In AutoMapper 8.0 missing ResolveUsing

by

Replace ResolveUsing with MapFrom, and add one more input parameter to the lambda (TDestination).

.ForMember(d => d.UsersCount, map => map.MapFrom((s,d) => s.Users?.Count ?? 0))

EDIT November 2022

Newest version(s) of AutoMapper doesn’t support lambda expression in MapFrom. In this case you have to extract it to a method (Func) or do it inline if you can.
Also, null propagation isn’t allowed unless it’s a method.

.ForMember(d => d.UsersCount, map => map.MapFrom(s => MapUserCount(s))
--------------
private static int MapUserCount(ApplicationRole src) {
    return src.Users?.Count ?? 0;
}

or

.ForMember(d => d.UsersCount, map => map.MapFrom(s => s.Users == null ? 0 : s.Users.Count))

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