If users are expected to create UserDetails on every request/trace, we should consider making this a struct (maybe an immutable readonly struct?) to avoid the heap allocation. Since this would not be a tiny struct, we could also make the UserDetails parameter in the extension method an in parameter, so it's passed by reference.

If users decide to cache the UserDetails in the user session, it will end up in the heap, but that's still only one instance per user instead of one per request.

+1

I agree with the struct part, but I don't think we should use in when it's not a readonly struct.

From the docs:

Pass a struct as the argument for an in parameter only if it's declared with the readonly modifier or the method accesses only readonly members of the struct. Otherwise, the compiler must create defensive copies in many situations to ensure that arguments are not mutated. Consider the following example that calculates the distance of a 3D point from the origin:

Without some language guarantee, the compiler must create a temporary copy of the argument before calling any member not marked with the readonly modifier. The temporary storage is created on the stack, the values of the argument are copied to the temporary storage, and the value is copied to the stack for each member access as the this argument. In many situations, these copies harm performance enough that pass-by-value is faster than pass-by-read-only-reference when the argument type isn't a readonly struct and the method calls members that aren't marked readonly.

I think the idea is to use readonly struct and have best of both worlds, an struct that can be cached if the user wants (allocated in the heap) or avoid the heap allocation and do stack allocation if they want to use new in each request.

I mentioned readonly struct but I wasn't sure if we should enforce it. Thank you both for keeping me honest 🎉

The trouble with readonly struct here is that it makes the API worse IMO. No better than just having parameters passed to the method. Either way would be a breaking change if we want to add an extra parameter.

Struct is still an improvement, we just have to accept the copy I think 🤷‍♂️

No better than just having parameters passed to the method.

I forgot this is how UserDetails came to be in the first place 🤦🏽‍♂️