Why do you use "where T: struct" for generic enum extension methods in C#?

Question

I've seen examples of generic extension methods in C# for enumerations that use where T: struct, also another that uses where T: IComparable. For example, in the former case:

public static class EnumExtensionMethods
{
  public static string Description<T>(this T enumValue) where T : struct
  {
    // ...
  }
}

I'm confused why the constraint requires that type T must be a struct. I'd expect it to be where T : Enum. Can someone explain this to me? As a bonus item, maybe also explain why IComparable is also used in some examples.

FWIW, I did my research on this. I can find explanations on why IComparable is used, for example in this question, but it doesn't seem conclusive, nor do they explain why struct is used in conjunction.

Answer 1

I suspect you're looking at some code that was written before C# 7.3 - the ability to constrain a generic type parameter using Enum or Delegate was only introduced in C# 7.3.

But for extension methods targeting enums, you'd want both Enum and struct in the constraint, e.g.

public static string GetDescription<T>(this T enumValue) where T : struct, Enum
{
    // ...
}

That way it can only be called on concrete enum types, rather than GetDescription<Enum>(null).