c# - How do I implement caching in an immutable way? -

i've read , heard lot of things immutability, decided try out in 1 of hobby projects. declared of fields readonly, , made methods mutate object return new, modified version.

it worked great until ran situation method should, external protocol, return information object without modifying it, @ same time optimized modifying internal structure. in particular, happens tree path compression in union find algorithm.

when user calls int find(int n), object appears unmodified outsider. represents same entity conceptually, it's internal fields mutated optimize running time.

how can implement in immutable way?

short answer: have ensure thread-safety yourself.

the readonly keyword on field gives insurance field cannot modified after object containing field has been constructed. write can have field contained in constructor (or in field initialization), , read through method call cannot occur before object constructed, hence thread-safety of readonly.

if want implement caching, break assumption 1 write occurs (since "caching writes" can , occur during reads), , there can threading problems in bad cases (think you're reading lines file, 2 threads can call find method same parameter read 2 different lines , therefore different results). want implement observational immutability. related question memoization may elegant answer.