It is better to set the capacity of the list.

[behroozbc]

I just saw this code and noticed a small performance issue.

MQTTnet/Source/MQTTnet/Server/Internal/MqttClientSessionsManager.cs

Line 254 in 73d681b

var result = new List<MqttSessionStatus>();

If a list has a small number of items, it does not really matter to set the capacity of the list. However, when the list becomes large, it is important to set the capacity of the list. the cost of performance is O(log(n)) and happened because of the EnsureCapacity method of List<T>
There's a possibility that things might change like this

 var result = new List<MqttClientStatus>(_clients.Values.Count);

It would be my pleasure to find and fix the whole project if you would like.

[SeppPenner]

Sounds legit for me, what do you think of this @chkr1011?

[chkr1011]

Yes this is indeed a good optimization. I also applied it at several places but not yet everywhere.

@behroozbc When adding this please make sure to NOT replace the data types. List do support this but HashSet etc. may not.

[behroozbc]

Certainly, I'll begin to work on this in small chunks.