C# Inheritance misconception

Question

Let's say that we have 2 classes. The first one is Person

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;

    namespace People
    {
        class Person
        {
            public string FirstName { get; set; }
            public string LastName { get; set; }
        }
    }

The second one is Teacher

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace People
{
    class Teacher:Person
    {
        public string Position { get; set; }
    }
}

And I want to make a teacher object using this syntax Person teacher = new Teacher(); When I make it and I try to refer to the Position property, I can't... Why does this happen? I can instead use this syntax Person teacher = new Teacher() {Position="boss" }; but even though, after that I can't refer to the teacher position using "teacher.Position".

The variable is of type Person, which has no Position property. Did you mean to add that property to Person? Related to this, look into something called the Liskov Substitution Principle. Meaningful type inheritance is more than just what keywords are used where. — David
– David, Commented Mar 31, 2017 at 12:31
Related: What is polymorphism, what is it for, and how is it used? — Samir Aguiar
– Samir Aguiar, Commented Mar 31, 2017 at 12:31
Before asking this question you should read something about polymorphism, as @SamirAguiar proposed. — Gh61
– Gh61, Commented Mar 31, 2017 at 12:34
@peter basically this limitation is the compiler always ensuring that you never try to access memory that you should not be able to. As you can cross reference and assign variables from multiple assemblies that do not even need to be a concrete implementation like an interface, the compiler makes sure that what ever you declare as a variable can be safely accessed. As the Person class doesn't have a Position property, the compiler will not let you access it, even if the actual object acutally has that property. — Adwaenyth
– Adwaenyth, Commented Mar 31, 2017 at 12:41

teo van kot · Accepted Answer · 2017-03-31 12:26:28Z

3

You can reach it this way:

Person teacher = new Teacher() { Position="boss" };
var position = ((Teacher)teacher).Position;

answered Mar 31, 2017 at 12:26

teo van kot

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Comments

Codor · Accepted Answer · 2017-03-31 12:32:09Z

The problem is caused by the type of the reference. In the declaration

Person teacher = new Teacher() { Position="boss" };

the right-hand side creates a new object of type Teacher and also initializes the Position property. However, the reference teacher is of type Person. While the object referred to by teacher is actually of type Teacher, the reference itself is of the more general type Person.

Informally speaking, in the above declaration you state that you want to refer teacher to a Person, regardless of whether it is of the more derived type Teacher, so the reference itself does not 'know' that it refers to the more special type Teacher and therefor does not 'know' that there is a Position property.

More technically speaking, this is necessary because at runtime it would be possible to let teacher refer to other objects deriving from Person, say Student or Wizard, which would perhaps also would not have a Position property.

Codor · Accepted Answer · 2017-03-31 12:36:19Z

1

Doing this:

Person teacher = new Teacher();

You create object of type Teacher but your reference type(teacher) is of type Person - it means that using this reference you can access only all members of type Person.

To access Teacher members you should either create reference of type Teacher:

Teacher teacher = new Teacher();
var position = teacher.Position;

or you should cast your reference to Teacher type:

Person teacher = new Teacher();
var position = ((Teacher)teacher).Position;

edited Mar 31, 2017 at 12:36

Codor

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answered Mar 31, 2017 at 12:34

Roman

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Comments

Adwaenyth · Accepted Answer · 2017-03-31 12:30:24Z

0

The reason is, the compiler simply does not know the inherited type after you assigned it to a variable explicitly declared as the base type.

The reason for that is, that while the object is still clearly an instance of Teacher the compiler can only be sure that this object is a Person at runtime, thus it will not allow you to access any properties of the Teacher class.

Declaring the variable as var will let the compiler determine the type of that variable based on what was assigned to it.

answered Mar 31, 2017 at 12:30

Adwaenyth

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1 Comment

Gh61 Over a year ago

Declaring as var or as Teacher. If he wants to access Teacher properties.

Arkadiusz Raszeja · Accepted Answer · 2017-03-31 12:35:02Z

0

read about Abstract Factory design pattern :)
That will answer your question.

answered Mar 31, 2017 at 12:35

Arkadiusz Raszeja

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