Deallocate memory from C# dictionary contained in a static object

Dictionaries store items in a hash table. An array is used internally for this. Because of the way hash tables work, this array must always be larger than the actual number of items stored (at least about 30% larger). Microsoft uses a load factor of 72%, i.e. at least 28% of the array will be empty (see An Extensive Examination of Data Structures Using C# 2.0 and especially The System.Collections.Hashtable Class and The System.Collections.Generic.Dictionary Class) Therefore the null/null entries could just represent this free space.

If the array is too small, it will grow automatically; however, when items are removed, the array does not shrink, but the space that will be freed up should be reused when new items are inserted.

If you are in control of this dictionary, you could try to re-create it in order to shrink it:

theDict = new Dictionary<TKey, IEnumerable<KeyValuePair<TKey2, TVal>>>(theDict);

But the problem might arise from the actual (non empty) entries. Your dictionary is static and will therefore never be reclaimed automatically by the garbage collector, unless you assign it another dictionary or null (theDict = new ... or theDict = null). This is only true for the dictionary itself which is static, not for its entries. As long as references to removed entries exist somewhere else, they will persist. The GC will reclaim any object (earlier or later) which cannot be accessed any more through some reference. It makes no difference, whether this object was declared static or not. The objects themselves are not static, only their references.

---

As @RobertTausig kindly pointed out, there is the new Dictionary.TrimExcess() since .NET Core 2.1, which is what you actually wanted, but didn't exist back then.

Looks like you need to recycle space in that dict periodically. You can do that by creating a new one: new Dictionary<a,b>(oldDict). Be sure to do this in a thread-safe manner.

When to do this? Either on the tick of a timer (60sec?) or when a specific number of writes has occurred (100k?) (you'd need to keep a modification counter).