C# Inheritance & Casting

Question

I get the following exception:

InvalidCastException: Unable to cast object of type 'Employee' to type 'EmployeeProfile'.

I have the following code:

    private class Employee
    {
        public string Name { get; private set; }

        public Employee()
        {
            this.Name = "employee";
        }

        public override string ToString()
        {
            return this.Name;
        }
    }

    private class EmployeeProfile : Employee
    {
        public string Profile { get; private set; }

        public EmployeeProfile() : base()
        {
            this.Profile = string.Format("{0}'s profile", this.Name);
        }

        public override string ToString()
        {
            return this.Profile;
        }
    }

    public void RunTest()
    {
        Employee emp = new Employee();
        EmployeeProfile prof = (EmployeeProfile)emp; // InvalidCastException here

        System.Console.WriteLine(emp);
        System.Console.WriteLine(prof);
    }

Maybe my brain is burned out, but I thought you can cast a subtype to its base type? What am I missing here? Maybe it is a vacation... thank you!

Solution 1

You can cast a subtype to its base type. But you are casting an instance of the base type to the subtype.

An EmployeeProfile is-an Employee. Not necessarily the other way around.

So this would work:

EmployeeProfile prof = new EmployeeProfile();
Employee emp = prof;

However, this model reeks of bad design. An employee profile is not a special kind of an employee, is it? It makes more sense for an employee to have a profile. You are after the composition pattern here.

Solution 2

All the answers are correct...just providing a no frills simple explanation...

class Employee

class Female : Employee

class Male: Employee

Just because you are an Employee does not make you a Female...

Solution 3

Maybe my brain is burned out, but I thought you can cast a subtype to its base type?

You are attempting to cast a basetype to its subtype. Exactly the opposite of what you say:

Employee emp = new Employee();
EmployeeProfile prof = emp;

Comments

[Steven Striga]

I made a small mistake in my post - please see the updated like: EmployeeProfile prof = (EmployeeProfile)emp; // InvalidCastException here

[cdhowie]

@WeekendWarrior: That doesn't matter. You still can't cast an instance of the base type to the derived type. You could cast in the other direction.

[Asongfac Roland]

What cdhowie says still stands, you are still trying to cast a basetype into a subtype.

[Steven Striga]

This is an excellent way of thinking about it... this helps a lot. thanks so much.

[Steven Striga]

Composition pattern makes sense. EmployeeProfile has a one-to-one relationship with an Employee - it just contains additional properties. thank you for the tip!

[cdhowie]

@WeekendWarrior: Sure, no problem.

[Alex]

if you had a list of Employee and Male and Female had unique properties, how would you count for example, how many Females have given birth? or how many Males have had a vasectomy? LoL trying to come up with examples following the classes mentioned. I need to know how to cast appropriately down to their type to get to the unique properties. THANKS!

[JasonInVegas]

Is is not strictly true that you "need" a method in the derived class which assigns all properties from the base class. This is a long-winded way of performing the assignment of properties whereas an object-level assignment will work for all available, matching properties.

[A.R.SEIF]

not worked for me