How do I get all the values from my extended enum class?

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I decided one of the enums in my project might benefit from being more intelligent and followed the Microsoft example on the following page:

Microsoft Extended Enum

This is a cut-down version of my code:

public abstract class Enumeration : IComparable
{
    protected Enumeration() {}

    protected Enumeration( int id, string name )
    {
        Id = id;
        Name = name;
    }

    public int Id { get; }

    public string Name { get; }

    public int CompareTo( object other ) => Id.CompareTo( ((Enumeration)other).Id );

    public override string ToString() => Name;

    public static IEnumerable<T> GetAll<T>() where T : Enumeration, new()
    {
        Type type = typeof( T );
        FieldInfo[] fields = type.GetTypeInfo().GetFields( BindingFlags.Public | BindingFlags.Static | BindingFlags.DeclaredOnly );
        foreach( var info in fields )
        {
            var instance = new T();
            var locatedValue = info.GetValue( instance ) as T;
            if( locatedValue != null )
                yield return locatedValue;
        }
    }
    
    public override bool Equals(object obj)
    {
        if( !( obj is Enumeration otherValue ) ) return false;
        var typeMatches = GetType().Equals(obj.GetType());
        var valueMatches = Id.Equals(otherValue.Id);
        return typeMatches && valueMatches;
    }

    public override int GetHashCode()
    {
        return Id.GetHashCode();
    }
}


public class Fruit : Enumeration
{
    public static Fruit Apple = new AppleFruitType();
    public static Fruit Banana = new BananaFruitType();
    public static Fruit Orange = new OrangeFruitType();

    protected Fruit( int id, string name ) : base( id, name ) { }

    private class AppleFruitType : Fruit
    {
        public AppleFruitType() : base( 0, "Apple" ) {}
    }

    private class BananaFruitType : Fruit
    {
        public BananaFruitType() : base( 1, "Banana" ) {}
    }

    private class OrangeFruitType : Fruit
    {
        public OrangeFruitType() : base( 2, "Orange" ) {}
    }
}


public class MainApplication
{
    public void SomeCode()
    {
        // None of these three lines will compile.
        List<Fruit> AllFruits1 = Fruit.GetAll();
        List<Fruit> AllFruits2 = Fruit.GetAll<Fruit>();
        List<Fruit> AllFruits3 = Fruit.GetAll<Enumeration>();
    	List<Fruit> AllFruits4 = Fruit.GetAll().ToList<Fruit>;
    	List<Fruit> AllFruits5 = Fruit.GetAll<Fruit>().ToList<Fruit>;
    	List<Fruit> AllFruits6 = Fruit.GetAll<Enumeration>().ToList<Fruit>;
    }
}

Although the compiler information on why these six lines don't work seems to be clear, I can't figure out what I need to write to get this working.

Can anyone please explain this obvious glaring hole in my language knowledge?

What I have tried:

The examples shown in the above code.

Posted 21-Aug-18 22:29pm

Patrick Skelton

Updated 21-Aug-18 23:26pm

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2 solutions

Solution 2

To get a specific type e.g. Fruit, you can do this:

C#

IEnumerable<Fruit> AllFruits1 = Enumeration.GetAll<Fruit>();

If you have another extended enum type like CardType:

C#

public abstract class CardType : Enumeration
...


IEnumerable<CardType> AllCardType1 = Enumeration.GetAll<CardType>();
IEnumerable<Fruit> AllFruits1 = Enumeration.GetAll<Fruit>();

Basically the purpose of Enumeration class is that you can easily get your all enum types

Posted 21-Aug-18 23:27pm

Keviniano Gayo

Comments

Patrick Skelton 22-Aug-18 5:35am

Thanks for the explanation. You did state very clearly something I hadn't quite realised. I chose a different answer as solution because your reply didn't mention the public parameterless constructor that appears to be required. Thank you.

Keviniano Gayo 22-Aug-18 5:56am

No problem. Public parameter-less by the way is not required, unless you are going to instantiate a class without parameters. Although the compiler will add the default constructor(parameter-less) in the background if you didn't explicitly declare it, if no constructor override in your class declaration.

Patrick Skelton 22-Aug-18 6:02am

Mmm... I can't get it to work without explicitly adding a public parameter-less constructor. Doesn't the declaration of the protected constructor with parameters remove the default one?

Keviniano Gayo 22-Aug-18 6:50am

Yes it removed the default parameter-less constructor

Patrick Skelton 22-Aug-18 6:57am

So don't I need to put it back to get a list of all the enumeration values or is there a better way?

Keviniano Gayo 22-Aug-18 7:18am

The way the Enumeration class is designed, you don't need to add the default constructor. You don't have to call Fruit method directly e.g. GetAll because it is used as a type (to identify a type of enum).
This code in your example: List<fruit> AllFruits2 = Fruit.GetAll<fruit>(); doesn't make any sense because you will always get a Fruit type since you are "getting" all types from the "Fruit" itself.
This is how you call to get specific Enumerations type:
IEnumerable<cardtype> AllCardType1 = Enumeration.GetAll<cardtype>();
IEnumerable<fruit> AllFruits1 = Enumeration.GetAll<fruit>();

Or enum checking:
Enumeration cardCurrent = CardType.Amex;

if (cardCurrent is CardType)
{
int isEqual = cardCurrent.CompareTo(CardType.MasterCard);
bool ismatch = cardCurrent == CardType.Visa;
}

Patrick Skelton 23-Aug-18 5:36am

Thanks for the explanation. Isn't this hopelessly inefficient? If I don't cache the result of the IEnumerable in a List somewhere, what will happen if I data bind a UI element to the enumeration? Won't the IEnumerable (which involves reflection) be executed every time the UI element is displayed? Maybe I am being paranoid. I have worked in video games for a lot of years and things like reflection being called repeatedly make me nervous.

Keviniano Gayo 23-Aug-18 10:39am

that kind of low level stuff cautious to memory is somewhat i'm not good at. But i know that using IEnumerable as a variable or to pass around is efficient compared to List because IEnumerable has fewer methods and the base class of List. Displaying UI in LOB application is not as intensive as video games. IMO, in LOB apps usability and intuitive UI is important then you can later improve its performance..

Patrick Skelton 23-Aug-18 12:15pm

So, I think the short answer is, yes, I am being paranoid! :)

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willifritz · Accepted Answer · 2018-08-21T22:55:00

Solution 1

Fruit must have a parameterless constructor.
Add

C#

public Fruit()
{
}

to your Fruit class and

C#

var AllFruits2 = Fruit.GetAll<Fruit>();

should work.

Posted 21-Aug-18 22:55pm

willifritz

Comments

Patrick Skelton 22-Aug-18 5:32am

That is indeed all that was needed. the compiler told me as such but I didn't add one because I foolishly thought, "If it needed a public parameterless constructor, surely Microsoft's example code would have one."

Now all of the following compile fine:

var AllFruits1 = Fruit.GetAll<fruit>();
IEnumerable<fruit> AllFruits2 = Fruit.GetAll<fruit>();
List<fruit> AllFruits3 = Fruit.GetAll<fruit>().ToList();

Thank you!