Enum “Inheritance”

Question

I have an enum in a low level namespace. I'd like to provide a class or enum in a mid level namespace that "inherits" the low level enum.

namespace low
{
   public enum base
   {
      x, y, z
   }
}

namespace mid
{
   public enum consume : low.base
   {
   }
}

I'm hoping that this is possible, or perhaps some kind of class that can take the place of the enum consume which will provide a layer of abstraction for the enum, but still let an instance of that class access the enum.

Thoughts?

EDIT: One of the reasons I haven't just switched this to consts in classes is that the low level enum is needed by a service that I must consume. I have been given the WSDLs and the XSDs, which define the structure as an enum. The service cannot be changed.

Answer 1

This is not possible. Enums cannot inherit from other enums. In fact all enums must actually inherit from System.Enum. C# allows syntax to change the underlying representation of the enum values which looks like inheritance, but in actuality they still inherit from System.enum.

See section 8.5.2 of the CLI spec for the full details. Relevant information from the spec

• All enums must derive from System.Enum
• Because of the above, all enums are value types and hence sealed

Answer 2

You can achieve what you want with classes:

public class Base
{
    public const int A = 1;
    public const int B = 2;
    public const int C = 3;
}
public class Consume : Base
{
    public const int D = 4;
    public const int E = 5;
}

Now you can use these classes similar as when they were enums:

int i = Consume.B;

Update (after your update of the question):

If you assign the same int values to the constants as defined in the existing enum, then you can cast between the enum and the constants, e.g:

public enum SomeEnum // this is the existing enum (from WSDL)
{
    A = 1,
    B = 2,
    ...
}
public class Base
{
    public const int A = (int)SomeEnum.A;
    //...
}
public class Consume : Base
{
    public const int D = 4;
    public const int E = 5;
}

// where you have to use the enum, use a cast:
SomeEnum e = (SomeEnum)Consume.B;

Answer 3

The short answer is no. You can play a bit, if you want:

You can always do something like this:

private enum Base
{
    A,
    B,
    C
}

private enum Consume
{
    A = Base.A,
    B = Base.B,
    C = Base.C,
    D,
    E
}

But, it doesn't work all that great because Base.A != Consume.A

You can always do something like this, though:

public static class Extensions
{
    public static T As<T>(this Consume c) where T : struct
    {
        return (T)System.Enum.Parse(typeof(T), c.ToString(), false);
    }
}

In order to cross between Base and Consume...

You could also cast the values of the enums as ints, and compare them as ints instead of enum, but that kind of sucks too.

The extension method return should type cast it type T.

Answer 4

The solutions above using classes with int constants lack type-safety. I.e. you could invent new values actually not defined in the class. Furthermore it is not possible for example to write a method taking one of these classes as input.

You would need to write

public void DoSomethingMeaningFull(int consumeValue) ...

However, there is a class based solution of the old days of Java, when there were no enums available. This provides an almost enum-like behaviour. The only caveat is that these constants cannot be used within a switch-statement.

public class MyBaseEnum
{
    public static readonly MyBaseEnum A = new MyBaseEnum( 1 );
    public static readonly MyBaseEnum B = new MyBaseEnum( 2 );
    public static readonly MyBaseEnum C = new MyBaseEnum( 3 );

    public int InternalValue { get; protected set; }

    protected MyBaseEnum( int internalValue )
    {
        this.InternalValue = internalValue;
    }
}

public class MyEnum : MyBaseEnum
{
    public static readonly MyEnum D = new MyEnum( 4 );
    public static readonly MyEnum E = new MyEnum( 5 );

    protected MyEnum( int internalValue ) : base( internalValue )
    {
        // Nothing
    }
}

[TestMethod]
public void EnumTest()
{
    this.DoSomethingMeaningful( MyEnum.A );
}

private void DoSomethingMeaningful( MyBaseEnum enumValue )
{
    // ...
    if( enumValue == MyEnum.A ) { /* ... */ }
    else if (enumValue == MyEnum.B) { /* ... */ }
    // ...
}

Answer 5

Ignoring the fact that base is a reserved word you cannot do inheritance of enum.

The best thing you could do is something like that:

public enum Baseenum
{
   x, y, z
}

public enum Consume
{
   x = Baseenum.x,
   y = Baseenum.y,
   z = Baseenum.z
}

public void Test()
{
   Baseenum a = Baseenum.x;
   Consume newA = (Consume) a;

   if ((Int32) a == (Int32) newA)
   {
   MessageBox.Show(newA.ToString());
   }
}

Since they're all the same base type (ie: int) you could assign the value from an instance of one type to the other which a cast. Not ideal but it work.

Answer 6

I know this answer is kind of late but this is what I ended up doing:

public class BaseAnimal : IEquatable<BaseAnimal>
{
    public string Name { private set; get; }
    public int Value { private set; get; }

    public BaseAnimal(int value, String name)
    {
        this.Name = name;
        this.Value = value;
    }

    public override String ToString()
    {
        return Name;
    }

    public bool Equals(BaseAnimal other)
    {
        return other.Name == this.Name && other.Value == this.Value;
    }
}

public class AnimalType : BaseAnimal
{
    public static readonly BaseAnimal Invertebrate = new BaseAnimal(1, "Invertebrate");

    public static readonly BaseAnimal Amphibians = new BaseAnimal(2, "Amphibians");

    // etc        
}

public class DogType : AnimalType
{
    public static readonly BaseAnimal Golden_Retriever = new BaseAnimal(3, "Golden_Retriever");

    public static readonly BaseAnimal Great_Dane = new BaseAnimal(4, "Great_Dane");

    // etc        
}

Then I am able to do things like:

public void SomeMethod()
{
    var a = AnimalType.Amphibians;
    var b = AnimalType.Amphibians;

    if (a == b)
    {
        // should be equal
    }

    // call method as
    Foo(a);

    // using ifs
    if (a == AnimalType.Amphibians)
    {
    }
    else if (a == AnimalType.Invertebrate)
    {
    }
    else if (a == DogType.Golden_Retriever)
    {
    }
    // etc          
}

public void Foo(BaseAnimal typeOfAnimal)
{
}

Answer 7

Alternative solution

In my company, we avoid "jumping over projects" to get to non-common lower level projects. For instance, our presentation/API layer can only reference our domain layer, and the domain layer can only reference the data layer.

However, this is a problem when there are enums that need to be referenced by both the presentation and the domain layers.

Here is the solution that we have implemented (so far). It is a pretty good solution and works well for us. The other answers were hitting all around this.

The basic premise is that enums cannot be inherited - but classes can. So...

// In the lower level project (or DLL)...
public abstract class BaseEnums
{
    public enum ImportanceType
    {
        None = 0,
        Success = 1,
        Warning = 2,
        Information = 3,
        Exclamation = 4
    }

    [Flags]
    public enum StatusType : Int32
    {
        None = 0,
        Pending = 1,
        Approved = 2,
        Canceled = 4,
        Accepted = (8 | Approved),
        Rejected = 16,
        Shipped = (32 | Accepted),
        Reconciled = (64 | Shipped)
    }

    public enum Conveyance
    {
        None = 0,
        Feet = 1,
        Automobile = 2,
        Bicycle = 3,
        Motorcycle = 4,
        TukTuk = 5,
        Horse = 6,
        Yak = 7,
        Segue = 8
    }

Then, to "inherit" the enums in another higher level project...

// Class in another project
public sealed class SubEnums: BaseEnums
{
   private SubEnums()
   {}
}

This has three real advantages...

1. The enum definitions are automatically the same in both projects - by definition.
2. Any changes to the enum definitions are automatically echoed in the second without having to make any modifications to the second class.
3. The enums are based on the same code - so the values can easily be compared (with some caveats).

To reference the enums in the first project, you can use the prefix of the class: BaseEnums.StatusType.Pending or add a "using static BaseEnums;" statement to your usings.

In the second project when dealing with the inherited class however, I could not get the "using static ..." approach to work, so all references to the "inherited enums" would be prefixed with the class, e.g. SubEnums.StatusType.Pending. If anyone comes up with a way to allow the "using static" approach to be used in the second project, let me know.

I am sure that this can be tweaked to make it even better - but this actually works and I have used this approach in working projects.

Answer 8

This is what I did. What I've done differently is use the same name and the new keyword on the "consuming" enum. Since the name of the enum is the same, you can just mindlessly use it and it will be right. Plus you get intellisense. You just have to manually take care when setting it up that the values are copied over from the base and keep them sync'ed. You can help that along with code comments. This is another reason why in the database when storing enum values I always store the string, not the value. Because if you are using automatically assigned increasing integer values those can change over time.

// Base Class for balls 
public class BaseBall
{
    // keep synced with subclasses!
    public enum Sizes
    {
        Small,
        Medium,
        Large
    }
}

public class VolleyBall : BaseBall
{
    // keep synced with base class!
    public new enum Sizes
    {
        Small = BaseBall.Sizes.Small,
        Medium = BaseBall.Sizes.Medium,
        Large = BaseBall.Sizes.Large,
        SmallMedium,
        MediumLarge,
        Ginormous
    }
}

Answer 9

I also wanted to overload Enums and created a mix of the answer of 'Seven' on this page and the answer of 'Merlyn Morgan-Graham' on a duplicate post of this, plus a couple of improvements.
Main advantages of my solution over the others:

• automatic increment of the underlying int value
• automatic naming

This is an out-of-the-box solution and may be directly inserted into your project. It is designed to my needs, so if you don't like some parts of it, just replace them with your own code.

First, there is the base class CEnum that all custom enums should inherit from. It has the basic functionality, similar to the .net Enum type:

public class CEnum
{
  protected static readonly int msc_iUpdateNames  = int.MinValue;
  protected static int          ms_iAutoValue     = -1;
  protected static List<int>    ms_listiValue     = new List<int>();

  public int Value
  {
    get;
    protected set;
  }

  public string Name
  {
    get;
    protected set;
  }

  protected CEnum ()
  {
    CommonConstructor (-1);
  }

  protected CEnum (int i_iValue)
  {
    CommonConstructor (i_iValue);
  }

  public static string[] GetNames (IList<CEnum> i_listoValue)
  {
    if (i_listoValue == null)
      return null;
    string[] asName = new string[i_listoValue.Count];
    for (int ixCnt = 0; ixCnt < asName.Length; ixCnt++)
      asName[ixCnt] = i_listoValue[ixCnt]?.Name;
    return asName;
  }

  public static CEnum[] GetValues ()
  {
    return new CEnum[0];
  }

  protected virtual void CommonConstructor (int i_iValue)
  {
    if (i_iValue == msc_iUpdateNames)
    {
      UpdateNames (this.GetType ());
      return;
    }
    else if (i_iValue > ms_iAutoValue)
      ms_iAutoValue = i_iValue;
    else
      i_iValue = ++ms_iAutoValue;

    if (ms_listiValue.Contains (i_iValue))
      throw new ArgumentException ("duplicate value " + i_iValue.ToString ());
    Value = i_iValue;
    ms_listiValue.Add (i_iValue);
  }

  private static void UpdateNames (Type i_oType)
  {
    if (i_oType == null)
      return;
    FieldInfo[] aoFieldInfo = i_oType.GetFields (BindingFlags.Public | BindingFlags.Static);

    foreach (FieldInfo oFieldInfo in aoFieldInfo)
    {
      CEnum oEnumResult = oFieldInfo.GetValue (null) as CEnum;
      if (oEnumResult == null)
        continue;
      oEnumResult.Name = oFieldInfo.Name;
    }
  }
}

Secondly, here are 2 derived Enum classes. All derived classes need some basic methods in order to work as expected. It's always the same boilerplate code; I haven't found a way yet to outsource it to the base class. The code of the first level of inheritance differs slightly from all subsequent levels.

public class CEnumResult : CEnum
{
  private   static List<CEnumResult>  ms_listoValue = new List<CEnumResult>();

  public    static readonly CEnumResult Nothing         = new CEnumResult (  0);
  public    static readonly CEnumResult SUCCESS         = new CEnumResult (  1);
  public    static readonly CEnumResult UserAbort       = new CEnumResult ( 11);
  public    static readonly CEnumResult InProgress      = new CEnumResult (101);
  public    static readonly CEnumResult Pausing         = new CEnumResult (201);
  private   static readonly CEnumResult Dummy           = new CEnumResult (msc_iUpdateNames);

  protected CEnumResult () : base ()
  {
  }

  protected CEnumResult (int i_iValue) : base (i_iValue)
  {
  }

  protected override void CommonConstructor (int i_iValue)
  {
    base.CommonConstructor (i_iValue);

    if (i_iValue == msc_iUpdateNames)
      return;
    if (this.GetType () == System.Reflection.MethodBase.GetCurrentMethod ().DeclaringType)
      ms_listoValue.Add (this);
  }

  public static new CEnumResult[] GetValues ()
  {
    List<CEnumResult> listoValue = new List<CEnumResult> ();
    listoValue.AddRange (ms_listoValue);
    return listoValue.ToArray ();
  }
}

public class CEnumResultClassCommon : CEnumResult
{
  private   static List<CEnumResultClassCommon> ms_listoValue = new List<CEnumResultClassCommon>();

  public    static readonly CEnumResult Error_InternalProgramming           = new CEnumResultClassCommon (1000);

  public    static readonly CEnumResult Error_Initialization                = new CEnumResultClassCommon ();
  public    static readonly CEnumResult Error_ObjectNotInitialized          = new CEnumResultClassCommon ();
  public    static readonly CEnumResult Error_DLLMissing                    = new CEnumResultClassCommon ();
  // ... many more
  private   static readonly CEnumResult Dummy                               = new CEnumResultClassCommon (msc_iUpdateNames);

  protected CEnumResultClassCommon () : base ()
  {
  }

  protected CEnumResultClassCommon (int i_iValue) : base (i_iValue)
  {
  }

  protected override void CommonConstructor (int i_iValue)
  {
    base.CommonConstructor (i_iValue);

    if (i_iValue == msc_iUpdateNames)
      return;
    if (this.GetType () == System.Reflection.MethodBase.GetCurrentMethod ().DeclaringType)
      ms_listoValue.Add (this);
  }

  public static new CEnumResult[] GetValues ()
  {
    List<CEnumResult> listoValue = new List<CEnumResult> (CEnumResult.GetValues ());
    listoValue.AddRange (ms_listoValue);
    return listoValue.ToArray ();
  }
}

The classes have been successfully tested with follwing code:

private static void Main (string[] args)
{
  CEnumResult oEnumResult = CEnumResultClassCommon.Error_Initialization;
  string sName = oEnumResult.Name;   // sName = "Error_Initialization"

  CEnum[] aoEnumResult = CEnumResultClassCommon.GetValues ();   // aoEnumResult = {testCEnumResult.Program.CEnumResult[9]}
  string[] asEnumNames = CEnum.GetNames (aoEnumResult);
  int ixValue = Array.IndexOf (aoEnumResult, oEnumResult);    // ixValue = 6
}

Answer 10

I realize I'm a bit late to this party, but here's my two cents.

We're all clear that Enum inheritance is not supported by the framework. Some very interesting workarounds have been suggested in this thread, but none of them felt quite like what I was looking for, so I had a go at it myself.

Introducing: ObjectEnum

You can check the code and documentation here: https://github.com/dimi3tron/ObjectEnum.

And the package here: https://www.nuget.org/packages/ObjectEnum

Or just install it: Install-Package ObjectEnum

In short, ObjectEnum<TEnum> acts as a wrapper for any enum. By overriding the GetDefinedValues() in subclasses, one can specify which enum values are valid for this specific class.

A number of operator overloads have been added to make an ObjectEnum<TEnum> instance behave as if it were an instance of the underlying enum, keeping in mind the defined value restrictions. This means you can easily compare the instance to an int or enum value, and thus use it in a switch case or any other conditional.

I'd like to refer to the github repo mentioned above for examples and further info.

I hope you find this useful. Feel free to comment or open an issue on github for further thoughts or comments.

Here are a few short examples of what you can do with ObjectEnum<TEnum>:

var sunday = new WorkDay(DayOfWeek.Sunday); //throws exception
var monday = new WorkDay(DayOfWeek.Monday); //works fine
var label = $"{monday} is day {(int)monday}." //produces: "Monday is day 1."
var mondayIsAlwaysMonday = monday == DayOfWeek.Monday; //true, sorry...

var friday = new WorkDay(DayOfWeek.Friday);

switch((DayOfWeek)friday){
    case DayOfWeek.Monday:
        //do something monday related
        break;
        /*...*/
    case DayOfWeek.Friday:
        //do something friday related
        break;
}

Answer 11

Enums are not actual classes, even if they look like it. Internally, they are treated just like their underlying type (by default Int32). Therefore, you can only do this by "copying" single values from one enum to another and casting them to their integer number to compare them for equality.

Answer 12

Enums cannot be derrived from other enums, but only from int, uint, short, ushort, long, ulong, byte and sbyte.

Like Pascal said, you can use other enum's values or constants to initialize an enum value, but that's about it.

Answer 13

another possible solution:

public enum @base
{
    x,
    y,
    z
}

public enum consume
{
    x = @base.x,
    y = @base.y,
    z = @base.z,

    a,b,c
}

// TODO: Add a unit-test to check that if @base and consume are aligned

HTH

Answer 14

This is not possible (as @JaredPar already mentioned). Trying to put logic to work around this is a bad practice. In case you have a base class that have an enum, you should list of all possible enum-values there, and the implementation of class should work with the values that it knows.

E.g. Supposed you have a base class BaseCatalog, and it has an enum ProductFormats (Digital, Physical). Then you can have a MusicCatalog or BookCatalog that could contains both Digital and Physical products, But if the class is ClothingCatalog, it should only contains Physical products.

Answer 15

You can perform inheritance in enum, however it's limited to following types only . int, uint, byte, sbyte, short, ushort, long, ulong

E.g.

public enum Car:int{
Toyota,
Benz,
}