How to take a lock for similar request for a userid in C#?

Question

I have a function which Get the value from API and then update inside the cache. This function is used by all users but I do not want to update the value to cache multiple times. how to avoid multiple writes ? how to take locks for multiple users (if it is possible)? if lock is taken by user A user B should not be blocked and continue to update the value and take its own lock. All request for A should be updated only once to the cache.

Void foo(Guid userId){
    bool isFound = GetfromCache(userId);
    if(!isFound)
     var val = GetValueFromAPI();
    UpdateInsideCache(val, userId); // Here we could have multiple requests updating the same entry, since it is high scale system ~ Assume 100 Million request per day.
}