generic constrains for enum

[ali50m]

I think the generic constrain should not accept the Yoo format. Since you can define it, but can not use it as type.

Could someone help to point out how to use this Yoo generic since I can define it.

public class Foo<T> where T : struct, Enum
{
    public T Value { get; set; }
}


public class Yoo<T> where T : class, Enum
{
    public T Value { get; set; }
}

public class Bar
{
    public Foo<Goo> Goo { get; set; } = new();

    // this line will cause compiling error!
    public Yoo<Goo> Yoo { get; set; } = new();
}

public enum Goo
{
    A,
    B,
}

[CyrusNajmabadi]

Could someone help to point out how to use this Yoo generic since I can define it.

you can do Yoo<System.Enum>. You'd also be able to use any other reference type subclass of System.Enum if any are added by the BCL in the future.

[ali50m]

@CyrusNajmabadi thanks for your reply. But I can`t define Foo<System.Enum> as type now. Maybe the Foo should not have the constrains both for struct and Enum at the same time.

[CyrusNajmabadi]

thanks for your reply. But I can`t define Foo<System.Enum> as type now.

Correct. But you asked how to use this Yoo generic. You can use the Yoo generic with the System.Enum type. You can use the Foo generic with any actual enum type.

Maybe the Foo should not have the constrains both for struct and Enum at the same time.

The Foo generic makes complete sense. It is saying that to be instantiated the type must be a subtype of System.Enum, and that it must be a struct. In .net today there are tons of types that satisfy this. Namely, every single enum.

The Yoo generic is odd, but still makes sense. It is saying that to be instantiated the type must be a subtype of System.ENum (or System.Enum itself), and that it must be a class. In .net today only one type that satisfies this (System.Enum itself). Perhaps in the future there will be more. For now though, it's probably not something anyone would want to do.

[zacharylayne]

Perhaps in the future there will be more.

CONFIRMED: C# 13 introducing Enum classes.

[ali50m]

@zacharylayne OMG! Do you have the link?

[huoyaoyuan]

CONFIRMED: C# 13 introducing Enum classes.

No. C# 13 doesn't contain any change related to enums.

Even if some concepts are going to happen in the future, it's still very unlikely to see any new class inheriting from System.Enum, because it inherits from System.ValueType, which blocks non-special classes inheriting from it.

[ali50m]

I found if I remove Enum from Foo constrains, then Foo<int> will possiable now.

[CyrusNajmabadi]

Correct. Because now you're not constraining to subtypes of System.Enum.