of that, a Beam can be stored in a StructuralMember variable:

IsA.cs
StructuralMember m = new Beam { Mark = "B2" };   // OK: a Beam is a memberConsole.WriteLine(m.Describe());// Console.WriteLine(m.Profile);    // error: through m we only see member things StructuralMember[] frame = { new Beam(), new Column(), new Beam() };   // mix them in one array

Constructors and base(...)

Constructors are not inherited. A derived constructor must make sure the base part is built first, by calling a base constructor with : base(...). The base constructor always runs before the derived one.

BaseCtor.cs
public class StructuralMember{    public string Mark { get; }    public StructuralMember(string mark)    {        Mark = mark;        Console.WriteLine($"StructuralMember({mark})");    }} public class Beam : StructuralMember{    public string Profile { get; }    public Beam(string mark, string profile) : base(mark)   // pass mark up    {        Profile = profile;        Console.WriteLine($"Beam({profile})");    }} var b = new Beam("B7", "HEA240");
Output
StructuralMember(B7)
Beam(HEA240)

protected: for children only

Protected.cs
public class StructuralMember{    protected double SelfWeightKn;     // children may use it, outside code may not} public class Beam : StructuralMember{    public double TotalLoad(double liveKn) => SelfWeightKn + liveKn;   // OK}

Rules and limits

  • A class can inherit from only one base class (single inheritance). Interfaces, coming soon, cover the “many” case.
  • Every class ultimately inherits from object, which is why everything has ToString(), Equals() and GetType().
  • Mark a class sealed to stop others from inheriting from it: public sealed class Bolt.
  • Private members of the base exist in the object but can’t be used by the derived class’s code.
Quick check

Which line compiles, given class Beam : StructuralMember and class Column : StructuralMember?

Quick check

With new Beam(...), which constructor body runs first?

Practice

Create a base class Shape with a Name property and a constructor that takes the name. Derive Circle (Radius) and Square (Side) that pass their names to the base constructor and each add a method double Area(). Create one of each and print name and area.

Show the solution
Solution.cs
public class Shape{    public string Name { get; }    public Shape(string name) => Name = name;} public class Circle : Shape{    public double Radius { get; }    public Circle(double r) : base("Circle") => Radius = r;    public double Area() => Math.PI * Radius * Radius;} public class Square : Shape{    public double Side { get; }    public Square(double s) : base("Square") => Side = s;    public double Area() => Side * Side;} var c = new Circle(1);var s = new Square(3);Console.WriteLine($"{c.Name}: {c.Area():F2}");Console.WriteLine($"{s.Name}: {s.Area()}");
Output
Circle: 3.14
Square: 9

Remember

  • class Child : Parent reuses and extends the parent’s members.
  • Inherit only for true is-a relationships; prefer composition for has-a.
  • Base constructors run first; pass arguments with : base(...).
  • Single inheritance only; every class derives from object; sealed stops inheritance.
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