FormLinks Target Getter Interfaces
FormLinks are FormKeys with typing information mixed in as to which record type they should associate with. As such, they require you specify the typing you want to target.
Take for example that you just wanted to target Npcs, there are still a few options:
Npc-> The direct classINpc-> The setter interfaceINpcGetter-> The readonly interface
The correct usage (99% of the time) is to always use the INpcGetter readonly interface.
IFormLinkGetter<INpcGetter>
Or more rarely the mutable version:
FormLink<INpcGetter>
In both, the generic is targeting the getter interface INpcGetter, which is the important part.
Use Setters With Caution
There are very limited and intentional scenarios where having a FormLink target a mutable type is desirable, but for the most part it's a pitfall trap for new users.
Complication
Consider a LinkCache Resolve. A FormLink and a LinkCache are combined to look up a record with a specific FormKey and Type.
You are allowed to use any record type in a resolve query, but using a non-getter interface is often less ideal. This is because it limits the scope that the LinkCache can match against. This can result in a confusing failure to match where it might find the Npc with the target FormKey, but not be able to satisfy the more restrictive type.
IFormLinkGetter<INpc> myTargetNpc = ...;
if (myTargetNpc.TryResolve(myLinkCache, out var npc))
{
// Found a INpc!
}
TryResolve call wants to return an INpc type to you. But if all it can find is a readonly INpcGetter, it cannot pretend that it's settable, and so fails to match. This is the result of you asking the system to find an Npc that is settable, when the ones that exist are only getters.
You can solve this issue by modifying the TryResolve scope:
IFormLinkGetter<INpc> myTargetNpc = ...;
if (myTargetNpc.TryResolve<INpcGetter>(myLinkCache, out var npc))
{
// Found a INpcGetter!
}