The best way to regain memory footprint for collections like HashSet and List in .NET 6 and above is to utilize the Clear() method for the collection to remove all elements, which allows the collection to release references to those elements. However, if you want to reclaim the memory immediately, you can also consider creating a new instance of the collection instead of just clearing it, as this forces garbage collection of the old instance.

Another approach is to set the collection to null if it is no longer needed, which allows the garbage collector to reclaim memory when it determines that the collection is no longer referenced. Finally, for large collections, you may also want to consider using the GC.Collect() method, although this should be used sparingly as it forces garbage collection and can impact performance.

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