This is the solution to the Can You Spot the DeadLock 2.

The problem

This code can deadlock. It actually deadlocks pretty fast on my testbed. The deadlock occurs between the notification mechanism and the unsubscription method in the client class!

Where does it occur?

The notification thread is locked trying to gets its notification through

public class Client: IDisposable {
...
private void OnEvent(object sender, EventArgs arguments) {
lock (this) // notification thread is locked here

The main thread is locked trying to unsubscribe properly
public class EventSource {
...
public event EventHandler EventOccured{
remove {
lock (this) // main thread is locked here
}

The source of the issue is that we have two conflicting resource acquisition paths:
1) The notification path, where first (the lock for) the event is acquired (to prevent modification on the fly of the subscribers list) and then the client (lock) is acquired (to ensure exclusive access to its internals).
2) The unsubscription path wich first acquire the client (to ensure exclusive access to its internals) and then acquire the event for safe unsubscription.

How can it be fixed?

That is the impossible part. There is no proper solution to this problem! That one of the reason I like it so much. But the keyword here is proper. You can resolve the deadlock assuming you are ok to relax the contract a bit.

What is the contract here?

This code (unsuccessful) implements a strong contract: The Client will not receive any notification before the end of the subscription method and it will not receive any notifications after the end of unsubscription method. The first assumption is in fact trivial: how could a client receive notifications from a source it has not subscribed to yet :-). The second one looks similar and quite harmless, it even looks like very desirable. But its a trap: every implementation of this contract offers deadlock opportunities!

You need to relax it a bit and remove the second requirement: you must accept to (potentialy) receive notifications even after a succesful unsubscription. Then you can drop the locks in EventSource.