How free memory used by a large list in C#?

The problem may be that Clear is not doing what you think it is. Clear simply marks the List as being empty without resizing the internal array that it uses behind the scenes. It will, however, remove all references to the individual BasePopulation instances. So if no other data structure has a reference to them they will be eligible for garbage collection. But, it will not reduce the size of the List directly. I just verified this using ILSpy.

You have two options.

1. Set Population = null. This will unroot the entire object instance making it eligible for garbage collection.

2. Call TrimExcess on this List. This will resize the internal array.

Well, since the garbage collector (GC) is taking care of the memory management for you, the first thing you can do is getting rid of all references to the list (and the contained elements) so that the GC is able to remove it on the next occasion. You can do this, for example, by explicitly setting

Population = null;

If this isn't enough for you, for example because you're really eager to get rid of the objects now and you can accept non-optimal run-time behaviour, you can tell the GC to start collecting objects now via

GC.Collect();

More information about this method can be found here.

As indicated above, this practice can induce a performance penalty since it forces the GC to clean up resources at a point in the program where it usually wouldn't. Directly calling the method is thus often discouraged, but it might serve your needs if this is really a special point in your application. As a practical example, I've successfully improved peak memory usage in a program that requires a lot of objects during an initialization that can be discarded once the actual program execution has started. Here, the small performance penalty for calling GC.Collect() after the initialization was justifiable.