C# Accessing a subclass property

Question

I have a few classes and I am having problems accessing properties defined in subclasses from other class methods.

I have a base class called Section and a few subclasses, e.g. SectionPlane : Section. In each subclass, a different set of fields and properties are defined (in SectionPlane, private field _t and public property t can be found, whereas in SectionExtruded : Section I have private field _A and public property ´A´).

Class Section

// General section object
public abstract class Section
{
    public Section()
    {}
}

Class Plane Section

// Section object representing a plane with thickness t
public class SectionPlane : Section
{
    private double _t;

    public SectionPlane(double t)
    {
        this.t = t;
    }

    public double t
    {
        get
        {
            return _t;
        }
        set
        {
            _t = value;
        }
    }
}

Class Extruded Section

// Section object of some geometry with cross section area A extruded along the element it is assigned to.
public class SectionExtruded : Section
{
    private double _A;

    public SectionExtruded(double A)
    {
        this.A = A;
    }

    public double A
    {
        get
        {
            return _A;
        }
        set
        {
            _A = value;
        }
    }
}

The problem occurs when I from any subclass of the class Element tries to access the properties, since theese are not set in the base class Section, e.g. in the element Solid2D : Element:

Class Element

public abstract class Element
{
    private Section _section;

    public Element(Section section)
    {
        this.section = section;
    }

    public Section section
        {
            get 
            {
                return _section;
            }
            set
            {
                _section = value;
            }
        }
    }
}

Class Solid 2D Element

// Solid2D elements can only have sections of type SectionPlane
public class Solid2D : Element
{
    public Solid2D(SectionPlane section)
        : base(section)
    {
    }

    public void Calc()
    {
        double t = section.t;    // This results in error 'Section' does not contain a definition for 't' and no extension method 't' accepting a first argument of type 'Section' could be found (are you missing a using directive or an assembly reference?)
    }
}

Bar Element

// Bar elements can only have sections of type SectionExtruded
public class Solid2D : Element
{
    public Solid2D(SectionExtruded section)
        : base(section)
    {
    }

    public void Calc()
    {
        double A = section.A;    // This results in error 'Section' does not contain a definition for 'A' and no extension method 'A' accepting a first argument of type 'Section' could be found (are you missing a using directive or an assembly reference?)
    }
}

Is there any way to access my property t without having to include it in the base class Section? This would be very helpfull since not all of the sections that I will use have the same properties.

Answer 1

Since you know that it can only be a SectionPlane you can cast it

double t = ((SectionPlane)section).t;

If you are not sure whether you have a section of the right type, you can use the as keyword

double t = 0;
var sectionPane = section as SectionPlane;
if (sectionPane != null) {
    t = sectionPane.t;
}

as does not throw an exception if the section has another type, but returns null instead.

Or you can simply test

double t = 0;
if(section is SectionPlane) {
    t = ((SectionPlane)section).t;
}

but this is less elegant than using as, since you have to test the type and then cast it; but casting does this test again internally.

Using the new pattern matching introduced in C# 7.0 you can write:

double t = 0;
if(section is SectionPlane sp) {
    t = sp.t;
}

---

But if you have to perfrom such a test, the question is, whether your approach is right in the object-oriented sense. If you move the Calc-method to the abstract class Section and let each class perform its own calculation, then no type test or casting will be required.

In Section:

public abstract void Calc();

In SectionPlane

public override void Calc()
{
    var x = t;
}

...

section.Calc();  // No casting or type test required.