sehlstrom sehlstrom - 1 year ago 53
C# Question

C# Accessing a subclass property

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 Source

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 not type test or casting will be required.

In Section:

public abstract void Calc();

In SectionPlane

public override void Calc()
{
    var x = t; // No casting or type test required.
}
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