Why enumeration types in generic constraints are not unmanaged and struct?

[kyoyama-kazusa]

Hello everyone. I found the documentation for unmanaged types. The documentation said that the enumeration types are also be unmanaged types. However, when I use generic constraint, I cannot use sizeof(TEnum) even if I wrote where TEnum : Enum. The compiler says, "Cannot take the address of, get the size of, or declare a pointer to a managed type ('TEnum')", which makes me confused.

If and only if I wrote the both constraint Enum and unmanaged, sizeof(TEnum) will work. But why? Is runtime problem?

void F<TEnum>(TEnum a) where TEnum : unmanaged, Enum
{
    // ...
}

In addtion, Enums cannot be reference types, but why we should add struct constraint to make some features work?

[HaloFour]

System.Enum is a weird beast in the runtime. Even though it is the "base type" for all enums as far as the type system is concerned it is technically a reference type. That's why you must combine it with the struct (or unmanaged) constraint which eliminates System.Enum itself as being used as the generic type argument.

[kyoyama-kazusa]

I got it. Thank you!