Question: I have a list called `Population`, is a great list of very many positions and at some point I stop using it. How I can free the resources?
Then this is part of the code:

```
 private List <BasePopulation> Population=new List <BasePopulation>();
 Population.SomeMethod();
 Population.Clear();

```

I used the Clear method, but not works. Any idea?

Answer: 
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](https://msdn.microsoft.com/en-gb/library/xe0c2357(v=vs.110).aspx).

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.