Feature request: C# 7.3 enum generic constraint not being able to compare to default()

[douglasg14b]

I am unable to compare a restricted type parameter to it's default().

Rough Example:

public class MyClass<TEnum> where TEnum : System.Enum
    {
        public bool SomeMethod(TEnum myEnum)
        {
            return default(TEnum) == myEnum;
        }
}

This throws the compiler error:

Error CS0019 Operator '==' cannot be applied to operands of type 'TEnum' and 'TEnum'

Do I need to do something else?

[CyrusNajmabadi]

Do I need to do something else?

That code will not work since the size of the enum isn't known at compile time (and the type literally could be System.Enum not a subclass), so there's no way to know which == to call. You can replace the above with default(TEnum).Equals(myEnum) though.

[douglasg14b]

@CyrusNajmabadi

Thanks for the explanation.

Are there additional generic constraints that could clear that up at compile time? Either existing or proposed/planned?

[bbarry]

Does default(TEnum).Equals(myEnum) box? And it could throw an exception if you instantiated it as new MyClass<System.Enum>()...

I think you want this:

using System.Collections.Generic;

public class MyClass<TEnum> where TEnum : struct, System.Enum
    {
        public bool SomeMethod(TEnum myEnum)
        {
            return EqualityComparer<TEnum>.Default.Equals(default, myEnum);
        }
}

[yaakov-h]

Does default(TEnum).Equals(myEnum) box?

It sure seems like it. sharplab

[CyrusNajmabadi]

Are there additional generic constraints that could clear that up at compile time? Either existing or proposed/planned?

It's possible that the 'Shapes' proposal would cover this.