Generic enum as method parameter

Question

Given a constructor

public MyObject(int id){
    ID = id;
}

And two enums:

public enum MyEnum1{
    Something = 1,
    Anotherthing = 2
}

public enum MyEnum2{
    Dodo = 1,
    Moustache= 2
}

Is it possible to pass in a generic enum as a parameter of the constructor? I'm looking for a solution along the lines of:

public MyObject(enum someEnum){
    ID = (int)someEnum;
}

So you can do:

var newObject = new MyObject(MyEnum1.Something);
var anotherObject = new MyObject(MyEnum2.Dodo);

Risarise

You can receive the parameter as Integer, and convert your enums to Integers when you pass them. – Fiduciary

You can't do a generic constructor in C#. The best you could do is to write a nongeneric base class with a generic subclass, then make the enum type a type parameter for the generic subclass. Awkward. MyBase b = new MySub<EnumA>(EnumA.SomeValue); int n = b.ID; – Guibert

MyObject(MyEnum1 enum1Selection){} but integer is better – Belen

new MyObject((int)MyEnum1.Something); ? – Clanton

upvote @SuperPeanut, but, I would create a "objectType" enum and pass that to a factory that in turn creates the right thing with the right enum parameter. var newObject = myFactory.Create(ObjectType.One). – Nipissing

... Alternatively, overload the constructor. – Nipissing

Answer 1

Why do you want to pass the enums, while you could pass integers ?

var newObject = new MyObject((int)MyEnum1.Something);
var anotherObject = new MyObject((int)MyEnum2.Dodo);

and use your first constructor :

public MyObject(int id){
    ID = id;
}

Fiduciary

Yup this is what I was doing already, would just look more readable if I could pass the enum in directly – Risarise

If it is about readability, you can use @HimBromBeere solution with a generic constructor, you have to write the enum name twice but it looks all right... All depends on what you want – Fiduciary

Because Enum can have attribute Flags – Steib

Answer 2

Another option would be:

public MyObject(Enum someEnum){
    ID = Convert.ToInt32(someEnum);
}

This way you can use it like you requested without having to cast to int each time you call your contstructors:

var newObject = new MyObject(MyEnum1.Something);
var anotherObject = new MyObject(MyEnum2.Dodo);

Percutaneous

I'm trying to do this in a ctor of a class and there it seems it won't allow that. Are methods and ctors different in that point? Edit: it seems Attributes are special there and it's not allowed there. – Foregather

Answer 3

Just use a generic constructor:

class MyObject<T> {

    public MyObject(T someEnum) where T : struct, IConvertible
    {
        if (!typeof(T).IsEnum) 
            throw new ArgumentException("Not an enum");
        ID = Convert.ToInt32(someEnum);
    }
}

Now you can easily call it like this:

var m = new MyObject<MyEnum>(MyEnum1.Something);

But easier would be to pass the enum as integer to the constructor as mentioned in other answers.

EDIT: As of C# 7.3 you can use an enum-constraint right away:

class MyObject<T> where T: Enum { ... }

Embarrassment

You know you can't use Enum as a constraint? Life would be easier if you could but it won't compile. – Linton

@Linton You´re right, I updated the answer appropriately. Unfortunately you´re loosing compiler-type check against Enum with this approach making it possible to pass instances of int to the method as well. – Embarrassment

I am painfully aware of this. My day has been spent trying to find as robust of a solution as possible. But sadly the above is about as good as you'll get! – Linton

Answer 4

I have tried to do this with Attributes and it seems Enum as parameter in Attributes are not allowed. Therefore I had to do this workaround:

public MyCustomAttribute(object myEnumParameter)
{
    if (myEnumParameteris not Enum)
    {
        throw new ArgumentException("myEnumParameter is not an enum", 
            nameof(myEnumParameter));
    }

    MyEnumProperty = (Enum)myEnumParameter;
}

Foregather

Answer 5

Well, if you really need to make this call generic for a wide variety of types, then IMHO you should use:

1. Type.IsEnum to check if your argument is really an Enum;
2. Enum.GetUnderlyingType to know what type is your argument is based on (it's not necessarily an Int32);

3. Now cast your object.

   public static Int32 GetAnInt<T>(T arg) where T : struct
   {
       if ((typeof(T).IsEnum))
       {
           var underlyingType = typeof(T).GetEnumUnderlyingType();
           if (underlyingType == typeof(Int32)
           || underlyingType == typeof(Int16)) //etc.
           {
               try
               {
                   dynamic value = arg;
                   var result = (Int32)value; // can throw InvalidCast!
                   return result;
               }
               catch
               {
                   throw;
               }
           }
           else
           {
               throw new InvalidCastException("Underlying type
                            is certainly not castable to Int32!");
           }
       }
       else
       {
           throw new InvalidCastException("Not an Enum!");
       }
   }
   

   That way you achieve the beautiful syntax of: var j = GetAnInt(MyEnum.FirstValue);

Peccable

And I'm almost certain you may add a lot of different checks and polish this code for years =) – Peccable

And yes, it's easier to pass an integer. – Peccable

Answer 6

Are you using properties enum or parameters.

public enum Enum1{}
public Enum1 enum1 { get;set; }
public MyObject()
{
   ID = (int)enum1;
}

Just try it. I hope it is useful.

Commix

How exactly does it answer his question? – Breadwinner

And what will you get accessing a member property from a constructor? – Peccable