Abstract Property In Base Class To Force Programmer To Define It

Question

I'm coding with a state pattern for an embedded device. I have a base/abstract class called State and then each discrete(concrete) state class implements the abstract State Class.

In the State Class I have several Abstract Methods. If I don't implement the abstract methods in the discrete(concrete) class, Visual Studio will give an error something like this:

...Error 1 'myConcreteState' does not implement inherited abstract member 'myAbstractState'

Now: I'm trying to create a String property for each State called StateName. Whenever I create a new concrete class, I need to define StateName. I want VS to throw an error if I don't use it. Is there a simple way to do this?

I've tried this in the abstract/base class:

public abstract string StateName { get; set; }

But I don't need to implement the Get and Set methods in each State.

Revised Question: In an ideal situation, each State Class would be required to have StateName defines and be inherited from the abstract base class.

StateName = "MyState1"; //or whatever the state's name is

If that statement is missing then Visual Studio will generate an error as described above. Is this possible and if so, how?

Answer 1

I guess the "correct" way to do this is to have a protected constructor on the base class which requires the state name as a parameter.

public abstract class State
{
    private readonly string _name;

    protected State(string name)
    {
        if(String.IsNullOrEmpty(name))
            throw new ArgumentException("Must not be empty", "name");

        _name = name;
    }

    public string Name { get { return _name; } }
}

The concrete states then provide a public constructor which implicitly calls the base class constructor with the appropriate name.

public abstract class SomeState : State
{
    public SomeState() : base("The name of this state")
    {
        // ...
    }
}

Since the base class does not expose any other constructors (neither protected nor public) each inheriting class needs to go through this single constructor and thus needs to define a name.

Note that you don't need to provide the name when you instantiate a concrete state because its constructor takes care of that:

var someState = new SomeState(); // No need to define the name here
var name = someState.Name; // returns "The name of this state"

Answer 2

As of C# 6 (I believe - C# : The New and Improved C# 6.0) you are able to create getter only properties. So you could declare your base class like so -

public abstract class State
{
    public abstract string name { get; }

    // Your other functions....
}

And then in your sub-class you can implement State like so -

public class SomeState : State
{
    public override string name { get { return "State_1"; } }
}

Or even neater using the lambda operator -

public class SomeState : State
{
    public override string name => "State_1";
}

Both would always return "State_1" and be immutable.

Answer 3

Your requirements are a contradiction:

I'm trying to create a String property for each State called StateName.

vs.

But I don't need to implement all of that in each State.

There's no language feature that allows you to force the existence of a member in only a few sub classes. After all, the point of using a super class is to rely on the fact that all subclasses will have all the members of the super class (and possibly more). If you want to create classes that act as a State but dont have a name, then by definition, they should (/can) not be subclasses of State.

You either have to change your requirements or use something other than simple inheritance.

A possible solution with the code as it is could be to make the method in State non abstract and return an empty string.

Answer 4

You can't do this with an abstract class, but you can do it with an interface.

public interface IState
{
    public string Name { get; }
}

// This won't compile: 'SomeState' does not implement interface member 'IState.Name'
public class SomeState : IState
{
}

// This will compile
public class SomeState : IState
{
    public string Name => "SomeState's Name";
}

See here for more: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/interface-properties